perf tools: Provide backward compatibility with previous perf.data version

We have merged the trace.info file into perf.data by adding one
section in the perf headers. This makes it incompatible with
previous version: the new perf tools can't read the older
perf.data.

To support the previous format, we check the headers size. If they
have the same size than in the previous format, then ignore the
trace info section that doesn't exist.
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <1255032449-12022-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

tools/perf/util/header.c

@@ -287,10 +287,16 @@ struct perf_header *perf_header__read(int fd)
 	do_read(fd, &f_header, sizeof(f_header));
 
 	if (f_header.magic	!= PERF_MAGIC		||
-	    f_header.size	!= sizeof(f_header)	||
 	    f_header.attr_size	!= sizeof(f_attr))
 		die("incompatible file format");
 
+	if (f_header.size != sizeof(f_header)) {
+		/* Support the previous format */
+		if (f_header.size == offsetof(typeof(f_header), trace_info))
+			f_header.trace_info.size = 0;
+		else
+			die("incompatible file format");
+	}
 	nr_attrs = f_header.attrs.size / sizeof(f_attr);
 	lseek(fd, f_header.attrs.offset, SEEK_SET);