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

[Mark Rutland]

On Wed, May 27, 2020 at 09:33:00AM +0800, Jiping Ma wrote:
> 
> 
> 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.

I must be missing something here.

The core code perf_reg_abi(task) is called with the task being sampled,
and the regs are from the task being sampled. For a userspace sample for
a compat task, compat_user_mode(regs) should be equivalent to the
is_compat_thread(task_thread_info(task)) check.

What am I missing?

Thanks,
Mark.