Re: [PATCH] arm64: vdso32: force vdso32 to be compiled as -marm

Nick Desaulniers
2020-05-27T18:17:47Z

On Wed, May 27, 2020 at 11:08 AM Will Deacon <will@kernel.org> wrote:
>
> On Wed, May 27, 2020 at 10:55:24AM -0700, Nick Desaulniers wrote:
> > On Wed, May 27, 2020 at 6:45 AM Robin Murphy <robin.murphy@arm.com> wrote:
> > >
> > > On 2020-05-26 18:31, Nick Desaulniers wrote:
> > > > Custom toolchains that modify the default target to -mthumb cannot
> > > > compile the arm64 compat vdso32, as
> > > > arch/arm64/include/asm/vdso/compat_gettimeofday.h
> > > > contains assembly that's invalid in -mthumb. Force the use of -marm,
> > > > always.
> > >
> > > FWIW, this seems suspicious - the only assembly instructions I see there
> > > are SWI(SVC), MRRC, and a MOV, all of which exist in Thumb for the
> > > -march=armv7a baseline that we set.
> > >
> > > On a hunch, I've just bodged "VDSO_CFLAGS += -mthumb" into my tree and
> > > built a Thumb VDSO quite happily with Ubuntu 19.04's
> > > gcc-arm-linux-gnueabihf. What was the actual failure you saw?
> >
> > From the link in the commit message: `write to reserved register 'R7'`
> > https://godbolt.org/z/zwr7iZ
> > IIUC r7 is reserved for the frame pointer in THUMB?
> >
> > What is the implicit default of your gcc-arm-linux-gnueabihf at -O2?
> > -mthumb, or -marm?
>
> Hmm, but this *is* weird because if I build a 32-bit kernel then I get
> either an ARM or a Thumb-2 VDSO depending on CONFIG_THUMB2_KERNEL. I'm
> not sure if that's deliberate, but both build and appear to work.

That's because there's 3 VDSO's when it comes to ARM:
arm64's 64b vdso: arch/arm64/kernel/vdso
arm64's 32b vdso: arch/arm64/kernel/vdso32/
arm's 32b vdso: arch/arm/kernel/vdso.c

When you build a 32b kernel, you're only making use of the last of
those three; the arm64 vdso and vdso32 code is irrelevant.
This patch is specific to the second case, which is the 32b compat
vdso for a 64b kernel.

>
> I'll drop this patch for now, while we figure it out a bit more.
>
> Cheers,
>
> Will

--
Thanks,
~Nick Desaulniers