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

[Jiping Ma]

On 05/26/2020 06:26 PM, 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 reckon what we should do is have seperate functions for the two ABIs,
> to ensure we don't conflate them, e.g.
>
> u64 perf_reg_value_abi32(struct pt_regs *regs, int idx)
> {
> 	if ((u32)idx > PERF_REG_ARM32_PC)
> 		return 0;
> 	if (idx == PERF_REG_ARM32_PC)
> 		return regs->pc;
> 	
> 	/*
> 	 * Compat SP and LR already in-place
> 	 */
> 	return regs->regs[idx];
> }
>
> u64 perf_reg_value_abi64(struct pt_regs *regs, int idx)
> {
> 	if ((u32)idx > PERF_REG_ARM64_MAX)
> 		return 0;
> 	if ((u32)idx == PERF_REG_ARM64_SP)
> 		return regs->sp;
> 	if ((u32)idx == PERF_REG_ARM64_PC)
> 		return regs->pc;
> 	
> 	reutrn regs->regs[idx];
> }
>
> u64 perf_reg_value(struct pt_regs *regs, int idx)
> {
> 	if (compat_user_mode(regs))
> 		return perf_reg_value_abi32(regs, idx);
> 	else
> 		return perf_reg_value_abi64(regs, idx);
> }
This modification can not fix our issue, we need
perf_reg_abi(current) == PERF_SAMPLE_REGS_ABI_32 to judge if it is 
32-bit task or not,
then return the correct PC value.
> Thanks,
> Mark.