[PATCH] powerpc/vdso: Fix incorrect CFI in gettimeofday.S

[Alan Modra]

On Tue, May 17, 2022 at 10:32:09PM +1000, Michael Ellerman wrote:
> "Naveen N. Rao" <naveen.n.rao at linux.ibm.com> writes:
> > Michael Ellerman wrote:
> >>
> >> diff --git a/arch/powerpc/kernel/vdso/gettimeofday.S b/arch/powerpc/kernel/vdso/gettimeofday.S
> >> index eb9c81e1c218..0aee255e9cbb 100644
> >> --- a/arch/powerpc/kernel/vdso/gettimeofday.S
> >> +++ b/arch/powerpc/kernel/vdso/gettimeofday.S
> >> @@ -22,12 +22,15 @@
> >>  .macro cvdso_call funct call_time=0
> >>    .cfi_startproc
> >>  	PPC_STLU	r1, -PPC_MIN_STKFRM(r1)
> >> +  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
> >>  	mflr		r0
> >> -  .cfi_register lr, r0
> >>  	PPC_STLU	r1, -PPC_MIN_STKFRM(r1)
> >> +  .cfi_adjust_cfa_offset PPC_MIN_STKFRM
> >>  	PPC_STL		r0, PPC_MIN_STKFRM + PPC_LR_STKOFF(r1)
> >
> > <snip>
> >
> >> @@ -46,6 +50,7 @@
> >>  	mtlr		r0
> >>    .cfi_restore lr
> >>  	addi		r1, r1, 2 * PPC_MIN_STKFRM
> >> +  .cfi_def_cfa_offset 0
> >
> > Should this be .cfi_adjust_cfa_offset, given that we used that at the
> > start of the function?
>  
> AIUI "adjust x" is offset += x, whereas "def x" is offset = x.

Yes.

> So we could use adjust here, but we'd need to adjust by -(2 * PPC_MIN_STKFRM).

Yes.

> It seemed clearer to just set the offset back to 0, which is what it is
> at the start of the function.

Yes.  In detail, both .cfi_def_cfa_offset and .cfi_adjust_cfa_offset
are interpreteted by the assembler into DW_CFA_def_cfa_offset byte
codes, so you should get the same .eh_frame contents if using Naveen's
suggestion.  It boils down to style really, and the most common style
is to use ".cfi_def_cfa_offset 0" here.

-- 
Alan Modra
Australia Development Lab, IBM