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SubjectRe: [PATCH v2]: perf/x86: store user space frame-pointer value on a sample
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Hi Andy,

On 09.05.2018 17:54, Peter Zijlstra wrote:
> 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@linux.intel.com>
>> ---
>> 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.

Are there any thoughts so far? Feedback on the matter above is highly appreciated.

Thanks,
Alexey

>
>> */
>> 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;
>

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