[PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Dmitry Osipenko]

The vfp_kmode_exception() function now is unreachable using relative
branching in THUMB2 kernel configuration, resulting in a "relocation
truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
linker error. Let's use long jump in order to fix the issue.

Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
---
 arch/arm/vfp/vfphw.S | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
index 4fcff9f59947..6e2b29f0c48d 100644
--- a/arch/arm/vfp/vfphw.S
+++ b/arch/arm/vfp/vfphw.S
@@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
 	ldr	r3, [sp, #S_PSR]	@ Neither lazy restore nor FP exceptions
 	and	r3, r3, #MODE_MASK	@ are supported in kernel mode
 	teq	r3, #USR_MODE
-	bne	vfp_kmode_exception	@ Returns through lr
+	ldr	r1, =vfp_kmode_exception
+	bxne	r1			@ Returns through lr
 
 	VFPFMRX	r1, FPEXC		@ Is the VFP enabled?
 	DBGSTR1	"fpexc %08x", r1
-- 
2.27.0

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Kees Cook]

On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> The vfp_kmode_exception() function now is unreachable using relative
> branching in THUMB2 kernel configuration, resulting in a "relocation
> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> linker error. Let's use long jump in order to fix the issue.

Eek. Is this with gcc or clang?

> 
> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")

Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
That commit may have implicitly moved the location of .vfp11_veneer,
though I thought I had chosen the correct position.

> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> ---
>  arch/arm/vfp/vfphw.S | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> index 4fcff9f59947..6e2b29f0c48d 100644
> --- a/arch/arm/vfp/vfphw.S
> +++ b/arch/arm/vfp/vfphw.S
> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
>  	ldr	r3, [sp, #S_PSR]	@ Neither lazy restore nor FP exceptions
>  	and	r3, r3, #MODE_MASK	@ are supported in kernel mode
>  	teq	r3, #USR_MODE
> -	bne	vfp_kmode_exception	@ Returns through lr
> +	ldr	r1, =vfp_kmode_exception
> +	bxne	r1			@ Returns through lr
>  
>  	VFPFMRX	r1, FPEXC		@ Is the VFP enabled?
>  	DBGSTR1	"fpexc %08x", r1

This seems like a workaround though? I suspect the vfp11_veneer needs
moving?

-- 
Kees Cook

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Dmitry Osipenko]

22.10.2020 02:40, Kees Cook пишет:
> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
>> The vfp_kmode_exception() function now is unreachable using relative
>> branching in THUMB2 kernel configuration, resulting in a "relocation
>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
>> linker error. Let's use long jump in order to fix the issue.
> 
> Eek. Is this with gcc or clang?

GCC 9.3.0

>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> 
> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> That commit may have implicitly moved the location of .vfp11_veneer,
> though I thought I had chosen the correct position.

I re-checked that the fixes tag is correct.

>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>> ---
>>  arch/arm/vfp/vfphw.S | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
>> index 4fcff9f59947..6e2b29f0c48d 100644
>> --- a/arch/arm/vfp/vfphw.S
>> +++ b/arch/arm/vfp/vfphw.S
>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
>>  	ldr	r3, [sp, #S_PSR]	@ Neither lazy restore nor FP exceptions
>>  	and	r3, r3, #MODE_MASK	@ are supported in kernel mode
>>  	teq	r3, #USR_MODE
>> -	bne	vfp_kmode_exception	@ Returns through lr
>> +	ldr	r1, =vfp_kmode_exception
>> +	bxne	r1			@ Returns through lr
>>  
>>  	VFPFMRX	r1, FPEXC		@ Is the VFP enabled?
>>  	DBGSTR1	"fpexc %08x", r1
> 
> This seems like a workaround though? I suspect the vfp11_veneer needs
> moving?
> 

I don't know where it needs to be moved. Please feel free to make a
patch if you have a better idea, I'll be glad to test it.

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Kees Cook]

On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> 22.10.2020 02:40, Kees Cook пишет:
> > On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> >> The vfp_kmode_exception() function now is unreachable using relative
> >> branching in THUMB2 kernel configuration, resulting in a "relocation
> >> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> >> linker error. Let's use long jump in order to fix the issue.
> > 
> > Eek. Is this with gcc or clang?
> 
> GCC 9.3.0
> 
> >> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > 
> > Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > That commit may have implicitly moved the location of .vfp11_veneer,
> > though I thought I had chosen the correct position.
> 
> I re-checked that the fixes tag is correct.
> 
> >> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> >> ---
> >>  arch/arm/vfp/vfphw.S | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> >> index 4fcff9f59947..6e2b29f0c48d 100644
> >> --- a/arch/arm/vfp/vfphw.S
> >> +++ b/arch/arm/vfp/vfphw.S
> >> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> >>  	ldr	r3, [sp, #S_PSR]	@ Neither lazy restore nor FP exceptions
> >>  	and	r3, r3, #MODE_MASK	@ are supported in kernel mode
> >>  	teq	r3, #USR_MODE
> >> -	bne	vfp_kmode_exception	@ Returns through lr
> >> +	ldr	r1, =vfp_kmode_exception
> >> +	bxne	r1			@ Returns through lr
> >>  
> >>  	VFPFMRX	r1, FPEXC		@ Is the VFP enabled?
> >>  	DBGSTR1	"fpexc %08x", r1
> > 
> > This seems like a workaround though? I suspect the vfp11_veneer needs
> > moving?
> > 
> 
> I don't know where it needs to be moved. Please feel free to make a
> patch if you have a better idea, I'll be glad to test it.

I might have just been distracted by the common "vfp" prefix. It's
possible that the text section shuffling just ended up being very large,
so probably this patch is right then!

-- 
Kees Cook

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > 22.10.2020 02:40, Kees Cook пишет:
> > > On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > >> The vfp_kmode_exception() function now is unreachable using relative
> > >> branching in THUMB2 kernel configuration, resulting in a "relocation
> > >> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > >> linker error. Let's use long jump in order to fix the issue.
> > >
> > > Eek. Is this with gcc or clang?
> >
> > GCC 9.3.0
> >
> > >> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > >
> > > Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > That commit may have implicitly moved the location of .vfp11_veneer,
> > > though I thought I had chosen the correct position.
> >
> > I re-checked that the fixes tag is correct.
> >
> > >> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > >> ---
> > >>  arch/arm/vfp/vfphw.S | 3 ++-
> > >>  1 file changed, 2 insertions(+), 1 deletion(-)
> > >>
> > >> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > >> index 4fcff9f59947..6e2b29f0c48d 100644
> > >> --- a/arch/arm/vfp/vfphw.S
> > >> +++ b/arch/arm/vfp/vfphw.S
> > >> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > >>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > >>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > >>    teq     r3, #USR_MODE
> > >> -  bne     vfp_kmode_exception     @ Returns through lr
> > >> +  ldr     r1, =vfp_kmode_exception
> > >> +  bxne    r1                      @ Returns through lr
> > >>
> > >>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > >>    DBGSTR1 "fpexc %08x", r1
> > >
> > > This seems like a workaround though? I suspect the vfp11_veneer needs
> > > moving?
> > >
> >
> > I don't know where it needs to be moved. Please feel free to make a
> > patch if you have a better idea, I'll be glad to test it.
>
> I might have just been distracted by the common "vfp" prefix. It's
> possible that the text section shuffling just ended up being very large,
> so probably this patch is right then!
>

I already sent a fix for this issue:

https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Russell King - ARM Linux admin]

On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> The vfp_kmode_exception() function now is unreachable using relative
> branching in THUMB2 kernel configuration, resulting in a "relocation
> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> linker error. Let's use long jump in order to fix the issue.
> 
> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>

I guess the problem is that vfp_support_entry is in .text which tends
to be at the beginning of the text section, but vfp_kmode_exception
is in something like .text.vfp_kmode_exception ?

Would it be an idea just to change the section name that stuff like
vfp_support_entry ends up in, rather than making the code less
efficient?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Dmitry Osipenko]

22.10.2020 17:57, Russell King - ARM Linux admin пишет:
> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
>> The vfp_kmode_exception() function now is unreachable using relative
>> branching in THUMB2 kernel configuration, resulting in a "relocation
>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
>> linker error. Let's use long jump in order to fix the issue.
>>
>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> 
> I guess the problem is that vfp_support_entry is in .text which tends
> to be at the beginning of the text section, but vfp_kmode_exception
> is in something like .text.vfp_kmode_exception ?

Indeed, vfp_support_entry is at beginning of .text, while
vfp_kmode_exception is near the end.

> Would it be an idea just to change the section name that stuff like
> vfp_support_entry ends up in, rather than making the code less
> efficient?
> 

This works:

diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
index 4fcff9f59947..65f0a2ef3613 100644
--- a/arch/arm/vfp/vfphw.S
+++ b/arch/arm/vfp/vfphw.S
@@ -65,6 +65,7 @@
 #endif
 	.endm

+	.pushsection .vfp11_veneer, "ax"

 @ VFP hardware support entry point.
 @
diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c
index 8c9e7f9f0277..30b83b191174 100644
--- a/arch/arm/vfp/vfpmodule.c
+++ b/arch/arm/vfp/vfpmodule.c
@@ -642,7 +642,7 @@ static int vfp_starting_cpu(unsigned int unused)
 	return 0;
 }

-void vfp_kmode_exception(void)
+void __section(".vfp11_veneer") vfp_kmode_exception(void)
 {
 	/*
 	 * If we reach this point, a floating point exception has been raised

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Dmitry Osipenko]

22.10.2020 10:06, Ard Biesheuvel пишет:
> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
>>
>> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
>>> 22.10.2020 02:40, Kees Cook пишет:
>>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
>>>>> The vfp_kmode_exception() function now is unreachable using relative
>>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
>>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
>>>>> linker error. Let's use long jump in order to fix the issue.
>>>>
>>>> Eek. Is this with gcc or clang?
>>>
>>> GCC 9.3.0
>>>
>>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
>>>>
>>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
>>>> That commit may have implicitly moved the location of .vfp11_veneer,
>>>> though I thought I had chosen the correct position.
>>>
>>> I re-checked that the fixes tag is correct.
>>>
>>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>>>>> ---
>>>>>  arch/arm/vfp/vfphw.S | 3 ++-
>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
>>>>> index 4fcff9f59947..6e2b29f0c48d 100644
>>>>> --- a/arch/arm/vfp/vfphw.S
>>>>> +++ b/arch/arm/vfp/vfphw.S
>>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
>>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
>>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
>>>>>    teq     r3, #USR_MODE
>>>>> -  bne     vfp_kmode_exception     @ Returns through lr
>>>>> +  ldr     r1, =vfp_kmode_exception
>>>>> +  bxne    r1                      @ Returns through lr
>>>>>
>>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
>>>>>    DBGSTR1 "fpexc %08x", r1
>>>>
>>>> This seems like a workaround though? I suspect the vfp11_veneer needs
>>>> moving?
>>>>
>>>
>>> I don't know where it needs to be moved. Please feel free to make a
>>> patch if you have a better idea, I'll be glad to test it.
>>
>> I might have just been distracted by the common "vfp" prefix. It's
>> possible that the text section shuffling just ended up being very large,
>> so probably this patch is right then!
>>
> 
> I already sent a fix for this issue:
> 
> https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> 

The offending commit contains stable tag, so I assume that fixes tag is
mandatory. Yours patch misses the fixes tag.

