[PATCH v4 14/18] perf: fix endianness detection in perf.data

[Stephane Eranian <eranian@google.com>]

The current version of perf detects whether or not
the perf.data file is written in a different endianness
using the attr_size field in the header of the file. This
field represents sizeof(struct perf_event_attr) as known
to perf record. If the sizes do not match, then perf tries
the byte-swapped version. If they match, then the tool assumes
a different endianness.

The issue with the approach is that it assumes the size of
perf_event_attr always has to match between perf record and
perf report. However, the kernel perf_event ABI is extensible.
New fields can be added to struct perf_event_attr. Consequently,
it is not possible to use attr_size to detect endianness.

This patch takes another approach by using the magic number
written at the beginning of the perf.data file to detect
endianness. The magic number is an eight-byte signature.
It's primary purpose is to identify (signature) a perf.data
file. But it could also be used to encode the endianness.

The patch introduces a new value for this signature. The key
difference is that the signature is written differently in
the file depending on the endianness. Thus, by comparing the
signature from the file with the tool's own signature it is
possible to detect endianness. The new signature is "PERFILE2".

Backward compatiblity with existing perf.data file is
ensured.

Signed-off-by: Stephane Eranian <eranian@google.com>
---
 tools/perf/util/header.c |   77 ++++++++++++++++++++++++++++++++++++++--------
 1 files changed, 64 insertions(+), 13 deletions(-)

diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index ecd7f4d..6f4187d 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -63,9 +63,20 @@ char *perf_header__find_event(u64 id)
 	return NULL;
 }
 
-static const char *__perf_magic = "PERFFILE";
+/*
+ * magic2 = "PERFILE2"
+ * must be a numerical value to let the endianness
+ * determine the memory layout. That way we are able
+ * to detect endianness when reading the perf.data file
+ * back.
+ *
+ * we check for legacy (PERFFILE) format.
+ */
+static const char *__perf_magic1 = "PERFFILE";
+static const u64 __perf_magic2    = 0x32454c4946524550ULL;
+static const u64 __perf_magic2_sw = 0x50455246494c4532ULL;
 
-#define PERF_MAGIC	(*(u64 *)__perf_magic)
+#define PERF_MAGIC	__perf_magic2
 
 struct perf_file_attr {
 	struct perf_event_attr	attr;
@@ -1620,24 +1631,59 @@ int perf_header__process_sections(struct perf_header *header, int fd,
 	return err;
 }
 
+static int check_magic_endian(u64 *magic, struct perf_file_header *header,
+			      struct perf_header *ph)
+{
+	int ret;
+
+	/* check for legacy format */
+	ret = memcmp(magic, __perf_magic1, sizeof(*magic));
+	if (ret == 0) {
+		pr_debug("legacy perf.data format\n");
+		if (!header)
+			return -1;
+
+		if (header->attr_size != sizeof(struct perf_file_attr)) {
+			u64 attr_size = bswap_64(header->attr_size);
+
+			if (attr_size != sizeof(struct perf_file_attr))
+				return -1;
+
+			ph->needs_swap = true;
+		}
+		return 0;
+	}
+
+	/* check magic number with same endianness */
+	if (*magic == __perf_magic2)
+		return 0;
+
+	/* check magic number but opposite endianness */
+	if (*magic != __perf_magic2_sw)
+		return -1;
+
+	ph->needs_swap = true;
+
+	return 0;
+}
+
 int perf_file_header__read(struct perf_file_header *header,
 			   struct perf_header *ph, int fd)
 {
+	int ret;
+
 	lseek(fd, 0, SEEK_SET);
 
-	if (readn(fd, header, sizeof(*header)) <= 0 ||
-	    memcmp(&header->magic, __perf_magic, sizeof(header->magic)))
+	ret = readn(fd, header, sizeof(*header));
+	if (ret <= 0)
 		return -1;
 
-	if (header->attr_size != sizeof(struct perf_file_attr)) {
-		u64 attr_size = bswap_64(header->attr_size);
-
-		if (attr_size != sizeof(struct perf_file_attr))
-			return -1;
+	if (check_magic_endian(&header->magic, header, ph) < 0)
+		return -1;
 
+	if (ph->needs_swap) {
 		mem_bswap_64(header, offsetof(struct perf_file_header,
-					    adds_features));
-		ph->needs_swap = true;
+			     adds_features));
 	}
 
 	if (header->size != sizeof(*header)) {
@@ -1873,8 +1919,13 @@ static int perf_file_header__read_pipe(struct perf_pipe_file_header *header,
 				       struct perf_header *ph, int fd,
 				       bool repipe)
 {
-	if (readn(fd, header, sizeof(*header)) <= 0 ||
-	    memcmp(&header->magic, __perf_magic, sizeof(header->magic)))
+	int ret;
+
+	ret = readn(fd, header, sizeof(*header));
+	if (ret <= 0)
+		return -1;
+
+	 if (check_magic_endian(&header->magic, NULL, ph) < 0)
 		return -1;
 
 	if (repipe && do_write(STDOUT_FILENO, header, sizeof(*header)) < 0)
-- 
1.7.1