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

[Mark Rutland]

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);
}

Thanks,
Mark.