Messages in this thread | | | Subject | Re: [PATCH 3/3] Fix insn.c misaligned address error | From | Adrian Hunter <> | Date | Tue, 30 Jul 2019 10:50:03 +0300 |
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On 29/07/19 10:32 PM, Ian Rogers wrote: > On Mon, Jul 29, 2019 at 1:24 AM Adrian Hunter <adrian.hunter@intel.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@kernel.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.
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