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
>
> 22.10.2020 10:06, Ard Biesheuvel пишет:
> > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> >>
> >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> >>> 22.10.2020 02:40, Kees Cook пишет:
> >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> >>>>> The vfp_kmode_exception() function now is unreachable using relative
> >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> >>>>> linker error. Let's use long jump in order to fix the issue.
> >>>>
> >>>> Eek. Is this with gcc or clang?
> >>>
> >>> GCC 9.3.0
> >>>
> >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> >>>>
> >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> >>>> though I thought I had chosen the correct position.
> >>>
> >>> I re-checked that the fixes tag is correct.
> >>>
> >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> >>>>> ---
> >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> >>>>> --- a/arch/arm/vfp/vfphw.S
> >>>>> +++ b/arch/arm/vfp/vfphw.S
> >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> >>>>>    teq     r3, #USR_MODE
> >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> >>>>> +  ldr     r1, =vfp_kmode_exception
> >>>>> +  bxne    r1                      @ Returns through lr
> >>>>>
> >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> >>>>>    DBGSTR1 "fpexc %08x", r1
> >>>>
> >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> >>>> moving?
> >>>>
> >>>
> >>> I don't know where it needs to be moved. Please feel free to make a
> >>> patch if you have a better idea, I'll be glad to test it.
> >>
> >> I might have just been distracted by the common "vfp" prefix. It's
> >> possible that the text section shuffling just ended up being very large,
> >> so probably this patch is right then!
> >>
> >
> > I already sent a fix for this issue:
> >
> > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> >
>
> The offending commit contains stable tag, so I assume that fixes tag is
> mandatory. Yours patch misses the fixes tag.

Russell, mind adding that? Or would you like me to update the patch in
the patch system?

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Russell King - ARM Linux admin]

On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> >
> > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > >>
> > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > >>> 22.10.2020 02:40, Kees Cook пишет:
> > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > >>>>> linker error. Let's use long jump in order to fix the issue.
> > >>>>
> > >>>> Eek. Is this with gcc or clang?
> > >>>
> > >>> GCC 9.3.0
> > >>>
> > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > >>>>
> > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > >>>> though I thought I had chosen the correct position.
> > >>>
> > >>> I re-checked that the fixes tag is correct.
> > >>>
> > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > >>>>> ---
> > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > >>>>>
> > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > >>>>> --- a/arch/arm/vfp/vfphw.S
> > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > >>>>>    teq     r3, #USR_MODE
> > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > >>>>> +  ldr     r1, =vfp_kmode_exception
> > >>>>> +  bxne    r1                      @ Returns through lr
> > >>>>>
> > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > >>>>>    DBGSTR1 "fpexc %08x", r1
> > >>>>
> > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > >>>> moving?
> > >>>>
> > >>>
> > >>> I don't know where it needs to be moved. Please feel free to make a
> > >>> patch if you have a better idea, I'll be glad to test it.
> > >>
> > >> I might have just been distracted by the common "vfp" prefix. It's
> > >> possible that the text section shuffling just ended up being very large,
> > >> so probably this patch is right then!
> > >>
> > >
> > > I already sent a fix for this issue:
> > >
> > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > >
> >
> > The offending commit contains stable tag, so I assume that fixes tag is
> > mandatory. Yours patch misses the fixes tag.
> 
> Russell, mind adding that? Or would you like me to update the patch in
> the patch system?

Rather than adding the IT, I'm suggesting that we solve it a different
way - ensuring that the two bits of code are co-located. There's no
reason for them to be separated, and the assembly code entry point is
already called indirectly.

The problem is the assembly ends up in the .text section which ends up
at the start of the binary, but depending on the compiler, functions
in .c files end up in their own sections. It would be good if, as
Dmitry has shown that it is indeed possible, to have them co-located.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:
>
> On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > >
> > > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > >>
> > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > >>> 22.10.2020 02:40, Kees Cook пишет:
> > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > >>>>> linker error. Let's use long jump in order to fix the issue.
> > > >>>>
> > > >>>> Eek. Is this with gcc or clang?
> > > >>>
> > > >>> GCC 9.3.0
> > > >>>
> > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > >>>>
> > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > >>>> though I thought I had chosen the correct position.
> > > >>>
> > > >>> I re-checked that the fixes tag is correct.
> > > >>>
> > > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > >>>>> ---
> > > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >>>>>
> > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > >>>>> --- a/arch/arm/vfp/vfphw.S
> > > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > >>>>>    teq     r3, #USR_MODE
> > > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > >>>>> +  ldr     r1, =vfp_kmode_exception
> > > >>>>> +  bxne    r1                      @ Returns through lr
> > > >>>>>
> > > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > >>>>>    DBGSTR1 "fpexc %08x", r1
> > > >>>>
> > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > >>>> moving?
> > > >>>>
> > > >>>
> > > >>> I don't know where it needs to be moved. Please feel free to make a
> > > >>> patch if you have a better idea, I'll be glad to test it.
> > > >>
> > > >> I might have just been distracted by the common "vfp" prefix. It's
> > > >> possible that the text section shuffling just ended up being very large,
> > > >> so probably this patch is right then!
> > > >>
> > > >
> > > > I already sent a fix for this issue:
> > > >
> > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > >
> > >
> > > The offending commit contains stable tag, so I assume that fixes tag is
> > > mandatory. Yours patch misses the fixes tag.
> >
> > Russell, mind adding that? Or would you like me to update the patch in
> > the patch system?
>
> Rather than adding the IT, I'm suggesting that we solve it a different
> way - ensuring that the two bits of code are co-located. There's no
> reason for them to be separated, and the assembly code entry point is
> already called indirectly.
>
> The problem is the assembly ends up in the .text section which ends up
> at the start of the binary, but depending on the compiler, functions
> in .c files end up in their own sections. It would be good if, as
> Dmitry has shown that it is indeed possible, to have them co-located.
>

Why is that better? I provided a minimal fix which has zero impact on
ARM builds, and minimal impact on Thumb2 builds, given that it retains
the exact same semantics as before, but using a different opcode.

R_ARM_THM_JUMP19 only has -/+ 1 MB range, i.e., smaller than the size
of .text, and so using it for calls to external symbols is still going
to be risky, unless we create a completely separate code section for
VFP related routines. So we may need the IT instruction anyway, at
which point we don't need anything more.

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Russell King - ARM Linux admin]

On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> <linux@armlinux.org.uk> wrote:
> >
> > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > >
> > > > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > > >>
> > > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > > >>> 22.10.2020 02:40, Kees Cook пишет:
> > > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > > >>>>> linker error. Let's use long jump in order to fix the issue.
> > > > >>>>
> > > > >>>> Eek. Is this with gcc or clang?
> > > > >>>
> > > > >>> GCC 9.3.0
> > > > >>>
> > > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > > >>>>
> > > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > > >>>> though I thought I had chosen the correct position.
> > > > >>>
> > > > >>> I re-checked that the fixes tag is correct.
> > > > >>>
> > > > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > > >>>>> ---
> > > > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > >>>>>
> > > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > > >>>>> --- a/arch/arm/vfp/vfphw.S
> > > > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > > >>>>>    teq     r3, #USR_MODE
> > > > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > > >>>>> +  ldr     r1, =vfp_kmode_exception
> > > > >>>>> +  bxne    r1                      @ Returns through lr
> > > > >>>>>
> > > > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > > >>>>>    DBGSTR1 "fpexc %08x", r1
> > > > >>>>
> > > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > > >>>> moving?
> > > > >>>>
> > > > >>>
> > > > >>> I don't know where it needs to be moved. Please feel free to make a
> > > > >>> patch if you have a better idea, I'll be glad to test it.
> > > > >>
> > > > >> I might have just been distracted by the common "vfp" prefix. It's
> > > > >> possible that the text section shuffling just ended up being very large,
> > > > >> so probably this patch is right then!
> > > > >>
> > > > >
> > > > > I already sent a fix for this issue:
> > > > >
> > > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > > >
> > > >
> > > > The offending commit contains stable tag, so I assume that fixes tag is
> > > > mandatory. Yours patch misses the fixes tag.
> > >
> > > Russell, mind adding that? Or would you like me to update the patch in
> > > the patch system?
> >
> > Rather than adding the IT, I'm suggesting that we solve it a different
> > way - ensuring that the two bits of code are co-located. There's no
> > reason for them to be separated, and the assembly code entry point is
> > already called indirectly.
> >
> > The problem is the assembly ends up in the .text section which ends up
> > at the start of the binary, but depending on the compiler, functions
> > in .c files end up in their own sections. It would be good if, as
> > Dmitry has shown that it is indeed possible, to have them co-located.
> 
> Why is that better? I provided a minimal fix which has zero impact on
> ARM builds, and minimal impact on Thumb2 builds, given that it retains
> the exact same semantics as before, but using a different opcode.

