Re: [PATCH v2]: perf/x86: store user space frame-pointer value on a sample

From: Peter Zijlstra
Date: Wed May 09 2018 - 10:54:53 EST


On Tue, May 08, 2018 at 06:21:36PM +0300, Alexey Budankov wrote:
>
> Store user space frame-pointer value (BP register) into Perf trace
> on a sample for a process so the value becomes available when
> unwinding call stacks for functions gaining event samples.
>
> Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
> ---
> arch/x86/kernel/perf_regs.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/perf_regs.c b/arch/x86/kernel/perf_regs.c
> index e47b2dbbdef3..8d68658eff7f 100644
> --- a/arch/x86/kernel/perf_regs.c
> +++ b/arch/x86/kernel/perf_regs.c
> @@ -156,7 +156,13 @@ void perf_get_regs_user(struct perf_regs *regs_user,


> * Most system calls don't save these registers, don't report them.

^^^ that worries me and is the reason for the '-1's below. However I
think with all the PTI rework this might no longer be true.

The Changelog needs to state that user_regs->bp is in fact valid and
ideally point to the commits that makes it so. Also this patch should
update that comment.

Cc Andy who keeps better track of all that than me.

> */
> regs_user_copy->bx = -1;
> - regs_user_copy->bp = -1;
> + /*
> + * Store user space frame-pointer value on sample
> + * to facilitate stack unwinding for cases when
> + * user space executable code has such support
> + * enabled at compile time;
> + */
> + regs_user_copy->bp = user_regs->bp;
> regs_user_copy->r12 = -1;
> regs_user_copy->r13 = -1;
> regs_user_copy->r14 = -1;