Subject: Re: [PATCH] perf/core: Fix the mask in perf_output_sample_regs

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Mon, 4 Jul 2016 19:41:36 -0300

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 <[email-masked]>

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

- Arnaldo
 
> > 
> > Suggested-by: Yury Norov <[email-masked]>
> > Cc: Yury Norov <[email-masked]>
> > Cc: Peter Zijlstra <[email-masked]>
> > Cc: Ingo Molnar <[email-masked]>
> > Cc: Arnaldo Carvalho de Melo <[email-masked]>
> > Cc: Alexander Shishkin <[email-masked]>
> > Cc: Jiri Olsa <[email-masked]>
> > Cc: Michael Ellerman <[email-masked]>
> > Signed-off-by: Madhavan Srinivasan <[email-masked]>
> > ---
> >  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