I think you just described the reason there. Why should we force
everything to use a different opcode when a short jump _should_
suffice?

Your patch may be a single line, but it has a slightly greater
impact than the alternative two line solution.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 18:23, Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:
>
> On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > <linux@armlinux.org.uk> wrote:
> > >
> > > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > > >
> > > > > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > > > >>
> > > > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > > > >>> 22.10.2020 02:40, Kees Cook пишет:
> > > > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > > > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > > > >>>>> linker error. Let's use long jump in order to fix the issue.
> > > > > >>>>
> > > > > >>>> Eek. Is this with gcc or clang?
> > > > > >>>
> > > > > >>> GCC 9.3.0
> > > > > >>>
> > > > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > > > >>>>
> > > > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > > > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > > > >>>> though I thought I had chosen the correct position.
> > > > > >>>
> > > > > >>> I re-checked that the fixes tag is correct.
> > > > > >>>
> > > > > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > > > >>>>> ---
> > > > > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > > > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > >>>>>
> > > > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > > > >>>>> --- a/arch/arm/vfp/vfphw.S
> > > > > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > > > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > > > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > > > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > > > >>>>>    teq     r3, #USR_MODE
> > > > > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > > > >>>>> +  ldr     r1, =vfp_kmode_exception
> > > > > >>>>> +  bxne    r1                      @ Returns through lr
> > > > > >>>>>
> > > > > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > > > >>>>>    DBGSTR1 "fpexc %08x", r1
> > > > > >>>>
> > > > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > > > >>>> moving?
> > > > > >>>>
> > > > > >>>
> > > > > >>> I don't know where it needs to be moved. Please feel free to make a
> > > > > >>> patch if you have a better idea, I'll be glad to test it.
> > > > > >>
> > > > > >> I might have just been distracted by the common "vfp" prefix. It's
> > > > > >> possible that the text section shuffling just ended up being very large,
> > > > > >> so probably this patch is right then!
> > > > > >>
> > > > > >
> > > > > > I already sent a fix for this issue:
> > > > > >
> > > > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > > > >
> > > > >
> > > > > The offending commit contains stable tag, so I assume that fixes tag is
> > > > > mandatory. Yours patch misses the fixes tag.
> > > >
> > > > Russell, mind adding that? Or would you like me to update the patch in
> > > > the patch system?
> > >
> > > Rather than adding the IT, I'm suggesting that we solve it a different
> > > way - ensuring that the two bits of code are co-located. There's no
> > > reason for them to be separated, and the assembly code entry point is
> > > already called indirectly.
> > >
> > > The problem is the assembly ends up in the .text section which ends up
> > > at the start of the binary, but depending on the compiler, functions
> > > in .c files end up in their own sections. It would be good if, as
> > > Dmitry has shown that it is indeed possible, to have them co-located.
> >
> > Why is that better? I provided a minimal fix which has zero impact on
> > ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > the exact same semantics as before, but using a different opcode.
>
> I think you just described the reason there. Why should we force
> everything to use a different opcode when a short jump _should_
> suffice?
>

Why should a short jump suffice? The call is to vfp_kmode_exception(),
which we only call in exceptional cases. Why would we want to keep
that in close proximity?

> Your patch may be a single line, but it has a slightly greater
> impact than the alternative two line solution.
>

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Dmitry Osipenko]

22.10.2020 19:23, Russell King - ARM Linux admin пишет:
> On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
>> On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
>> <linux@armlinux.org.uk> wrote:
>>>
>>> On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
>>>> On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
>>>>>
>>>>> 22.10.2020 10:06, Ard Biesheuvel пишет:
>>>>>> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
>>>>>>>
>>>>>>> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
>>>>>>>> 22.10.2020 02:40, Kees Cook пишет:
>>>>>>>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
>>>>>>>>>> The vfp_kmode_exception() function now is unreachable using relative
>>>>>>>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
>>>>>>>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
>>>>>>>>>> linker error. Let's use long jump in order to fix the issue.
>>>>>>>>>
>>>>>>>>> Eek. Is this with gcc or clang?
>>>>>>>>
>>>>>>>> GCC 9.3.0
>>>>>>>>
>>>>>>>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
>>>>>>>>>
>>>>>>>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
>>>>>>>>> That commit may have implicitly moved the location of .vfp11_veneer,
>>>>>>>>> though I thought I had chosen the correct position.
>>>>>>>>
>>>>>>>> I re-checked that the fixes tag is correct.
>>>>>>>>
>>>>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
>>>>>>>>>> ---
>>>>>>>>>>  arch/arm/vfp/vfphw.S | 3 ++-
>>>>>>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>>>>>>
>>>>>>>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
>>>>>>>>>> index 4fcff9f59947..6e2b29f0c48d 100644
>>>>>>>>>> --- a/arch/arm/vfp/vfphw.S
>>>>>>>>>> +++ b/arch/arm/vfp/vfphw.S
>>>>>>>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
>>>>>>>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
>>>>>>>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
>>>>>>>>>>    teq     r3, #USR_MODE
>>>>>>>>>> -  bne     vfp_kmode_exception     @ Returns through lr
>>>>>>>>>> +  ldr     r1, =vfp_kmode_exception
>>>>>>>>>> +  bxne    r1                      @ Returns through lr
>>>>>>>>>>
>>>>>>>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
>>>>>>>>>>    DBGSTR1 "fpexc %08x", r1
>>>>>>>>>
>>>>>>>>> This seems like a workaround though? I suspect the vfp11_veneer needs
>>>>>>>>> moving?
>>>>>>>>>
>>>>>>>>
>>>>>>>> I don't know where it needs to be moved. Please feel free to make a
>>>>>>>> patch if you have a better idea, I'll be glad to test it.
>>>>>>>
>>>>>>> I might have just been distracted by the common "vfp" prefix. It's
>>>>>>> possible that the text section shuffling just ended up being very large,
>>>>>>> so probably this patch is right then!
>>>>>>>
>>>>>>
>>>>>> I already sent a fix for this issue:
>>>>>>
>>>>>> https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
>>>>>>
>>>>>
>>>>> The offending commit contains stable tag, so I assume that fixes tag is
>>>>> mandatory. Yours patch misses the fixes tag.
>>>>
>>>> Russell, mind adding that? Or would you like me to update the patch in
>>>> the patch system?
>>>
>>> Rather than adding the IT, I'm suggesting that we solve it a different
>>> way - ensuring that the two bits of code are co-located. There's no
>>> reason for them to be separated, and the assembly code entry point is
>>> already called indirectly.
>>>
>>> The problem is the assembly ends up in the .text section which ends up
>>> at the start of the binary, but depending on the compiler, functions
>>> in .c files end up in their own sections. It would be good if, as
>>> Dmitry has shown that it is indeed possible, to have them co-located.
>>
>> Why is that better? I provided a minimal fix which has zero impact on
>> ARM builds, and minimal impact on Thumb2 builds, given that it retains
>> the exact same semantics as before, but using a different opcode.
> 
> I think you just described the reason there. Why should we force
> everything to use a different opcode when a short jump _should_
> suffice?
> 
> Your patch may be a single line, but it has a slightly greater
> impact than the alternative two line solution.
> 

But the two line change isn't portable to stable kernels as-is, isn't it?

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Russell King - ARM Linux admin]

On Thu, Oct 22, 2020 at 07:34:38PM +0300, Dmitry Osipenko wrote:
> 22.10.2020 19:23, Russell King - ARM Linux admin пишет:
> > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> >> On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> >> <linux@armlinux.org.uk> wrote:
> >>>
> >>> On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> >>>> On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> >>>>>
> >>>>> 22.10.2020 10:06, Ard Biesheuvel пишет:
> >>>>>> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> >>>>>>>
> >>>>>>> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> >>>>>>>> 22.10.2020 02:40, Kees Cook пишет:
> >>>>>>>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> >>>>>>>>>> The vfp_kmode_exception() function now is unreachable using relative
> >>>>>>>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> >>>>>>>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> >>>>>>>>>> linker error. Let's use long jump in order to fix the issue.
> >>>>>>>>>
> >>>>>>>>> Eek. Is this with gcc or clang?
> >>>>>>>>
> >>>>>>>> GCC 9.3.0
> >>>>>>>>
> >>>>>>>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> >>>>>>>>>
> >>>>>>>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> >>>>>>>>> That commit may have implicitly moved the location of .vfp11_veneer,
> >>>>>>>>> though I thought I had chosen the correct position.
> >>>>>>>>
> >>>>>>>> I re-checked that the fixes tag is correct.
> >>>>>>>>
> >>>>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> >>>>>>>>>> ---
> >>>>>>>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> >>>>>>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>>>>>>>>>
> >>>>>>>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> >>>>>>>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> >>>>>>>>>> --- a/arch/arm/vfp/vfphw.S
> >>>>>>>>>> +++ b/arch/arm/vfp/vfphw.S
> >>>>>>>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> >>>>>>>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> >>>>>>>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> >>>>>>>>>>    teq     r3, #USR_MODE
> >>>>>>>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> >>>>>>>>>> +  ldr     r1, =vfp_kmode_exception
> >>>>>>>>>> +  bxne    r1                      @ Returns through lr
> >>>>>>>>>>
> >>>>>>>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> >>>>>>>>>>    DBGSTR1 "fpexc %08x", r1
> >>>>>>>>>
> >>>>>>>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> >>>>>>>>> moving?
> >>>>>>>>>
> >>>>>>>>
> >>>>>>>> I don't know where it needs to be moved. Please feel free to make a
> >>>>>>>> patch if you have a better idea, I'll be glad to test it.
> >>>>>>>
> >>>>>>> I might have just been distracted by the common "vfp" prefix. It's
> >>>>>>> possible that the text section shuffling just ended up being very large,
> >>>>>>> so probably this patch is right then!
> >>>>>>>
> >>>>>>
> >>>>>> I already sent a fix for this issue:
> >>>>>>
> >>>>>> https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> >>>>>>
> >>>>>
> >>>>> The offending commit contains stable tag, so I assume that fixes tag is
> >>>>> mandatory. Yours patch misses the fixes tag.
> >>>>
> >>>> Russell, mind adding that? Or would you like me to update the patch in
> >>>> the patch system?
> >>>
> >>> Rather than adding the IT, I'm suggesting that we solve it a different
> >>> way - ensuring that the two bits of code are co-located. There's no
> >>> reason for them to be separated, and the assembly code entry point is
> >>> already called indirectly.
> >>>
> >>> The problem is the assembly ends up in the .text section which ends up
> >>> at the start of the binary, but depending on the compiler, functions
> >>> in .c files end up in their own sections. It would be good if, as
> >>> Dmitry has shown that it is indeed possible, to have them co-located.
> >>
> >> Why is that better? I provided a minimal fix which has zero impact on
> >> ARM builds, and minimal impact on Thumb2 builds, given that it retains
> >> the exact same semantics as before, but using a different opcode.
> > 
> > I think you just described the reason there. Why should we force
> > everything to use a different opcode when a short jump _should_
> > suffice?
> > 
> > Your patch may be a single line, but it has a slightly greater
> > impact than the alternative two line solution.
> > 
> 
> But the two line change isn't portable to stable kernels as-is, isn't it?

