tracing: Simplify & fix saved_tgids logic

commit b81b3e9 upstream.

The tgid_map array records a mapping from pid to tgid, where the index
of an entry within the array is the pid & the value stored at that index
is the tgid.

The saved_tgids_next() function iterates over pointers into the tgid_map
array & dereferences the pointers which results in the tgid, but then it
passes that dereferenced value to trace_find_tgid() which treats it as a
pid & does a further lookup within the tgid_map array. It seems likely
that the intent here was to skip over entries in tgid_map for which the
recorded tgid is zero, but instead we end up skipping over entries for
which the thread group leader hasn't yet had its own tgid recorded in
tgid_map.

A minimal fix would be to remove the call to trace_find_tgid, turning:

  if (trace_find_tgid(*ptr))

into:

  if (*ptr)

..but it seems like this logic can be much simpler if we simply let
seq_read() iterate over the whole tgid_map array & filter out empty
entries by returning SEQ_SKIP from saved_tgids_show(). Here we take that
approach, removing the incorrect logic here entirely.

Link: https://lkml.kernel.org/r/20210630003406.4013668-1-paulburton@google.com

Fixes: d914ba3 ("tracing: Add support for recording tgid of tasks")
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Joel Fernandes <joelaf@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Paul Burton <paulburton@google.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/trace/trace.c

@@ -5284,37 +5284,20 @@ static const struct file_operations tracing_readme_fops = {
 
 static void *saved_tgids_next(struct seq_file *m, void *v, loff_t *pos)
 {
-	int *ptr = v;
+	int pid = ++(*pos);
 
-	if (*pos || m->count)
-		ptr++;
-
-	(*pos)++;
-
-	for (; ptr <= &tgid_map[PID_MAX_DEFAULT]; ptr++) {
-		if (trace_find_tgid(*ptr))
-			return ptr;
-	}
+	if (pid > PID_MAX_DEFAULT)
+		return NULL;
 
-	return NULL;
+	return &tgid_map[pid];
 }
 
 static void *saved_tgids_start(struct seq_file *m, loff_t *pos)
 {
-	void *v;
-	loff_t l = 0;
-
-	if (!tgid_map)
+	if (!tgid_map || *pos > PID_MAX_DEFAULT)
 		return NULL;
 
-	v = &tgid_map[0];
-	while (l <= *pos) {
-		v = saved_tgids_next(m, v, &l);
-		if (!v)
-			return NULL;
-	}
-
-	return v;
+	return &tgid_map[*pos];
 }
 
 static void saved_tgids_stop(struct seq_file *m, void *v)
@@ -5323,9 +5306,14 @@ static void saved_tgids_stop(struct seq_file *m, void *v)
 
 static int saved_tgids_show(struct seq_file *m, void *v)
 {
-	int pid = (int *)v - tgid_map;
+	int *entry = (int *)v;
+	int pid = entry - tgid_map;
+	int tgid = *entry;
+
+	if (tgid == 0)
+		return SEQ_SKIP;
 
-	seq_printf(m, "%d %d\n", pid, trace_find_tgid(pid));
+	seq_printf(m, "%d %d\n", pid, tgid);
 	return 0;
 }