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

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Date:   Tue, 30 Jul 2019 09:17:21 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Adrian Hunter' <adrian.hunter@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
CC:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Numfor Mbiziwo-Tiapo <nums@...gle.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...hat.com" <jolsa@...hat.com>,
        "namhyung@...nel.org" <namhyung@...nel.org>,
        "songliubraving@...com" <songliubraving@...com>,
        "mbd@...com" <mbd@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "irogers@...gle.com" <irogers@...gle.com>,
        "eranian@...gle.com" <eranian@...gle.com>
Subject: RE: [PATCH 3/3] Fix insn.c misaligned address error

From: Adrian Hunter
> Sent: 30 July 2019 08:53
> On 30/07/19 3:47 AM, Masami Hiramatsu wrote:
> > On Mon, 29 Jul 2019 11:22:34 +0300
> > 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.
> >>
> >> 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().
> >>
> >> Obviously it would be better for a patch to be accepted to
> >> arch/x86/lib/insn.c also.
> >
> > Hmm, then I rather like memcpy() for arch/x86/lib/insn.c, because it runs only
> > on x86.
> 
> Yes, I was wrong about memcpy, and it is simpler for perf tools than
> dragging out get_unaligned().

It may well make the generated code worse because some optimisations
won't happen because they would need to be done before memcpy()
gets inlined.
I've certainly seen cases where a #define generates significantly
better code than an inline function.

	David

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