Re: [PATCH v3 1/1] ARM: kprobes: Explicitly reserve r7 for local variables

[Nathan Chancellor]

On Tue, Nov 21, 2023 at 11:11:56AM -0500, Ard Biesheuvel wrote:
> On Sun, 19 Nov 2023 at 22:29, Maria Yu <quic_aiquny@xxxxxxxxxxx> 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 this
> > file, 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.
> >
> > Fixes: dd12e97f3c72 ("ARM: kprobes: treat R7 as the frame pointer register in Thumb2 builds")
> > Suggested-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> > Signed-off-by: Maria Yu <quic_aiquny@xxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >  arch/arm/probes/kprobes/Makefile | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/arm/probes/kprobes/Makefile b/arch/arm/probes/kprobes/Makefile
> > index 6159010dac4a..b1f21e78950b 100644
> > --- a/arch/arm/probes/kprobes/Makefile
> > +++ b/arch/arm/probes/kprobes/Makefile
> > @@ -8,6 +8,7 @@ test-kprobes-objs               := test-core.o
> >
> >  ifdef CONFIG_THUMB2_KERNEL
> >  obj-$(CONFIG_KPROBES)          += actions-thumb.o checkers-thumb.o
> > +CFLAGS_actions-thumb.o         += -fno-omit-frame-pointer
> >  test-kprobes-objs              += test-thumb.o
> >  else
> >  obj-$(CONFIG_KPROBES)          += actions-arm.o checkers-arm.o
> >
> 
> If Nathan is happy with this, I think we can drop this into the patch tracker.

I have no qualms with this approach, there are no issues with the couple
of LLVM versions that I tested.

Cheers,
Nathan