perf trace: Handle legacy syscalls tracepoints

Currently the code skips the first field with the expectation that it is 'nr'.
But older kernels do not have the 'nr' field:

field:int nr; offset:8; size:4; signed:1;

Change perf-trace to drop the field if it exists after parsing the format file.

This fixes the off-by-one problem with older kernels (e.g., RHEL6). e.g,
perf-trace shows this for write:

1.515 ( 0.006 ms): dd/4245 write(buf: 2</dev/pts/0>, count: 140733837536224 ) = 26

where 2 is really the fd, the huge number is really the buf address, etc. With
this patch you get the more appropriate:

1.813 ( 0.003 ms): dd/6330 write(fd: 2</dev/pts/0>, buf: 0x7fff22fc81f0, count: 25) = 25

Based-on-a-patch-by: David Ahern <[e-mail]>
Acked-by: David Ahern <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Don Zickus <[e-mail]>
Cc: Frederic Weisbecker <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: http://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-trace.c

···
 
 struct syscall {
 	struct event_format *tp_format;
+	int		    nr_args;
+	struct format_field *args;
 	const char	    *name;
 	bool		    filtered;
 	bool		    is_exit;
···
 	struct format_field *field;
 	int idx = 0;
 
-	sc->arg_scnprintf = calloc(sc->tp_format->format.nr_fields - 1, sizeof(void *));
+	sc->arg_scnprintf = calloc(sc->nr_args, sizeof(void *));
 	if (sc->arg_scnprintf == NULL)
 		return -1;
 
 	if (sc->fmt)
 		sc->arg_parm = sc->fmt->arg_parm;
 
-	for (field = sc->tp_format->format.fields->next; field; field = field->next) {
+	for (field = sc->args; field; field = field->next) {
 		if (sc->fmt && sc->fmt->arg_scnprintf[idx])
 			sc->arg_scnprintf[idx] = sc->fmt->arg_scnprintf[idx];
 		else if (field->flags & FIELD_IS_POINTER)
···
 	if (sc->tp_format == NULL)
 		return -1;
 
+	sc->args = sc->tp_format->format.fields;
+	sc->nr_args = sc->tp_format->format.nr_fields;
+	/* drop nr field - not relevant here; does not exist on older kernels */
+	if (sc->args && strcmp(sc->args->name, "nr") == 0) {
+		sc->args = sc->args->next;
+		--sc->nr_args;
+	}
+
 	sc->is_exit = !strcmp(name, "exit_group") || !strcmp(name, "exit");
 
 	return syscall__set_arg_fmts(sc);
···
 	unsigned char *p;
 	unsigned long val;
 
-	if (sc->tp_format != NULL) {
+	if (sc->args != NULL) {
 		struct format_field *field;
 		u8 bit = 1;
 		struct syscall_arg arg = {
···
 			.thread = thread,
 		};
 
-		for (field = sc->tp_format->format.fields->next; field;
+		for (field = sc->args; field;
 		     field = field->next, ++arg.idx, bit <<= 1) {
 			if (arg.mask & bit)
 				continue;