Why not?

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 19:38, Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:
>
> On Thu, Oct 22, 2020 at 07:34:38PM +0300, Dmitry Osipenko wrote:
> > 22.10.2020 19:23, Russell King - ARM Linux admin пишет:
> > > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > >> On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > >> <linux@armlinux.org.uk> wrote:
> > >>>
> > >>> On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > >>>> On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > >>>>>
> > >>>>> 22.10.2020 10:06, Ard Biesheuvel пишет:
> > >>>>>> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > >>>>>>>
> > >>>>>>> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > >>>>>>>> 22.10.2020 02:40, Kees Cook пишет:
> > >>>>>>>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > >>>>>>>>>> The vfp_kmode_exception() function now is unreachable using relative
> > >>>>>>>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > >>>>>>>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > >>>>>>>>>> linker error. Let's use long jump in order to fix the issue.
> > >>>>>>>>>
> > >>>>>>>>> Eek. Is this with gcc or clang?
> > >>>>>>>>
> > >>>>>>>> GCC 9.3.0
> > >>>>>>>>
> > >>>>>>>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > >>>>>>>>>
> > >>>>>>>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > >>>>>>>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > >>>>>>>>> though I thought I had chosen the correct position.
> > >>>>>>>>
> > >>>>>>>> I re-checked that the fixes tag is correct.
> > >>>>>>>>
> > >>>>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > >>>>>>>>>> ---
> > >>>>>>>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > >>>>>>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > >>>>>>>>>>
> > >>>>>>>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > >>>>>>>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > >>>>>>>>>> --- a/arch/arm/vfp/vfphw.S
> > >>>>>>>>>> +++ b/arch/arm/vfp/vfphw.S
> > >>>>>>>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > >>>>>>>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > >>>>>>>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > >>>>>>>>>>    teq     r3, #USR_MODE
> > >>>>>>>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > >>>>>>>>>> +  ldr     r1, =vfp_kmode_exception
> > >>>>>>>>>> +  bxne    r1                      @ Returns through lr
> > >>>>>>>>>>
> > >>>>>>>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > >>>>>>>>>>    DBGSTR1 "fpexc %08x", r1
> > >>>>>>>>>
> > >>>>>>>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > >>>>>>>>> moving?
> > >>>>>>>>>
> > >>>>>>>>
> > >>>>>>>> I don't know where it needs to be moved. Please feel free to make a
> > >>>>>>>> patch if you have a better idea, I'll be glad to test it.
> > >>>>>>>
> > >>>>>>> I might have just been distracted by the common "vfp" prefix. It's
> > >>>>>>> possible that the text section shuffling just ended up being very large,
> > >>>>>>> so probably this patch is right then!
> > >>>>>>>
> > >>>>>>
> > >>>>>> I already sent a fix for this issue:
> > >>>>>>
> > >>>>>> https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > >>>>>>
> > >>>>>
> > >>>>> The offending commit contains stable tag, so I assume that fixes tag is
> > >>>>> mandatory. Yours patch misses the fixes tag.
> > >>>>
> > >>>> Russell, mind adding that? Or would you like me to update the patch in
> > >>>> the patch system?
> > >>>
> > >>> Rather than adding the IT, I'm suggesting that we solve it a different
> > >>> way - ensuring that the two bits of code are co-located. There's no
> > >>> reason for them to be separated, and the assembly code entry point is
> > >>> already called indirectly.
> > >>>
> > >>> The problem is the assembly ends up in the .text section which ends up
> > >>> at the start of the binary, but depending on the compiler, functions
> > >>> in .c files end up in their own sections. It would be good if, as
> > >>> Dmitry has shown that it is indeed possible, to have them co-located.
> > >>
> > >> Why is that better? I provided a minimal fix which has zero impact on
> > >> ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > >> the exact same semantics as before, but using a different opcode.
> > >
> > > I think you just described the reason there. Why should we force
> > > everything to use a different opcode when a short jump _should_
> > > suffice?
> > >
> > > Your patch may be a single line, but it has a slightly greater
> > > impact than the alternative two line solution.
> > >
> >
> > But the two line change isn't portable to stable kernels as-is, isn't it?
>
> Why not?
>

In any case, I'd prefer not to dump VFP exception handling code into
the .vfp11_veneer section, which is documented as below, and typically
empty in our case, given that the only FP code we have in the kernel
is NEON code.



"""
   The '--vfp11-denorm-fix' switch enables a link-time workaround for a
bug in certain VFP11 coprocessor hardware, which sometimes allows
instructions with denorm operands (which must be handled by support
code) to have those operands overwritten by subsequent instructions
before the support code can read the intended values.

   The bug may be avoided in scalar mode if you allow at least one
intervening instruction between a VFP11 instruction which uses a
register and another instruction which writes to the same register, or
at least two intervening instructions if vector mode is in use.  The bug
only affects full-compliance floating-point mode: you do not need this
workaround if you are using "runfast" mode.  Please contact ARM for
further details.

   If you know you are using buggy VFP11 hardware, you can enable this
workaround by specifying the linker option '--vfp-denorm-fix=scalar' if
you are using the VFP11 scalar mode only, or '--vfp-denorm-fix=vector'
if you are using vector mode (the latter also works for scalar code).
The default is '--vfp-denorm-fix=none'.

   If the workaround is enabled, instructions are scanned for
potentially-troublesome sequences, and a veneer is created for each such
sequence which may trigger the erratum.  The veneer consists of the
first instruction of the sequence and a branch back to the subsequent
instruction.  The original instruction is then replaced with a branch to
the veneer.  The extra cycles required to call and return from the
veneer are sufficient to avoid the erratum in both the scalar and vector
cases.
"""

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Russell King - ARM Linux admin]

On Thu, Oct 22, 2020 at 06:33:17PM +0200, Ard Biesheuvel wrote:
> On Thu, 22 Oct 2020 at 18:23, Russell King - ARM Linux admin
> <linux@armlinux.org.uk> wrote:
> >
> > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > > On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > > <linux@armlinux.org.uk> wrote:
> > > >
> > > > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > > > >
> > > > > > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > > > > >>
> > > > > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > > > > >>> 22.10.2020 02:40, Kees Cook пишет:
> > > > > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > > > > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > > > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > > > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > > > > >>>>> linker error. Let's use long jump in order to fix the issue.
> > > > > > >>>>
> > > > > > >>>> Eek. Is this with gcc or clang?
> > > > > > >>>
> > > > > > >>> GCC 9.3.0
> > > > > > >>>
> > > > > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > > > > >>>>
> > > > > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > > > > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > > > > >>>> though I thought I had chosen the correct position.
> > > > > > >>>
> > > > > > >>> I re-checked that the fixes tag is correct.
> > > > > > >>>
> > > > > > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > > > > >>>>> ---
> > > > > > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > > > > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > > >>>>>
> > > > > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > > > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > > > > >>>>> --- a/arch/arm/vfp/vfphw.S
> > > > > > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > > > > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > > > > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > > > > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > > > > >>>>>    teq     r3, #USR_MODE
> > > > > > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > > > > >>>>> +  ldr     r1, =vfp_kmode_exception
> > > > > > >>>>> +  bxne    r1                      @ Returns through lr
> > > > > > >>>>>
> > > > > > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > > > > >>>>>    DBGSTR1 "fpexc %08x", r1
> > > > > > >>>>
> > > > > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > > > > >>>> moving?
> > > > > > >>>>
> > > > > > >>>
> > > > > > >>> I don't know where it needs to be moved. Please feel free to make a
> > > > > > >>> patch if you have a better idea, I'll be glad to test it.
> > > > > > >>
> > > > > > >> I might have just been distracted by the common "vfp" prefix. It's
> > > > > > >> possible that the text section shuffling just ended up being very large,
> > > > > > >> so probably this patch is right then!
> > > > > > >>
> > > > > > >
> > > > > > > I already sent a fix for this issue:
> > > > > > >
> > > > > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > > > > >
> > > > > >
> > > > > > The offending commit contains stable tag, so I assume that fixes tag is
> > > > > > mandatory. Yours patch misses the fixes tag.
> > > > >
> > > > > Russell, mind adding that? Or would you like me to update the patch in
> > > > > the patch system?
> > > >
> > > > Rather than adding the IT, I'm suggesting that we solve it a different
> > > > way - ensuring that the two bits of code are co-located. There's no
> > > > reason for them to be separated, and the assembly code entry point is
> > > > already called indirectly.
> > > >
> > > > The problem is the assembly ends up in the .text section which ends up
> > > > at the start of the binary, but depending on the compiler, functions
> > > > in .c files end up in their own sections. It would be good if, as
> > > > Dmitry has shown that it is indeed possible, to have them co-located.
> > >
> > > Why is that better? I provided a minimal fix which has zero impact on
> > > ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > > the exact same semantics as before, but using a different opcode.
> >
> > I think you just described the reason there. Why should we force
> > everything to use a different opcode when a short jump _should_
> > suffice?
> >
> 
> Why should a short jump suffice? The call is to vfp_kmode_exception(),
> which we only call in exceptional cases. Why would we want to keep
> that in close proximity?

