Re: [PATCH v2 1/1] ARM: kprobes: Explicitly reserve r7 for local variables

[Aiqun(Maria) Yu]

On 11/17/2023 1:36 AM, Ard Biesheuvel wrote:
> On Fri, 17 Nov 2023 at 03:24, Nathan Chancellor <nathan@xxxxxxxxxx> wrote:
> > On Wed, Nov 15, 2023 at 05:58:30PM +0800, Maria Yu wrote:
> > > Registers r7 is removed in clobber list, so compiler may choose r7 for
> > > local variables usage, while r7 will be actually updated by the inline asm
> > > code. This caused the runtime behavior wrong.
> > > While those kind of reserved registers cannot be set to clobber list
> > > because of error like "inline asm clobber list contains reserved
> > > registers".
> > > Explicitly reserve r7 by adding attribute no-omit-frame-pointer for needed
> > > function, then in T32 asm code r7 is used as a frame pointer and is not
> > > available for use as a general-purpose register.
> > > Note that "no-omit-frame-pointer" will make the code size a little bigger
> > > to store the stack frame pointer. So limited to needed functions can have
> > > the less impact than the full source file.
> > >
> > > Fixes: dd12e97f3c72 ("ARM: kprobes: treat R7 as the frame pointer register in Thumb2 builds")
> > > Signed-off-by: Maria Yu <quic_aiquny@xxxxxxxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx
> >
> > This causes warnings with clang:
> >
> >    arch/arm/probes/kprobes/actions-thumb.c:441:47: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
> >      441 | static unsigned long __kprobes __attribute__((optimize("no-omit-frame-pointer")))
> >          |                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    arch/arm/probes/kprobes/actions-thumb.c:524:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
> >      524 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
> >          |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    arch/arm/probes/kprobes/actions-thumb.c:560:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
> >      560 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
> >          |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    arch/arm/probes/kprobes/actions-thumb.c:579:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
> >      579 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
> >          |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >    4 warnings generated.
> >
> > Furthermore, as far as I am aware, the optimize attribute has other issues so
> > its use is discouraged, see commits 080b6f407635 ("bpf: Don't rely on GCC
> > __attribute__((optimize)) to disable GCSE") and a7223f5bfcae ("powerpc: Avoid
> > broken GCC __attribute__((optimize))").

Thx for trying this. I was tested with arm-linux-gnueabihf-gcc and not 
notice this.
> >
>
> Ah yes, apalogies for missing that. I did a test build with Clang but
> I did not spot the errors. So
>
> Unreviewed-by: ....
>
> Maria, please use the Makefile based per-file CFLAGS override that I
> suggested before. There is really no reason to make this per-function.
Ok. I will upload a new patchset.

--
Thx and BRs,
Aiqun(Maria) Yu