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

[Jiping Ma]

On 05/27/2020 03:54 AM, Will Deacon wrote:
> 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?
Yes, it is for 64-bit perf profiling a 32-bit task, not for a compat thread.

> Will