You're thinking about it in terms of what happens when the branch is
taken, rather than also considering that this code path is also
traversed for _every_ single time that we enter the support code
not just for kernel mode.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Russell King - ARM Linux admin]

On Thu, Oct 22, 2020 at 07:47:57PM +0200, Ard Biesheuvel wrote:
> On Thu, 22 Oct 2020 at 19:38, Russell King - ARM Linux admin
> <linux@armlinux.org.uk> wrote:
> >
> > On Thu, Oct 22, 2020 at 07:34:38PM +0300, Dmitry Osipenko wrote:
> > > 22.10.2020 19:23, Russell King - ARM Linux admin пишет:
> > > > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > > >> On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > > >> <linux@armlinux.org.uk> wrote:
> > > >>>
> > > >>> On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > >>>> On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > >>>>>
> > > >>>>> 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > >>>>>> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > >>>>>>>
> > > >>>>>>> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > >>>>>>>> 22.10.2020 02:40, Kees Cook пишет:
> > > >>>>>>>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > >>>>>>>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > >>>>>>>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > >>>>>>>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > >>>>>>>>>> linker error. Let's use long jump in order to fix the issue.
> > > >>>>>>>>>
> > > >>>>>>>>> Eek. Is this with gcc or clang?
> > > >>>>>>>>
> > > >>>>>>>> GCC 9.3.0
> > > >>>>>>>>
> > > >>>>>>>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > >>>>>>>>>
> > > >>>>>>>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > >>>>>>>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > >>>>>>>>> though I thought I had chosen the correct position.
> > > >>>>>>>>
> > > >>>>>>>> I re-checked that the fixes tag is correct.
> > > >>>>>>>>
> > > >>>>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > >>>>>>>>>> ---
> > > >>>>>>>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > >>>>>>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >>>>>>>>>>
> > > >>>>>>>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > >>>>>>>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > >>>>>>>>>> --- a/arch/arm/vfp/vfphw.S
> > > >>>>>>>>>> +++ b/arch/arm/vfp/vfphw.S
> > > >>>>>>>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > >>>>>>>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > >>>>>>>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > >>>>>>>>>>    teq     r3, #USR_MODE
> > > >>>>>>>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > >>>>>>>>>> +  ldr     r1, =vfp_kmode_exception
> > > >>>>>>>>>> +  bxne    r1                      @ Returns through lr
> > > >>>>>>>>>>
> > > >>>>>>>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > >>>>>>>>>>    DBGSTR1 "fpexc %08x", r1
> > > >>>>>>>>>
> > > >>>>>>>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > >>>>>>>>> moving?
> > > >>>>>>>>>
> > > >>>>>>>>
> > > >>>>>>>> I don't know where it needs to be moved. Please feel free to make a
> > > >>>>>>>> patch if you have a better idea, I'll be glad to test it.
> > > >>>>>>>
> > > >>>>>>> I might have just been distracted by the common "vfp" prefix. It's
> > > >>>>>>> possible that the text section shuffling just ended up being very large,
> > > >>>>>>> so probably this patch is right then!
> > > >>>>>>>
> > > >>>>>>
> > > >>>>>> I already sent a fix for this issue:
> > > >>>>>>
> > > >>>>>> https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > >>>>>>
> > > >>>>>
> > > >>>>> The offending commit contains stable tag, so I assume that fixes tag is
> > > >>>>> mandatory. Yours patch misses the fixes tag.
> > > >>>>
> > > >>>> Russell, mind adding that? Or would you like me to update the patch in
> > > >>>> the patch system?
> > > >>>
> > > >>> Rather than adding the IT, I'm suggesting that we solve it a different
> > > >>> way - ensuring that the two bits of code are co-located. There's no
> > > >>> reason for them to be separated, and the assembly code entry point is
> > > >>> already called indirectly.
> > > >>>
> > > >>> The problem is the assembly ends up in the .text section which ends up
> > > >>> at the start of the binary, but depending on the compiler, functions
> > > >>> in .c files end up in their own sections. It would be good if, as
> > > >>> Dmitry has shown that it is indeed possible, to have them co-located.
> > > >>
> > > >> Why is that better? I provided a minimal fix which has zero impact on
> > > >> ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > > >> the exact same semantics as before, but using a different opcode.
> > > >
> > > > I think you just described the reason there. Why should we force
> > > > everything to use a different opcode when a short jump _should_
> > > > suffice?
> > > >
> > > > Your patch may be a single line, but it has a slightly greater
> > > > impact than the alternative two line solution.
> > > >
> > >
> > > But the two line change isn't portable to stable kernels as-is, isn't it?
> >
> > Why not?
> >
> 
> In any case, I'd prefer not to dump VFP exception handling code into
> the .vfp11_veneer section, which is documented as below, and typically
> empty in our case, given that the only FP code we have in the kernel
> is NEON code.

This is getting out of hand, and really getting beyond a joke. I
didn't say put it in the ".vfp11_veneer" section.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 19:50, Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:
>
> On Thu, Oct 22, 2020 at 07:47:57PM +0200, Ard Biesheuvel wrote:
> > On Thu, 22 Oct 2020 at 19:38, Russell King - ARM Linux admin
> > <linux@armlinux.org.uk> wrote:
> > >
> > > On Thu, Oct 22, 2020 at 07:34:38PM +0300, Dmitry Osipenko wrote:
> > > > 22.10.2020 19:23, Russell King - ARM Linux admin пишет:
> > > > > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > > > >> On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > > > >> <linux@armlinux.org.uk> wrote:
> > > > >>>
> > > > >>> On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > > >>>> On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > > >>>>>
> > > > >>>>> 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > >>>>>> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > > >>>>>>>
> > > > >>>>>>> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > > >>>>>>>> 22.10.2020 02:40, Kees Cook пишет:
> > > > >>>>>>>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > > >>>>>>>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > > >>>>>>>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > > >>>>>>>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > > >>>>>>>>>> linker error. Let's use long jump in order to fix the issue.
> > > > >>>>>>>>>
> > > > >>>>>>>>> Eek. Is this with gcc or clang?
> > > > >>>>>>>>
> > > > >>>>>>>> GCC 9.3.0
> > > > >>>>>>>>
> > > > >>>>>>>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > > >>>>>>>>>
> > > > >>>>>>>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > > >>>>>>>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > > >>>>>>>>> though I thought I had chosen the correct position.
> > > > >>>>>>>>
> > > > >>>>>>>> I re-checked that the fixes tag is correct.
> > > > >>>>>>>>
> > > > >>>>>>>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > > >>>>>>>>>> ---
> > > > >>>>>>>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > > >>>>>>>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > >>>>>>>>>>
> > > > >>>>>>>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > > >>>>>>>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > > >>>>>>>>>> --- a/arch/arm/vfp/vfphw.S
> > > > >>>>>>>>>> +++ b/arch/arm/vfp/vfphw.S
> > > > >>>>>>>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > > >>>>>>>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > > >>>>>>>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > > >>>>>>>>>>    teq     r3, #USR_MODE
> > > > >>>>>>>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > > >>>>>>>>>> +  ldr     r1, =vfp_kmode_exception
> > > > >>>>>>>>>> +  bxne    r1                      @ Returns through lr
> > > > >>>>>>>>>>
> > > > >>>>>>>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > > >>>>>>>>>>    DBGSTR1 "fpexc %08x", r1
> > > > >>>>>>>>>
> > > > >>>>>>>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > > >>>>>>>>> moving?
> > > > >>>>>>>>>
> > > > >>>>>>>>
> > > > >>>>>>>> I don't know where it needs to be moved. Please feel free to make a
> > > > >>>>>>>> patch if you have a better idea, I'll be glad to test it.
> > > > >>>>>>>
> > > > >>>>>>> I might have just been distracted by the common "vfp" prefix. It's
> > > > >>>>>>> possible that the text section shuffling just ended up being very large,
> > > > >>>>>>> so probably this patch is right then!
> > > > >>>>>>>
> > > > >>>>>>
> > > > >>>>>> I already sent a fix for this issue:
> > > > >>>>>>
> > > > >>>>>> https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > > >>>>>>
> > > > >>>>>
> > > > >>>>> The offending commit contains stable tag, so I assume that fixes tag is
> > > > >>>>> mandatory. Yours patch misses the fixes tag.
> > > > >>>>
> > > > >>>> Russell, mind adding that? Or would you like me to update the patch in
> > > > >>>> the patch system?
> > > > >>>
> > > > >>> Rather than adding the IT, I'm suggesting that we solve it a different
> > > > >>> way - ensuring that the two bits of code are co-located. There's no
> > > > >>> reason for them to be separated, and the assembly code entry point is
> > > > >>> already called indirectly.
> > > > >>>
> > > > >>> The problem is the assembly ends up in the .text section which ends up
> > > > >>> at the start of the binary, but depending on the compiler, functions
> > > > >>> in .c files end up in their own sections. It would be good if, as
> > > > >>> Dmitry has shown that it is indeed possible, to have them co-located.
> > > > >>
> > > > >> Why is that better? I provided a minimal fix which has zero impact on
> > > > >> ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > > > >> the exact same semantics as before, but using a different opcode.
> > > > >
> > > > > I think you just described the reason there. Why should we force
> > > > > everything to use a different opcode when a short jump _should_
> > > > > suffice?
> > > > >
> > > > > Your patch may be a single line, but it has a slightly greater
> > > > > impact than the alternative two line solution.
> > > > >
> > > >
> > > > But the two line change isn't portable to stable kernels as-is, isn't it?
> > >
> > > Why not?
> > >
> >
> > In any case, I'd prefer not to dump VFP exception handling code into
> > the .vfp11_veneer section, which is documented as below, and typically
> > empty in our case, given that the only FP code we have in the kernel
> > is NEON code.
>
> This is getting out of hand, and really getting beyond a joke. I
> didn't say put it in the ".vfp11_veneer" section.
>

No, but that is what Dmitry's patch proposes.

