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

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Message-Id: <EF4BA087-7DA4-41EF-A8F9-91EAB7FCF10E@amacapital.net>
Date:   Mon, 21 May 2018 10:23:31 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        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


> On May 21, 2018, at 9:51 AM, Alexey Budankov <alexey.budankov@...ux.intel.com> wrote:
> 
> 
> Hi Andy,
>> On 21.05.2018 17:14, Andy Lutomirski wrote:
>> 
>>> On May 21, 2018, at 5:44 AM, Alexey Budankov <alexey.budankov@...ux.intel.com> wrote:
>>> 
>>> Hi Peter,
>>> 
>>>> On 10.05.2018 13:14, Peter Zijlstra wrote:
>>>> On Thu, May 10, 2018 at 12:42:38PM +0300, Alexey Budankov wrote:
>>>>>> The Changelog needs to state that user_regs->bp is in fact valid and
>>>>> 
>>>>> That actually was tested on binaries compiled without and with BP exposed 
>>>>> and in the latter case proved the value of that change.
>>>> 
>>>> Mostly works is not the same as 'always initialized', if there are entry
>>>> paths that do not store that register, then using the value might leak
>>>> values from the kernel stack, which would be bad.
>>>> 
>>>> But like said, I think much of the kernel entry code was sanitized with
>>>> the PTI effort and I suspect things are in fact fine now, but lets wait
>>>> for Andy to confirm.
>>> 
>>> It looks like, these days, all registers are saved on system calls, just 
>>> like you anticipated.
>>> 
>>> So BP register value might be stored into the Perf trace on a sample. 
>>> 
>>> Andy?
>> 
>> Hmm, I thought I replied. Yes, they are indeed all saved, but I’m not very excited about committing to doing so forever. But storing BP should be fine.
> 
> Thanks for explicit confirmation regarding BP register.
> BTW, do you see any mean to prevent possible unattended regression?
> I guess it could be some compile time assertion or regression testing.

Write a selftest?

The whole perf user regs mechanism is buggy and fragile. I need to massively clean it up at some point.

> 
> Thanks,
> Alexey
> 
>>