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Subject: Re: [PATCH v2]: perf/x86: store user space frame-pointer value on a sample
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <36a2c137-8342-4ff4-ddcb-3ac1fe092810@linux.intel.com>
Date:   Mon, 21 May 2018 10:23:31 -0700
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-perf-users@vger.kernel.org
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Message-Id: <EF4BA087-7DA4-41EF-A8F9-91EAB7FCF10E@amacapital.net>
References: <31205dc8-b756-e12b-0249-2ed06c2db9c9@linux.intel.com> <20180509145436.GV12217@hirez.programming.kicks-ass.net> <3ecedd91-1c01-6fc5-4648-89eaf5ff0930@linux.intel.com> <20180510101406.GA12217@hirez.programming.kicks-ass.net> <0594eaea-0cab-ea04-c258-5f4770ffeee6@linux.intel.com> <71E4F816-7E0F-4984-B53C-06F8566EAB40@amacapital.net> <36a2c137-8342-4ff4-ddcb-3ac1fe092810@linux.intel.com>
To:     Alexey Budankov <alexey.budankov@linux.intel.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On May 21, 2018, at 9:51 AM, Alexey Budankov <alexey.budankov@linux.intel.=
com> wrote:
>=20
>=20
> Hi Andy,
>> On 21.05.2018 17:14, Andy Lutomirski wrote:
>>=20
>>> On May 21, 2018, at 5:44 AM, Alexey Budankov <alexey.budankov@linux.inte=
l.com> wrote:
>>>=20
>>> Hi Peter,
>>>=20
>>>> 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
>>>>>=20
>>>>> That actually was tested on binaries compiled without and with BP expo=
sed=20
>>>>> and in the latter case proved the value of that change.
>>>>=20
>>>> Mostly works is not the same as 'always initialized', if there are entr=
y
>>>> paths that do not store that register, then using the value might leak
>>>> values from the kernel stack, which would be bad.
>>>>=20
>>>> 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.
>>>=20
>>> It looks like, these days, all registers are saved on system calls, just=
=20
>>> like you anticipated.
>>>=20
>>> So BP register value might be stored into the Perf trace on a sample.=20=

>>>=20
>>> Andy?
>>=20
>> Hmm, I thought I replied. Yes, they are indeed all saved, but I=E2=80=99m=
 not very excited about committing to doing so forever. But storing BP shoul=
d be fine.
>=20
> 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.

>=20
> Thanks,
> Alexey
>=20
>>=20