[PATCH v2 2/2] perf/core: Fix the mask in perf_output_sample_regs

[Madhavan Srinivasan]

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.

Suggested-by: Yury Norov <ynorov at caviumnetworks.com>
Reviewed-by: Yury Norov <ynorov at caviumnetworks.com>
Cc: Yury Norov <ynorov at caviumnetworks.com>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: Arnaldo Carvalho de Melo <acme at kernel.org>
Cc: Alexander Shishkin <alexander.shishkin at linux.intel.com>
Cc: Jiri Olsa <jolsa at kernel.org>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy at linux.vnet.ibm.com>
---
Changelog v1:
1)bitmap_from_u64() is inline now

 include/linux/bitmap.h | 18 ++++++++++++++++++
 kernel/events/core.c   |  5 +++--
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 598bc999f4c2..a9ea992161a1 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -339,6 +339,24 @@ static inline int bitmap_parse(const char *buf, unsigned int buflen,
 	return __bitmap_parse(buf, buflen, 0, maskp, nmaskbits);
 }
 
+/*
+ * 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.
+ */
+static inline void bitmap_from_u64(unsigned long *dst, u64 mask)
+{
+	dst[0] = mask & ULONG_MAX;
+
+	if (sizeof(mask) > sizeof(unsigned long))
+		dst[1] = mask >> 32;
+}
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* __LINUX_BITMAP_H */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 1903b8f3a705..de32e9bd8189 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5292,9 +5292,10 @@ 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,
-			 sizeof(mask) * BITS_PER_BYTE) {
+	bitmap_from_u64(_mask, mask);
+	for_each_set_bit(bit, _mask, sizeof(mask) * BITS_PER_BYTE) {
 		u64 val;
 
 		val = perf_reg_value(regs, bit);
-- 
2.7.4