[PATCH] recordmcount: Support empty section from recent binutils

[PATCH] recordmcount: Support empty section from recent binutils

[Christophe Leroy]

Looks like recent binutils (2.36 and over ?) may empty some section,
leading to failure like:

	Cannot find symbol for section 11: .text.unlikely.
	kernel/kexec_file.o: failed
	make[1]: *** [scripts/Makefile.build:287: kernel/kexec_file.o] Error 1

In order to avoid that, ensure that the section has a content before
returning it's name in has_rel_mcount().

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Link: https://github.com/linuxppc/issues/issues/388
Link: https://lore.kernel.org/all/20210215162209.5e2a475b@gandalf.local.home/
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
---
 scripts/recordmcount.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
index 1e9baa5c4fc6..cc6600b729ae 100644
--- a/scripts/recordmcount.h
+++ b/scripts/recordmcount.h
@@ -575,6 +575,8 @@ static char const *has_rel_mcount(Elf_Shdr const *const relhdr,
 				  char const *const shstrtab,
 				  char const *const fname)
 {
+	if (!shdr0->sh_size)
+		return NULL;
 	if (w(relhdr->sh_type) != SHT_REL && w(relhdr->sh_type) != SHT_RELA)
 		return NULL;
 	return __has_rel_mcount(relhdr, shdr0, shstrtab, fname);
-- 
2.33.1

Re: [PATCH] recordmcount: Support empty section from recent binutils

[LEROY Christophe]

Le 24/11/2021 à 15:43, Christophe Leroy a écrit :
> Looks like recent binutils (2.36 and over ?) may empty some section,
> leading to failure like:
> 
> 	Cannot find symbol for section 11: .text.unlikely.
> 	kernel/kexec_file.o: failed
> 	make[1]: *** [scripts/Makefile.build:287: kernel/kexec_file.o] Error 1
> 
> In order to avoid that, ensure that the section has a content before
> returning it's name in has_rel_mcount().

This patch doesn't work, on PPC32 I get the following message with this 
patch applied:

[    0.000000] ftrace: No functions to be traced?

Without the patch I get:

[    0.000000] ftrace: allocating 22381 entries in 66 pages
[    0.000000] ftrace: allocated 66 pages with 2 groups

Christophe

> 
> Suggested-by: Steven Rostedt <rostedt@goodmis.org>
> Link: https://github.com/linuxppc/issues/issues/388
> Link: https://lore.kernel.org/all/20210215162209.5e2a475b@gandalf.local.home/
> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>   scripts/recordmcount.h | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
> index 1e9baa5c4fc6..cc6600b729ae 100644
> --- a/scripts/recordmcount.h
> +++ b/scripts/recordmcount.h
> @@ -575,6 +575,8 @@ static char const *has_rel_mcount(Elf_Shdr const *const relhdr,
>   				  char const *const shstrtab,
>   				  char const *const fname)
>   {
> +	if (!shdr0->sh_size)
> +		return NULL;
>   	if (w(relhdr->sh_type) != SHT_REL && w(relhdr->sh_type) != SHT_RELA)
>   		return NULL;
>   	return __has_rel_mcount(relhdr, shdr0, shstrtab, fname);
> 

Re: [PATCH] recordmcount: Support empty section from recent binutils

[Steven Rostedt]

On Fri, 26 Nov 2021 08:43:23 +0000
LEROY Christophe <christophe.leroy@csgroup.eu> wrote:

> Le 24/11/2021 à 15:43, Christophe Leroy a écrit :
> > Looks like recent binutils (2.36 and over ?) may empty some section,
> > leading to failure like:
> > 
> > 	Cannot find symbol for section 11: .text.unlikely.
> > 	kernel/kexec_file.o: failed
> > 	make[1]: *** [scripts/Makefile.build:287: kernel/kexec_file.o] Error 1
> > 
> > In order to avoid that, ensure that the section has a content before
> > returning it's name in has_rel_mcount().  
> 
> This patch doesn't work, on PPC32 I get the following message with this 
> patch applied:
> 
> [    0.000000] ftrace: No functions to be traced?
> 
> Without the patch I get:
> 
> [    0.000000] ftrace: allocating 22381 entries in 66 pages
> [    0.000000] ftrace: allocated 66 pages with 2 groups

Because of this report, I have not applied this patch (even though I was
about to push it to Linus).

