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

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Date:   Fri, 18 May 2018 10:39:42 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v2]: perf/x86: store user space frame-pointer value on a
 sample


Hi,
On 16.05.2018 11:42, Alexey Budankov wrote:
> Hi,
> On 15.05.2018 19:30, Andy Lutomirski wrote:
>>
>>> On May 15, 2018, at 1:08 AM, Alexey Budankov <alexey.budankov@...ux.intel.com> wrote:
>>>
>>>
>>> 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@...ux.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.
>>
>> Sorry, I missed this. Can you forward the original patch?  I don’t have it.

Just to make sure this and below didn't sneak out of your attention.

> 
> 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@...ux.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.
>  	 */
>  	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;
>>> These days, system calls should save all registers, but I’m not entirely sure I want to promise that they’ll continue to do so forever.

Thanks,
Alexey