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

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Message-ID: <20201022173843.GR1551@shell.armlinux.org.uk>
Date:   Thu, 22 Oct 2020 18:38:44 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Dmitry Osipenko <digetx@...il.com>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ingo Molnar <mingo@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel
 compilation error

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@...linux.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@...il.com> wrote:
> >>>>>
> >>>>> 22.10.2020 10:06, Ard Biesheuvel пишет:
> >>>>>> On Thu, 22 Oct 2020 at 05:30, Kees Cook <keescook@...omium.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@...il.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/
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