perf tools: Validate perf event header size

The 'size' variable includes the header so must be at least
'sizeof(struct perf_event_header)'. Error out immediately if that is
not the case. Also don't byte-swap the header until it is actually
"fetched" from the mmap region.

Signed-off-by: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Mike Galbraith <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/session.c

···
 		perf_event_header__bswap(&event->header);
 
 	size = event->header.size;
-	if (size == 0)
-		size = 8;
+	if (size < sizeof(struct perf_event_header)) {
+		pr_err("bad event header size\n");
+		goto out_err;
+	}
 
 	if (size > cur_size) {
 		void *new = realloc(buf, size);
···
 	if (session->header.needs_swap)
 		perf_event_header__bswap(&event->header);
 
-	if (head + event->header.size > mmap_size)
+	if (head + event->header.size > mmap_size) {
+		/* We're not fetching the event so swap back again */
+		if (session->header.needs_swap)
+			perf_event_header__bswap(&event->header);
 		return NULL;
+	}
 
 	return event;
 }
···
 
 	size = event->header.size;
 
-	if (size == 0 ||
+	if (size < sizeof(struct perf_event_header) ||
 	    perf_session__process_event(session, event, tool, file_pos) < 0) {
 		pr_err("%#" PRIx64 " [%#x]: failed to process type: %d\n",
 		       file_offset + head, event->header.size,