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 19:48, Russell King - ARM Linux admin
<linux@armlinux.org.uk> wrote:
>
> On Thu, Oct 22, 2020 at 06:33:17PM +0200, Ard Biesheuvel wrote:
> > On Thu, 22 Oct 2020 at 18:23, Russell King - ARM Linux admin
> > <linux@armlinux.org.uk> wrote:
> > >
> > > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > > > On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > > > <linux@armlinux.org.uk> wrote:
> > > > >
> > > > > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > > > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > > > > >
> > > > > > > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > > > > > >>
> > > > > > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > > > > > >>> 22.10.2020 02:40, Kees Cook пишет:
> > > > > > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > > > > > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > > > > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > > > > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > > > > > >>>>> linker error. Let's use long jump in order to fix the issue.
> > > > > > > >>>>
> > > > > > > >>>> Eek. Is this with gcc or clang?
> > > > > > > >>>
> > > > > > > >>> GCC 9.3.0
> > > > > > > >>>
> > > > > > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > > > > > >>>>
> > > > > > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > > > > > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > > > > > >>>> though I thought I had chosen the correct position.
> > > > > > > >>>
> > > > > > > >>> I re-checked that the fixes tag is correct.
> > > > > > > >>>
> > > > > > > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > > > > > >>>>> ---
> > > > > > > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > > > > > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > > > >>>>>
> > > > > > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > > > > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > > > > > >>>>> --- a/arch/arm/vfp/vfphw.S
> > > > > > > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > > > > > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > > > > > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > > > > > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > > > > > >>>>>    teq     r3, #USR_MODE
> > > > > > > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > > > > > >>>>> +  ldr     r1, =vfp_kmode_exception
> > > > > > > >>>>> +  bxne    r1                      @ Returns through lr
> > > > > > > >>>>>
> > > > > > > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > > > > > >>>>>    DBGSTR1 "fpexc %08x", r1
> > > > > > > >>>>
> > > > > > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > > > > > >>>> moving?
> > > > > > > >>>>
> > > > > > > >>>
> > > > > > > >>> I don't know where it needs to be moved. Please feel free to make a
> > > > > > > >>> patch if you have a better idea, I'll be glad to test it.
> > > > > > > >>
> > > > > > > >> I might have just been distracted by the common "vfp" prefix. It's
> > > > > > > >> possible that the text section shuffling just ended up being very large,
> > > > > > > >> so probably this patch is right then!
> > > > > > > >>
> > > > > > > >
> > > > > > > > I already sent a fix for this issue:
> > > > > > > >
> > > > > > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > > > > > >
> > > > > > >
> > > > > > > The offending commit contains stable tag, so I assume that fixes tag is
> > > > > > > mandatory. Yours patch misses the fixes tag.
> > > > > >
> > > > > > Russell, mind adding that? Or would you like me to update the patch in
> > > > > > the patch system?
> > > > >
> > > > > Rather than adding the IT, I'm suggesting that we solve it a different
> > > > > way - ensuring that the two bits of code are co-located. There's no
> > > > > reason for them to be separated, and the assembly code entry point is
> > > > > already called indirectly.
> > > > >
> > > > > The problem is the assembly ends up in the .text section which ends up
> > > > > at the start of the binary, but depending on the compiler, functions
> > > > > in .c files end up in their own sections. It would be good if, as
> > > > > Dmitry has shown that it is indeed possible, to have them co-located.
> > > >
> > > > Why is that better? I provided a minimal fix which has zero impact on
> > > > ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > > > the exact same semantics as before, but using a different opcode.
> > >
> > > I think you just described the reason there. Why should we force
> > > everything to use a different opcode when a short jump _should_
> > > suffice?
> > >
> >
> > Why should a short jump suffice? The call is to vfp_kmode_exception(),
> > which we only call in exceptional cases. Why would we want to keep
> > that in close proximity?
>
> You're thinking about it in terms of what happens when the branch is
> taken, rather than also considering that this code path is also
> traversed for _every_ single time that we enter the support code
> not just for kernel mode.
>

True. If 2 bytes of additional opcode are the concern here, we can
change the current sequence

   6:   f093 0f10       teq     r3, #16
   a:   f47f affe       bne.w   0 <vfp_kmode_exception>

to

   6:   2b10            cmp     r3, #16
   8:   bf18            it      ne
   a:   f7ff bffe       bne.w   0 <vfp_kmode_exception>

which takes up the exact same space.

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Dmitry Osipenko]

22.10.2020 20:38, Russell King - ARM Linux admin пишет:
...
>> But the two line change isn't portable to stable kernels as-is, isn't it?
> 
> Why not?
> 

I assume we will need to define a new section, no?

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 22 Oct 2020 at 19:59, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Thu, 22 Oct 2020 at 19:48, Russell King - ARM Linux admin
> <linux@armlinux.org.uk> wrote:
> >
> > On Thu, Oct 22, 2020 at 06:33:17PM +0200, Ard Biesheuvel wrote:
> > > On Thu, 22 Oct 2020 at 18:23, Russell King - ARM Linux admin
> > > <linux@armlinux.org.uk> wrote:
> > > >
> > > > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > > > > On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > > > > <linux@armlinux.org.uk> wrote:
> > > > > >
> > > > > > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > > > > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > > > > > >
> > > > > > > > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > > > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > > > > > > >>
> > > > > > > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > > > > > > >>> 22.10.2020 02:40, Kees Cook пишет:
> > > > > > > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > > > > > > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > > > > > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > > > > > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > > > > > > >>>>> linker error. Let's use long jump in order to fix the issue.
> > > > > > > > >>>>
> > > > > > > > >>>> Eek. Is this with gcc or clang?
> > > > > > > > >>>
> > > > > > > > >>> GCC 9.3.0
> > > > > > > > >>>
> > > > > > > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > > > > > > >>>>
> > > > > > > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > > > > > > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > > > > > > >>>> though I thought I had chosen the correct position.
> > > > > > > > >>>
> > > > > > > > >>> I re-checked that the fixes tag is correct.
> > > > > > > > >>>
> > > > > > > > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > > > > > > >>>>> ---
> > > > > > > > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > > > > > > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > > > > >>>>>
> > > > > > > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > > > > > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > > > > > > >>>>> --- a/arch/arm/vfp/vfphw.S
> > > > > > > > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > > > > > > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > > > > > > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > > > > > > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > > > > > > >>>>>    teq     r3, #USR_MODE
> > > > > > > > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > > > > > > >>>>> +  ldr     r1, =vfp_kmode_exception
> > > > > > > > >>>>> +  bxne    r1                      @ Returns through lr
> > > > > > > > >>>>>
> > > > > > > > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > > > > > > >>>>>    DBGSTR1 "fpexc %08x", r1
> > > > > > > > >>>>
> > > > > > > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > > > > > > >>>> moving?
> > > > > > > > >>>>
> > > > > > > > >>>
> > > > > > > > >>> I don't know where it needs to be moved. Please feel free to make a
> > > > > > > > >>> patch if you have a better idea, I'll be glad to test it.
> > > > > > > > >>
> > > > > > > > >> I might have just been distracted by the common "vfp" prefix. It's
> > > > > > > > >> possible that the text section shuffling just ended up being very large,
> > > > > > > > >> so probably this patch is right then!
> > > > > > > > >>
> > > > > > > > >
> > > > > > > > > I already sent a fix for this issue:
> > > > > > > > >
> > > > > > > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > > > > > > >
> > > > > > > >
> > > > > > > > The offending commit contains stable tag, so I assume that fixes tag is
> > > > > > > > mandatory. Yours patch misses the fixes tag.
> > > > > > >
> > > > > > > Russell, mind adding that? Or would you like me to update the patch in
> > > > > > > the patch system?
> > > > > >
> > > > > > Rather than adding the IT, I'm suggesting that we solve it a different
> > > > > > way - ensuring that the two bits of code are co-located. There's no
> > > > > > reason for them to be separated, and the assembly code entry point is
> > > > > > already called indirectly.
> > > > > >
> > > > > > The problem is the assembly ends up in the .text section which ends up
> > > > > > at the start of the binary, but depending on the compiler, functions
> > > > > > in .c files end up in their own sections. It would be good if, as
> > > > > > Dmitry has shown that it is indeed possible, to have them co-located.
> > > > >
> > > > > Why is that better? I provided a minimal fix which has zero impact on
> > > > > ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > > > > the exact same semantics as before, but using a different opcode.
> > > >
> > > > I think you just described the reason there. Why should we force
> > > > everything to use a different opcode when a short jump _should_
> > > > suffice?
> > > >
> > >
> > > Why should a short jump suffice? The call is to vfp_kmode_exception(),
> > > which we only call in exceptional cases. Why would we want to keep
> > > that in close proximity?
> >
> > You're thinking about it in terms of what happens when the branch is
> > taken, rather than also considering that this code path is also
> > traversed for _every_ single time that we enter the support code
> > not just for kernel mode.
> >
>
> True. If 2 bytes of additional opcode are the concern here, we can
> change the current sequence
>
>    6:   f093 0f10       teq     r3, #16
>    a:   f47f affe       bne.w   0 <vfp_kmode_exception>
>
> to
>
>    6:   2b10            cmp     r3, #16
>    8:   bf18            it      ne
>    a:   f7ff bffe       bne.w   0 <vfp_kmode_exception>
>
> which takes up the exact same space.

BTW this code path looks slightly broken for Thumb-2 in any case: if a
FP exception is taken in kernel mode on a Thumb2 kernel, we enter the
emulation sequence via call_fpe, which will use the wrong set of
value/mask pairs to match the opcode. The minimal fix is to move the
call_fpe label to the right place, but I think it might be better to
move the check for a FP exception in kernel mode to the handling of
__und_svc.

