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

[Ard Biesheuvel <ardb@kernel.org>]

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.