[PATCH] perf trace: Handle legacy syscalls

[David Ahern <dsahern@gmail.com>]

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

Signed-off-by: David Ahern <dsahern@gmail.com>
---
 tools/lib/traceevent/event-parse.c | 11 ++++++++---
 tools/lib/traceevent/event-parse.h |  1 +
 tools/perf/builtin-trace.c         | 16 +++++++++++++---
 3 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/tools/lib/traceevent/event-parse.c b/tools/lib/traceevent/event-parse.c
index afe20ed9fac8..e8a29e730dfb 100644
--- a/tools/lib/traceevent/event-parse.c
+++ b/tools/lib/traceevent/event-parse.c
@@ -6228,15 +6228,20 @@ void pevent_ref(struct pevent *pevent)
 	pevent->ref_count++;
 }
 
+void free_format_field(struct format_field *field)
+{
+	free(field->type);
+	free(field->name);
+	free(field);
+}
+
 static void free_format_fields(struct format_field *field)
 {
 	struct format_field *next;
 
 	while (field) {
 		next = field->next;
-		free(field->type);
-		free(field->name);
-		free(field);
+		free_format_field(field);
 		field = next;
 	}
 }
diff --git a/tools/lib/traceevent/event-parse.h b/tools/lib/traceevent/event-parse.h
index 5b4efc062320..a548fac646f6 100644
--- a/tools/lib/traceevent/event-parse.h
+++ b/tools/lib/traceevent/event-parse.h
@@ -619,6 +619,7 @@ enum pevent_errno pevent_parse_format(struct pevent *pevent,
 				      const char *buf,
 				      unsigned long size, const char *sys);
 void pevent_free_format(struct event_format *event);
+void free_format_field(struct format_field *field);
 
 void *pevent_get_field_raw(struct trace_seq *s, struct event_format *event,
 			   const char *name, struct pevent_record *record,
diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
index dcd950ef2fd7..5caeefeda48a 100644
--- a/tools/perf/builtin-trace.c
+++ b/tools/perf/builtin-trace.c
@@ -1442,14 +1442,24 @@ static int syscall__set_arg_fmts(struct syscall *sc)
 	struct format_field *field;
 	int idx = 0;
 
-	sc->arg_scnprintf = calloc(sc->tp_format->format.nr_fields - 1, sizeof(void *));
+	field = sc->tp_format->format.fields;
+	/* drop nr field - not relevant here; does not exist on older kernels */
+	if (field && strcmp(field->name, "nr") == 0) {
+		struct format_field *next = field->next;
+
+		free_format_field(field);
+		sc->tp_format->format.fields = next;
+		sc->tp_format->format.nr_fields--;
+	}
+
+	sc->arg_scnprintf = calloc(sc->tp_format->format.nr_fields, 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->tp_format->format.fields; 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)
@@ -1547,7 +1557,7 @@ static size_t syscall__scnprintf_args(struct syscall *sc, char *bf, size_t size,
 			.thread = thread,
 		};
 
-		for (field = sc->tp_format->format.fields->next; field;
+		for (field = sc->tp_format->format.fields; field;
 		     field = field->next, ++arg.idx, bit <<= 1) {
 			if (arg.mask & bit)
 				continue;
-- 
2.2.1