Re: [PATCH][V3] arm64: perf: Get the wrong PC value in REGS_ABI_32 mode

[Will Deacon]

On Tue, May 26, 2020 at 11:26:11AM +0100, Mark Rutland wrote:
> On Mon, May 11, 2020 at 10:52:07AM +0800, Jiping Ma wrote:
> > Modified the patch subject and the change description.
> > 
> > PC value is get from regs[15] in REGS_ABI_32 mode, but correct PC
> > is regs->pc(regs[PERF_REG_ARM64_PC]) in arm64 kernel, which caused
> > that perf can not parser the backtrace of app with dwarf mode in the 
> > 32bit system and 64bit kernel.
> > 
> > Signed-off-by: Jiping Ma <jiping.ma2@xxxxxxxxxxxxx>
> 
> Thanks for this.
> 
> 
> > ---
> >  arch/arm64/kernel/perf_regs.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> > 
> > diff --git a/arch/arm64/kernel/perf_regs.c b/arch/arm64/kernel/perf_regs.c
> > index 0bbac61..0ef2880 100644
> > --- a/arch/arm64/kernel/perf_regs.c
> > +++ b/arch/arm64/kernel/perf_regs.c
> > @@ -32,6 +32,10 @@ u64 perf_reg_value(struct pt_regs *regs, int idx)
> >  	if ((u32)idx == PERF_REG_ARM64_PC)
> >  		return regs->pc;
> >  
> > +	if (perf_reg_abi(current) == PERF_SAMPLE_REGS_ABI_32
> > +		&& idx == 15)
> > +		return regs->pc;
> 
> I think there are some more issues here, and we may need a more
> substantial rework. For a compat thread, we always expose
> PERF_SAMPLE_REGS_ABI_32 via per_reg_abi(), but for some reason
> perf_reg_value() also munges the compat SP/LR into their ARM64
> equivalents, which don't exist in the 32-bit sample ABI. We also don't
> zero the regs that don't exist in 32-bit (including the aliasing PC).

I think this was for the case where you have a 64-bit perf profiling a
32-bit task, and it was passing the registers off to libunwind. Won't that
break if we follow your suggestion?

Will