I'm pulling it from my queue until this gets resolved.

Thanks,

-- Steve

Re: [PATCH] recordmcount: Support empty section from recent binutils

[Christophe Leroy]

Le 29/11/2021 à 18:43, Steven Rostedt a écrit :
> On Fri, 26 Nov 2021 08:43:23 +0000
> LEROY Christophe <christophe.leroy@csgroup.eu> wrote:
> 
>> Le 24/11/2021 à 15:43, Christophe Leroy a écrit :
>>> Looks like recent binutils (2.36 and over ?) may empty some section,
>>> leading to failure like:
>>>
>>> 	Cannot find symbol for section 11: .text.unlikely.
>>> 	kernel/kexec_file.o: failed
>>> 	make[1]: *** [scripts/Makefile.build:287: kernel/kexec_file.o] Error 1
>>>
>>> In order to avoid that, ensure that the section has a content before
>>> returning it's name in has_rel_mcount().
>>
>> This patch doesn't work, on PPC32 I get the following message with this
>> patch applied:
>>
>> [    0.000000] ftrace: No functions to be traced?
>>
>> Without the patch I get:
>>
>> [    0.000000] ftrace: allocating 22381 entries in 66 pages
>> [    0.000000] ftrace: allocated 66 pages with 2 groups
> 
> Because of this report, I have not applied this patch (even though I was
> about to push it to Linus).
> 
> I'm pulling it from my queue until this gets resolved.
> 

I have no idea on how to fix that for the moment.

With GCC 10 (binutils 2.36) an objdump -x on kernel/kexec_file.o gives:

0000000000000000 l    d  .text.unlikely	0000000000000000 .text.unlikely
0000000000000000  w    F .text.unlikely	0000000000000038 
.arch_kexec_apply_relocations_add
0000000000000038  w    F .text.unlikely	0000000000000038 
.arch_kexec_apply_relocations


With GCC 11 (binutils 2.37) the same gives:

0000000000000000  w    F .text.unlikely	0000000000000038 
.arch_kexec_apply_relocations_add
0000000000000038  w    F .text.unlikely	0000000000000038 
.arch_kexec_apply_relocations


The problem is that recordmcount drops weak symbols, and it doesn't find 
any non-weak symbol in .text.unlikely

Explication given at 
https://elixir.bootlin.com/linux/v5.16-rc2/source/scripts/recordmcount.h#L506

I have no idea on what to do.

Thanks
Christophe

Re: [PATCH] recordmcount: Support empty section from recent binutils

[Joel Stanley]

On Mon, 29 Nov 2021 at 22:43, Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 29/11/2021 à 18:43, Steven Rostedt a écrit :
> > On Fri, 26 Nov 2021 08:43:23 +0000
> > LEROY Christophe <christophe.leroy@csgroup.eu> wrote:
> >
> >> Le 24/11/2021 à 15:43, Christophe Leroy a écrit :
> >>> Looks like recent binutils (2.36 and over ?) may empty some section,
> >>> leading to failure like:
> >>>
> >>>     Cannot find symbol for section 11: .text.unlikely.
> >>>     kernel/kexec_file.o: failed
> >>>     make[1]: *** [scripts/Makefile.build:287: kernel/kexec_file.o] Error 1
> >>>
> >>> In order to avoid that, ensure that the section has a content before
> >>> returning it's name in has_rel_mcount().
> >>
> >> This patch doesn't work, on PPC32 I get the following message with this
> >> patch applied:
> >>
> >> [    0.000000] ftrace: No functions to be traced?
> >>
> >> Without the patch I get:
> >>
> >> [    0.000000] ftrace: allocating 22381 entries in 66 pages
> >> [    0.000000] ftrace: allocated 66 pages with 2 groups
> >
> > Because of this report, I have not applied this patch (even though I was
> > about to push it to Linus).
> >
> > I'm pulling it from my queue until this gets resolved.
> >
>
> I have no idea on how to fix that for the moment.
>
> With GCC 10 (binutils 2.36) an objdump -x on kernel/kexec_file.o gives:
>
> 0000000000000000 l    d  .text.unlikely 0000000000000000 .text.unlikely
> 0000000000000000  w    F .text.unlikely 0000000000000038
> .arch_kexec_apply_relocations_add
> 0000000000000038  w    F .text.unlikely 0000000000000038
> .arch_kexec_apply_relocations
>
>
> With GCC 11 (binutils 2.37) the same gives:
>
> 0000000000000000  w    F .text.unlikely 0000000000000038
> .arch_kexec_apply_relocations_add
> 0000000000000038  w    F .text.unlikely 0000000000000038
> .arch_kexec_apply_relocations
>
>
> The problem is that recordmcount drops weak symbols, and it doesn't find
> any non-weak symbol in .text.unlikely
>
> Explication given at
> https://elixir.bootlin.com/linux/v5.16-rc2/source/scripts/recordmcount.h#L506
>
> I have no idea on what to do.

