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

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Message-ID: <CAMj1kXHCPYU2ak_n4y6KruteoF3vE5EeiWcWpYsfh6WKXU7xnw@mail.gmail.com>
Date:   Thu, 22 Oct 2020 19:51:12 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Dmitry Osipenko <digetx@...il.com>,
        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, 22 Oct 2020 at 19:50, Russell King - ARM Linux admin
<linux@...linux.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@...linux.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@...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?
> > >
> >
> > 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.