linux-kernel - Re: [PATCH 3/3] Fix insn.c misaligned address error

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Date:   Tue, 30 Jul 2019 10:50:03 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Numfor Mbiziwo-Tiapo <nums@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Song Liu <songliubraving@...com>, mbd@...com,
        LKML <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 3/3] Fix insn.c misaligned address error

On 29/07/19 10:32 PM, Ian Rogers wrote:
> On Mon, Jul 29, 2019 at 1:24 AM Adrian Hunter <adrian.hunter@...el.com> wrote:
>>
>> On 27/07/19 12:46 PM, Masami Hiramatsu wrote:
>>> On Fri, 26 Jul 2019 16:38:06 -0300
>>> Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
>>>
>>>> Em Wed, Jul 24, 2019 at 11:45:12AM -0700, Numfor Mbiziwo-Tiapo escreveu:
>>>>> The ubsan (undefined behavior sanitizer) version of perf throws an
>>>>> error on the 'x86 instruction decoder - new instructions' function
>>>>> of perf test.
>>>>>
>>>>> To reproduce this run:
>>>>> make -C tools/perf USE_CLANG=1 EXTRA_CFLAGS="-fsanitize=undefined"
>>>>>
>>>>> then run: tools/perf/perf test 62 -v
>>>>>
>>>>> The error occurs in the __get_next macro (line 34) where an int is
>>>>> read from a potentially unaligned address. Using memcpy instead of
>>>>> assignment from an unaligned pointer.
>>>>
>>>> Since this came from the kernel, don't we have to fix it there as well?
>>>> Masami, Adrian?
>>>
>>> I guess we don't need it, since x86 can access "unaligned address" and
>>> x86 insn decoder in kernel runs only on x86. I'm not sure about perf's
>>> that part. Maybe if we run it on other arch as cross-arch application,
>>> it may cause unaligned pointer issue.
> 
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
> "A pointer to an object or incomplete type may be converted to a
> pointer to a different object or incomplete type. If the resulting
> pointer is not correctly aligned for the pointed-to type, the behavior
> is undefined."
> I agree the code will generally run on x86.
> 
>> Yes, theoretically Intel PT decoding can be done on any arch.
>>
>> But the memcpy is probably sub-optimal for x86, so the patch as it stands
>> does not seem suitable.  I notice the kernel has get_unaligned() and
>> put_unaligned().
> 
> Why is a fixed sized memcpy suboptimal? The compiler can should turn
> into a load.

True, I didn't click it was fixed size.