bitmap.h, perf/core: Fix the mask in perf_output_sample_regs()

This change “bitmap.h, perf/core: Fix the mask in perf_output_sample_regs()” in Linux kernel is authored by Madhavan Srinivasan <maddy [at] linux.vnet.ibm.com> on Wed Aug 17 15:06:08 2016 +0530.

bitmap.h, perf/core: Fix the mask in perf_output_sample_regs()

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 exisiting code works fine in most of the case, the logic
is broken for big-endian 32-bit kernels.

When reading a u64 mask using (u32 *)(&val)[0], find_*_bit() assumes
that it gets the lower 32 bits of u64, but instead it gets the upper
32 bits - which is wrong.

The fix is to swap the words of the u64 to handle this case.
This is _not_ a regular endianness swap.

Suggested-by: Yury Norov <ynorov@caviumnetworks.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Yury Norov <ynorov@caviumnetworks.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Cc: linuxppc-dev@lists.ozlabs.org
Link: http://lkml.kernel.org/r/1471426568-31051-2-git-send-email-maddy@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

This Linux change may have been applied to various maintained Linux releases and you can find Linux releases including commit 29dd328.

There are 23 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 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 598bc99..3b77588 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 32-bit Big Endian kernel, when using (u32 *)(&val)[*]
+ * to read u64 mask, we will get the wrong word.
+ * That is "(u32 *)(&val)[0]" gets the upper 32 bits,
+ * but we expect the lower 32-bits 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 ca4fde5..849919c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5340,9 +5340,10 @@ int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
 			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);

The commit for this change in Linux stable tree is 29dd328 (patch).

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