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

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Message-Id: <71E4F816-7E0F-4984-B53C-06F8566EAB40@amacapital.net>
Date:   Mon, 21 May 2018 07:14:49 -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 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.