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

[Jiping Ma]

On 05/27/2020 11:19 PM, Mark Rutland wrote:
> 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?
This issue caused by PC value is not correct. regs are sampled in 
function perf_output_sample_regs, that call perf_reg_value(regs, bit) to 
get PC value.
PC value is regs[15] in perf_reg_value() function. it should be regs[32].

perf_output_sample_regs(struct perf_output_handle *handle,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ struct pt_regs *regs, u64 mask)
{
ÂÂÂÂÂÂÂ int bit;
ÂÂÂÂÂÂÂ DECLARE_BITMAP(_mask, 64);

ÂÂÂÂÂÂÂ bitmap_from_u64(_mask, mask);
ÂÂÂÂÂÂÂ for_each_set_bit(bit, _mask, sizeof(mask) * BITS_PER_BYTE) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ u64 val;

ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ val = perf_reg_value(regs, bit);
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ perf_output_put(handle, val);
ÂÂÂÂÂÂÂ }
}

> Thanks,
> Mark.