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

From: Jiping Ma
Date: Wed May 27 2020 - 21:06:19 EST

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

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