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Mon, 26 Oct 2020 at 09:58, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Thu, 22 Oct 2020 at 19:59, Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > On Thu, 22 Oct 2020 at 19:48, Russell King - ARM Linux admin
> > <linux@armlinux.org.uk> wrote:
> > >
> > > On Thu, Oct 22, 2020 at 06:33:17PM +0200, Ard Biesheuvel wrote:
> > > > On Thu, 22 Oct 2020 at 18:23, Russell King - ARM Linux admin
> > > > <linux@armlinux.org.uk> wrote:
> > > > >
> > > > > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > > > > > On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > > > > > <linux@armlinux.org.uk> wrote:
> > > > > > >
> > > > > > > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > > > > > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > > > > > > >
> > > > > > > > > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > > > > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > > > > > > > >>
> > > > > > > > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > > > > > > > >>> 22.10.2020 02:40, Kees Cook пишет:
> > > > > > > > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > > > > > > > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > > > > > > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > > > > > > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > > > > > > > >>>>> linker error. Let's use long jump in order to fix the issue.
> > > > > > > > > >>>>
> > > > > > > > > >>>> Eek. Is this with gcc or clang?
> > > > > > > > > >>>
> > > > > > > > > >>> GCC 9.3.0
> > > > > > > > > >>>
> > > > > > > > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > > > > > > > >>>>
> > > > > > > > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > > > > > > > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > > > > > > > >>>> though I thought I had chosen the correct position.
> > > > > > > > > >>>
> > > > > > > > > >>> I re-checked that the fixes tag is correct.
> > > > > > > > > >>>
> > > > > > > > > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > > > > > > > >>>>> ---
> > > > > > > > > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > > > > > > > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > > > > > >>>>>
> > > > > > > > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > > > > > > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > > > > > > > >>>>> --- a/arch/arm/vfp/vfphw.S
> > > > > > > > > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > > > > > > > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > > > > > > > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > > > > > > > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > > > > > > > >>>>>    teq     r3, #USR_MODE
> > > > > > > > > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > > > > > > > >>>>> +  ldr     r1, =vfp_kmode_exception
> > > > > > > > > >>>>> +  bxne    r1                      @ Returns through lr
> > > > > > > > > >>>>>
> > > > > > > > > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > > > > > > > >>>>>    DBGSTR1 "fpexc %08x", r1
> > > > > > > > > >>>>
> > > > > > > > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > > > > > > > >>>> moving?
> > > > > > > > > >>>>
> > > > > > > > > >>>
> > > > > > > > > >>> I don't know where it needs to be moved. Please feel free to make a
> > > > > > > > > >>> patch if you have a better idea, I'll be glad to test it.
> > > > > > > > > >>
> > > > > > > > > >> I might have just been distracted by the common "vfp" prefix. It's
> > > > > > > > > >> possible that the text section shuffling just ended up being very large,
> > > > > > > > > >> so probably this patch is right then!
> > > > > > > > > >>
> > > > > > > > > >
> > > > > > > > > > I already sent a fix for this issue:
> > > > > > > > > >
> > > > > > > > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > > > > > > > >
> > > > > > > > >
> > > > > > > > > The offending commit contains stable tag, so I assume that fixes tag is
> > > > > > > > > mandatory. Yours patch misses the fixes tag.
> > > > > > > >
> > > > > > > > Russell, mind adding that? Or would you like me to update the patch in
> > > > > > > > the patch system?
> > > > > > >
> > > > > > > Rather than adding the IT, I'm suggesting that we solve it a different
> > > > > > > way - ensuring that the two bits of code are co-located. There's no
> > > > > > > reason for them to be separated, and the assembly code entry point is
> > > > > > > already called indirectly.
> > > > > > >
> > > > > > > The problem is the assembly ends up in the .text section which ends up
> > > > > > > at the start of the binary, but depending on the compiler, functions
> > > > > > > in .c files end up in their own sections. It would be good if, as
> > > > > > > Dmitry has shown that it is indeed possible, to have them co-located.
> > > > > >
> > > > > > Why is that better? I provided a minimal fix which has zero impact on
> > > > > > ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > > > > > the exact same semantics as before, but using a different opcode.
> > > > >
> > > > > I think you just described the reason there. Why should we force
> > > > > everything to use a different opcode when a short jump _should_
> > > > > suffice?
> > > > >
> > > >
> > > > Why should a short jump suffice? The call is to vfp_kmode_exception(),
> > > > which we only call in exceptional cases. Why would we want to keep
> > > > that in close proximity?
> > >
> > > You're thinking about it in terms of what happens when the branch is
> > > taken, rather than also considering that this code path is also
> > > traversed for _every_ single time that we enter the support code
> > > not just for kernel mode.
> > >
> >
> > True. If 2 bytes of additional opcode are the concern here, we can
> > change the current sequence
> >
> >    6:   f093 0f10       teq     r3, #16
> >    a:   f47f affe       bne.w   0 <vfp_kmode_exception>
> >
> > to
> >
> >    6:   2b10            cmp     r3, #16
> >    8:   bf18            it      ne
> >    a:   f7ff bffe       bne.w   0 <vfp_kmode_exception>
> >
> > which takes up the exact same space.
>
> BTW this code path looks slightly broken for Thumb-2 in any case: if a
> FP exception is taken in kernel mode on a Thumb2 kernel, we enter the
> emulation sequence via call_fpe, which will use the wrong set of
> value/mask pairs to match the opcode. The minimal fix is to move the
> call_fpe label to the right place, but I think it might be better to
> move the check for a FP exception in kernel mode to the handling of
> __und_svc.

Do we have a resolution here? This is causing breakage in kernelci

https://kernelci.org/build/id/5f9a834c5ed3c05dd538101b/

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Thu, 29 Oct 2020 at 10:56, Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Mon, 26 Oct 2020 at 09:58, Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > On Thu, 22 Oct 2020 at 19:59, Ard Biesheuvel <ardb@kernel.org> wrote:
> > >
> > > On Thu, 22 Oct 2020 at 19:48, Russell King - ARM Linux admin
> > > <linux@armlinux.org.uk> wrote:
> > > >
> > > > On Thu, Oct 22, 2020 at 06:33:17PM +0200, Ard Biesheuvel wrote:
> > > > > On Thu, 22 Oct 2020 at 18:23, Russell King - ARM Linux admin
> > > > > <linux@armlinux.org.uk> wrote:
> > > > > >
> > > > > > On Thu, Oct 22, 2020 at 06:20:40PM +0200, Ard Biesheuvel wrote:
> > > > > > > On Thu, 22 Oct 2020 at 18:11, Russell King - ARM Linux admin
> > > > > > > <linux@armlinux.org.uk> wrote:
> > > > > > > >
> > > > > > > > On Thu, Oct 22, 2020 at 06:06:32PM +0200, Ard Biesheuvel wrote:
> > > > > > > > > On Thu, 22 Oct 2020 at 17:57, Dmitry Osipenko <digetx@gmail.com> wrote:
> > > > > > > > > >
> > > > > > > > > > 22.10.2020 10:06, Ard Biesheuvel пишет:
> > > > > > > > > > > On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@chromium.org> wrote:
> > > > > > > > > > >>
> > > > > > > > > > >> On Thu, Oct 22, 2020 at 03:00:06AM +0300, Dmitry Osipenko wrote:
> > > > > > > > > > >>> 22.10.2020 02:40, Kees Cook пишет:
> > > > > > > > > > >>>> On Thu, Oct 22, 2020 at 01:57:37AM +0300, Dmitry Osipenko wrote:
> > > > > > > > > > >>>>> The vfp_kmode_exception() function now is unreachable using relative
> > > > > > > > > > >>>>> branching in THUMB2 kernel configuration, resulting in a "relocation
> > > > > > > > > > >>>>> truncated to fit: R_ARM_THM_JUMP19 against symbol `vfp_kmode_exception'"
> > > > > > > > > > >>>>> linker error. Let's use long jump in order to fix the issue.
> > > > > > > > > > >>>>
> > > > > > > > > > >>>> Eek. Is this with gcc or clang?
> > > > > > > > > > >>>
> > > > > > > > > > >>> GCC 9.3.0
> > > > > > > > > > >>>
> > > > > > > > > > >>>>> Fixes: eff8728fe698 ("vmlinux.lds.h: Add PGO and AutoFDO input sections")
> > > > > > > > > > >>>>
> > > > > > > > > > >>>> Are you sure it wasn't 512dd2eebe55 ("arm/build: Add missing sections") ?
> > > > > > > > > > >>>> That commit may have implicitly moved the location of .vfp11_veneer,
> > > > > > > > > > >>>> though I thought I had chosen the correct position.
> > > > > > > > > > >>>
> > > > > > > > > > >>> I re-checked that the fixes tag is correct.
> > > > > > > > > > >>>
> > > > > > > > > > >>>>> Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
> > > > > > > > > > >>>>> ---
> > > > > > > > > > >>>>>  arch/arm/vfp/vfphw.S | 3 ++-
> > > > > > > > > > >>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > > > > > > >>>>>
> > > > > > > > > > >>>>> diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S
> > > > > > > > > > >>>>> index 4fcff9f59947..6e2b29f0c48d 100644
> > > > > > > > > > >>>>> --- a/arch/arm/vfp/vfphw.S
> > > > > > > > > > >>>>> +++ b/arch/arm/vfp/vfphw.S
> > > > > > > > > > >>>>> @@ -82,7 +82,8 @@ ENTRY(vfp_support_entry)
> > > > > > > > > > >>>>>    ldr     r3, [sp, #S_PSR]        @ Neither lazy restore nor FP exceptions
> > > > > > > > > > >>>>>    and     r3, r3, #MODE_MASK      @ are supported in kernel mode
> > > > > > > > > > >>>>>    teq     r3, #USR_MODE
> > > > > > > > > > >>>>> -  bne     vfp_kmode_exception     @ Returns through lr
> > > > > > > > > > >>>>> +  ldr     r1, =vfp_kmode_exception
> > > > > > > > > > >>>>> +  bxne    r1                      @ Returns through lr
> > > > > > > > > > >>>>>
> > > > > > > > > > >>>>>    VFPFMRX r1, FPEXC               @ Is the VFP enabled?
> > > > > > > > > > >>>>>    DBGSTR1 "fpexc %08x", r1
> > > > > > > > > > >>>>
> > > > > > > > > > >>>> This seems like a workaround though? I suspect the vfp11_veneer needs
> > > > > > > > > > >>>> moving?
> > > > > > > > > > >>>>
> > > > > > > > > > >>>
> > > > > > > > > > >>> I don't know where it needs to be moved. Please feel free to make a
> > > > > > > > > > >>> patch if you have a better idea, I'll be glad to test it.
> > > > > > > > > > >>
> > > > > > > > > > >> I might have just been distracted by the common "vfp" prefix. It's
> > > > > > > > > > >> possible that the text section shuffling just ended up being very large,
> > > > > > > > > > >> so probably this patch is right then!
> > > > > > > > > > >>
> > > > > > > > > > >
> > > > > > > > > > > I already sent a fix for this issue:
> > > > > > > > > > >
> > > > > > > > > > > https://www.armlinux.org.uk/developer/patches/viewpatch.php?id=9018/1
> > > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > The offending commit contains stable tag, so I assume that fixes tag is
> > > > > > > > > > mandatory. Yours patch misses the fixes tag.
> > > > > > > > >
> > > > > > > > > Russell, mind adding that? Or would you like me to update the patch in
> > > > > > > > > the patch system?
> > > > > > > >
> > > > > > > > Rather than adding the IT, I'm suggesting that we solve it a different
> > > > > > > > way - ensuring that the two bits of code are co-located. There's no
> > > > > > > > reason for them to be separated, and the assembly code entry point is
> > > > > > > > already called indirectly.
> > > > > > > >
> > > > > > > > The problem is the assembly ends up in the .text section which ends up
> > > > > > > > at the start of the binary, but depending on the compiler, functions
> > > > > > > > in .c files end up in their own sections. It would be good if, as
> > > > > > > > Dmitry has shown that it is indeed possible, to have them co-located.
> > > > > > >
> > > > > > > Why is that better? I provided a minimal fix which has zero impact on
> > > > > > > ARM builds, and minimal impact on Thumb2 builds, given that it retains
> > > > > > > the exact same semantics as before, but using a different opcode.
> > > > > >
> > > > > > I think you just described the reason there. Why should we force
> > > > > > everything to use a different opcode when a short jump _should_
> > > > > > suffice?
> > > > > >
> > > > >
> > > > > Why should a short jump suffice? The call is to vfp_kmode_exception(),
> > > > > which we only call in exceptional cases. Why would we want to keep
> > > > > that in close proximity?
> > > >
> > > > You're thinking about it in terms of what happens when the branch is
> > > > taken, rather than also considering that this code path is also
> > > > traversed for _every_ single time that we enter the support code
> > > > not just for kernel mode.
> > > >
> > >
> > > True. If 2 bytes of additional opcode are the concern here, we can
> > > change the current sequence
> > >
> > >    6:   f093 0f10       teq     r3, #16
> > >    a:   f47f affe       bne.w   0 <vfp_kmode_exception>
> > >
> > > to
> > >
> > >    6:   2b10            cmp     r3, #16
> > >    8:   bf18            it      ne
> > >    a:   f7ff bffe       bne.w   0 <vfp_kmode_exception>
> > >
> > > which takes up the exact same space.
> >
> > BTW this code path looks slightly broken for Thumb-2 in any case: if a
> > FP exception is taken in kernel mode on a Thumb2 kernel, we enter the
> > emulation sequence via call_fpe, which will use the wrong set of
> > value/mask pairs to match the opcode. The minimal fix is to move the
> > call_fpe label to the right place, but I think it might be better to
> > move the check for a FP exception in kernel mode to the handling of
> > __und_svc.
>
> Do we have a resolution here? This is causing breakage in kernelci
>
> https://kernelci.org/build/id/5f9a834c5ed3c05dd538101b/

