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

From: Ard Biesheuvel <ardb@kernel.org>
To: Dmitry Osipenko <digetx@gmail.com>
Cc: Kees Cook <keescook@chromium.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Ingo Molnar <mingo@kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v1] ARM: vfp: Use long jump to fix THUMB2 kernel compilation error
Date: Thu, 22 Oct 2020 18:06:32 +0200	[thread overview]
Message-ID: <CAMj1kXERX_Bv1MdfafOVmdmDXPio6Uj897ZZZ7qRERbCXYw_iQ@mail.gmail.com> (raw)
In-Reply-To: <1d2e2b5d-3035-238c-d2ca-14c0c209a6a1@gmail.com>

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?