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

[Ian Rogers <irogers@google.com>]

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.

Thanks,
Ian

> Obviously it would be better for a patch to be accepted to
> arch/x86/lib/insn.c also.
>
> >
> > Thank you,
> >
> >>
> >> [acme@quaco perf]$ find . -name insn.c
> >> ./arch/x86/lib/insn.c
> >> ./arch/arm/kernel/insn.c
> >> ./arch/arm64/kernel/insn.c
> >> ./tools/objtool/arch/x86/lib/insn.c
> >> ./tools/perf/util/intel-pt-decoder/insn.c
> >> [acme@quaco perf]$ diff -u ./tools/perf/util/intel-pt-decoder/insn.c ./arch/x86/lib/insn.c
> >> --- ./tools/perf/util/intel-pt-decoder/insn.c        2019-07-06 16:59:05.734265998 -0300
> >> +++ ./arch/x86/lib/insn.c    2019-07-06 16:59:01.369202998 -0300
> >> @@ -10,8 +10,8 @@
> >>  #else
> >>  #include <string.h>
> >>  #endif
> >> -#include "inat.h"
> >> -#include "insn.h"
> >> +#include <asm/inat.h>
> >> +#include <asm/insn.h>
> >>
> >>  /* Verify next sizeof(t) bytes can be on the same instruction */
> >>  #define validate_next(t, insn, n)   \
> >> [acme@quaco perf]$
> >>
> >>
> >> - Arnaldo
> >>
> >>> Signed-off-by: Numfor Mbiziwo-Tiapo <nums@google.com>
> >>> ---
> >>>  tools/perf/util/intel-pt-decoder/insn.c | 3 ++-
> >>>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/tools/perf/util/intel-pt-decoder/insn.c b/tools/perf/util/intel-pt-decoder/insn.c
> >>> index ca983e2bea8b..de1944c60aa9 100644
> >>> --- a/tools/perf/util/intel-pt-decoder/insn.c
> >>> +++ b/tools/perf/util/intel-pt-decoder/insn.c
> >>> @@ -31,7 +31,8 @@
> >>>     ((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
> >>>
> >>>  #define __get_next(t, insn)        \
> >>> -   ({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
> >>> +   ({ t r; memcpy(&r, insn->next_byte, sizeof(t)); \
> >>> +           insn->next_byte += sizeof(t); r; })
> >>>
> >>>  #define __peek_nbyte_next(t, insn, n)      \
> >>>     ({ t r = *(t*)((insn)->next_byte + n); r; })
> >>> --
> >>> 2.22.0.657.g960e92d24f-goog
> >>
> >> --
> >>
> >> - Arnaldo
> >
> >
>