Still broken today

https://kernelci.org/build/id/5fa0c1a74bdb1ea4063fe7e4/

So the options are

a) merge my patch that adds 2 bytes of opcode to the Thumb2 build
b) merge Dmitry's patch that adds an unconditional literal load to all builds
c) remove kernel mode handling from vfp_support_entry() [my other patch]
d) move sections around so that vfp_kmode_exception is guaranteed to
be in range.
e) do nothing

Given the lack of reports about this issue, it is pretty clear that
few people use the Thumb2 build (which I find odd, tbh, since it
really is much smaller). However, that means that a) is a reasonable
fix, since nobody will notice the potential performance hit either,
and it can easily be backported to wherever the breakage was
introduced. (Note that eff8728fe698, which created the problem is
marked cc:stable itself).

Going forward, I can refine d) so that we can get rid of the kernel
mode path entirely.

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Dmitry Osipenko]

03.11.2020 10:24, Ard Biesheuvel пишет:
> Still broken today
> 
> https://kernelci.org/build/id/5fa0c1a74bdb1ea4063fe7e4/
> 
> So the options are
> 
> a) merge my patch that adds 2 bytes of opcode to the Thumb2 build
> b) merge Dmitry's patch that adds an unconditional literal load to all builds
> c) remove kernel mode handling from vfp_support_entry() [my other patch]
> d) move sections around so that vfp_kmode_exception is guaranteed to
> be in range.
> e) do nothing
> 
> Given the lack of reports about this issue, it is pretty clear that
> few people use the Thumb2 build (which I find odd, tbh, since it
> really is much smaller).

I waited for about a month, hoping that somebody will fix this problem
before bothering with bisection, which took quite some time and effort
because intermediate commits were broken, and then with creating and
sending a patch :)

Thumb2 usually is untested by CI farms and in a case of personal use
it's easier to wait for a fix. Hence no much reports, I suppose.

> However, that means that a) is a reasonable
> fix, since nobody will notice the potential performance hit either,
> and it can easily be backported to wherever the breakage was
> introduced. (Note that eff8728fe698, which created the problem is
> marked cc:stable itself).

The performance argument is questionable to me, to be honest. In
practice the performance difference should be absolutely negligible for
either of the proposed options, it should stay in a noise even if
somebody thoroughly counting cycles, IMO.

I'm still thinking that the best option will be to apply a).

> Going forward, I can refine d) so that we can get rid of the kernel
> mode path entirely.

And then improve it using d).

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Tue, 3 Nov 2020 at 09:43, Dmitry Osipenko <digetx@gmail.com> wrote:
>
> 03.11.2020 10:24, Ard Biesheuvel пишет:
> > Still broken today
> >
> > https://kernelci.org/build/id/5fa0c1a74bdb1ea4063fe7e4/
> >

Still broken today

https://kernelci.org/build/id/5fa898baa00b5f3167db8857/

> > So the options are
> >
> > a) merge my patch that adds 2 bytes of opcode to the Thumb2 build
> > b) merge Dmitry's patch that adds an unconditional literal load to all builds
> > c) remove kernel mode handling from vfp_support_entry() [my other patch]
> > d) move sections around so that vfp_kmode_exception is guaranteed to
> > be in range.
> > e) do nothing
> >
> > Given the lack of reports about this issue, it is pretty clear that
> > few people use the Thumb2 build (which I find odd, tbh, since it
> > really is much smaller).
>
> I waited for about a month, hoping that somebody will fix this problem
> before bothering with bisection, which took quite some time and effort
> because intermediate commits were broken, and then with creating and
> sending a patch :)
>
> Thumb2 usually is untested by CI farms and in a case of personal use
> it's easier to wait for a fix. Hence no much reports, I suppose.
>
> > However, that means that a) is a reasonable
> > fix, since nobody will notice the potential performance hit either,
> > and it can easily be backported to wherever the breakage was
> > introduced. (Note that eff8728fe698, which created the problem is
> > marked cc:stable itself).
>
> The performance argument is questionable to me, to be honest. In
> practice the performance difference should be absolutely negligible for
> either of the proposed options, it should stay in a noise even if
> somebody thoroughly counting cycles, IMO.
>
> I'm still thinking that the best option will be to apply a).
>

Can we take that as an acked-by?

> > Going forward, I can refine d) so that we can get rid of the kernel
> > mode path entirely.
>
> And then improve it using d).

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Dmitry Osipenko]

09.11.2020 10:25, Ard Biesheuvel пишет:
...
>>> So the options are
>>>
>>> a) merge my patch that adds 2 bytes of opcode to the Thumb2 build
>>> b) merge Dmitry's patch that adds an unconditional literal load to all builds
>>> c) remove kernel mode handling from vfp_support_entry() [my other patch]
>>> d) move sections around so that vfp_kmode_exception is guaranteed to
>>> be in range.
>>> e) do nothing
...
>> The performance argument is questionable to me, to be honest. In
>> practice the performance difference should be absolutely negligible for
>> either of the proposed options, it should stay in a noise even if
>> somebody thoroughly counting cycles, IMO.
>>
>> I'm still thinking that the best option will be to apply a).
>>
> 
> Can we take that as an acked-by?

Are you asking me for the ack? I think this is a more appropriate
question to Russel. I'm not arm/ maintainer, but could give r-b and t-b.

If you're going to follow approach that I'm suggesting with a) + d),
then could you please resend the two patches in a single series? The
first one-line patch-fix should contain the fixes tag.

Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error

[Ard Biesheuvel]

On Tue, 10 Nov 2020 at 08:44, Dmitry Osipenko <digetx@gmail.com> wrote:
>
> 09.11.2020 10:25, Ard Biesheuvel пишет:
> ...
> >>> So the options are
> >>>
> >>> a) merge my patch that adds 2 bytes of opcode to the Thumb2 build
> >>> b) merge Dmitry's patch that adds an unconditional literal load to all builds
> >>> c) remove kernel mode handling from vfp_support_entry() [my other patch]
> >>> d) move sections around so that vfp_kmode_exception is guaranteed to
> >>> be in range.
> >>> e) do nothing
> ...
> >> The performance argument is questionable to me, to be honest. In
> >> practice the performance difference should be absolutely negligible for
> >> either of the proposed options, it should stay in a noise even if
> >> somebody thoroughly counting cycles, IMO.
> >>
> >> I'm still thinking that the best option will be to apply a).
> >>
> >
> > Can we take that as an acked-by?
>
> Are you asking me for the ack?

Yes.

> I think this is a more appropriate
> question to Russel. I'm not arm/ maintainer, but could give r-b and t-b.
>

R-b and acked-by are basically the same thing.

> If you're going to follow approach that I'm suggesting with a) + d),
> then could you please resend the two patches in a single series? The
> first one-line patch-fix should contain the fixes tag.

Ok