Did you end up finding a solution for this issue?

Cheers,

Joel

Re: [PATCH] recordmcount: Support empty section from recent binutils

[Christophe Leroy]

Le 29/03/2022 à 00:31, Joel Stanley a écrit :
> On Mon, 29 Nov 2021 at 22:43, Christophe Leroy
> <christophe.leroy@csgroup.eu> wrote:
>>
>>
>>
>> Le 29/11/2021 à 18:43, Steven Rostedt a écrit :
>>> On Fri, 26 Nov 2021 08:43:23 +0000
>>> LEROY Christophe <christophe.leroy@csgroup.eu> wrote:
>>>
>>>> Le 24/11/2021 à 15:43, Christophe Leroy a écrit :
>>>>> Looks like recent binutils (2.36 and over ?) may empty some section,
>>>>> leading to failure like:
>>>>>
>>>>>      Cannot find symbol for section 11: .text.unlikely.
>>>>>      kernel/kexec_file.o: failed
>>>>>      make[1]: *** [scripts/Makefile.build:287: kernel/kexec_file.o] Error 1
>>>>>
>>>>> In order to avoid that, ensure that the section has a content before
>>>>> returning it's name in has_rel_mcount().
>>>>
>>>> This patch doesn't work, on PPC32 I get the following message with this
>>>> patch applied:
>>>>
>>>> [    0.000000] ftrace: No functions to be traced?
>>>>
>>>> Without the patch I get:
>>>>
>>>> [    0.000000] ftrace: allocating 22381 entries in 66 pages
>>>> [    0.000000] ftrace: allocated 66 pages with 2 groups
>>>
>>> Because of this report, I have not applied this patch (even though I was
>>> about to push it to Linus).
>>>
>>> I'm pulling it from my queue until this gets resolved.
>>>
>>
>> I have no idea on how to fix that for the moment.
>>
>> With GCC 10 (binutils 2.36) an objdump -x on kernel/kexec_file.o gives:
>>
>> 0000000000000000 l    d  .text.unlikely 0000000000000000 .text.unlikely
>> 0000000000000000  w    F .text.unlikely 0000000000000038
>> .arch_kexec_apply_relocations_add
>> 0000000000000038  w    F .text.unlikely 0000000000000038
>> .arch_kexec_apply_relocations
>>
>>
>> With GCC 11 (binutils 2.37) the same gives:
>>
>> 0000000000000000  w    F .text.unlikely 0000000000000038
>> .arch_kexec_apply_relocations_add
>> 0000000000000038  w    F .text.unlikely 0000000000000038
>> .arch_kexec_apply_relocations
>>
>>
>> The problem is that recordmcount drops weak symbols, and it doesn't find
>> any non-weak symbol in .text.unlikely
>>
>> Explication given at
>> https://elixir.bootlin.com/linux/v5.16-rc2/source/scripts/recordmcount.h#L506
>>
>> I have no idea on what to do.
> 
> Did you end up finding a solution for this issue?
> 

Not really, my solution was to switch to the kernel compiler at 
https://mirrors.edge.kernel.org/pub/tools/crosstool/ which embeds 
binutils 2.36

But it looks like using objtool instead of recordmcount doesn't exhibit 
the problem.

https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220318105140.43914-4-sv@linux.ibm.com/

Christophe