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SubjectRe: [PATCH] perf/core: Fix the mask in perf_output_sample_regs
Em Mon, Jul 04, 2016 at 10:19:06AM +0300, Yury Norov escreveu:
> On Sun, Jul 03, 2016 at 11:31:58PM +0530, Madhavan Srinivasan wrote:
> > When decoding the perf_regs mask in perf_output_sample_regs(),
> > we loop through the mask using find_first_bit and find_next_bit functions.
> > While the exisitng code works fine in most of the case,
> > the logic is broken for 32bit kernel (Big Endian).
> > When reading u64 mask using (u32 *)(&val)[0], find_*_bit() assumes it gets
> > lower 32bits of u64 but instead gets upper 32bits which is wrong.
> > Proposed fix is to swap the words of the u64 to handle this case.
> > This is _not_ endianness swap.
>
>
> In fact, it's broken for 32-bit LE as well if mask is zero and next
> word on stack is not zero. The rest is OK.
>
> Reviewed-by: Yury Norov <ynorov@caviumnetworks.com>

Waiting a bit for more acks, as IIRC several other people had comments
on this patch, ok?

- Arnaldo

> >
> > Suggested-by: Yury Norov <ynorov@caviumnetworks.com>
> > Cc: Yury Norov <ynorov@caviumnetworks.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > Cc: Jiri Olsa <jolsa@kernel.org>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
> > ---
> > include/linux/bitmap.h | 2 ++
> > kernel/events/core.c | 4 +++-
> > lib/bitmap.c | 19 +++++++++++++++++++
> > 3 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
> > index e9b0b9ab07e5..d95b422db183 100644
> > --- a/include/linux/bitmap.h
> > +++ b/include/linux/bitmap.h
> > @@ -188,6 +188,8 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
> > #define small_const_nbits(nbits) \
> > (__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG)
> >
> > +extern void bitmap_from_u64(unsigned long *dst, u64 mask);
> > +
> > static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
> > {
> > if (small_const_nbits(nbits))
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 9c51ec3f0f44..613fec95ea4c 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -5205,8 +5205,10 @@ perf_output_sample_regs(struct perf_output_handle *handle,
> > struct pt_regs *regs, u64 mask)
> > {
> > int bit;
> > + DECLARE_BITMAP(_mask, 64);
> >
> > - for_each_set_bit(bit, (const unsigned long *) &mask,
> > + bitmap_from_u64(_mask, mask);
> > + for_each_set_bit(bit, _mask,
> > sizeof(mask) * BITS_PER_BYTE) {
> > u64 val;
> >
> > diff --git a/lib/bitmap.c b/lib/bitmap.c
> > index c66da508cbf7..522f1b4c6078 100644
> > --- a/lib/bitmap.c
> > +++ b/lib/bitmap.c
> > @@ -1170,3 +1170,22 @@ void bitmap_copy_le(unsigned long *dst, const unsigned long *src, unsigned int n
> > }
> > EXPORT_SYMBOL(bitmap_copy_le);
> > #endif
> > +
> > +/*
> > + * bitmap_from_u64 - Check and swap words within u64.
> > + * @mask: source bitmap
> > + * @dst: destination bitmap
> > + *
> > + * In 32bit Big Endian kernel, when using (u32 *)(&val)[*]
> > + * to read u64 mask, we will get wrong word.
> > + * That is "(u32 *)(&val)[0]" gets upper 32 bits,
> > + * but expected could be lower 32bits of u64.
> > + */
> > +void bitmap_from_u64(unsigned long *dst, u64 mask)
> > +{
> > + dst[0] = mask & ULONG_MAX;
> > +
> > + if (sizeof(mask) > sizeof(unsigned long))
> > + dst[1] = mask >> 32;
> > +}
> > +EXPORT_SYMBOL(bitmap_from_u64);
> > --
> > 1.9.1

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