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

[Yury Norov <ynorov@caviumnetworks.com>]

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>

> 
> 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