tracing: protect reader of cmdline output

Impact: fix to one cause of incorrect comm outputs in trace

The spinlock only protected the creation of a comm <=> pid pair.
But it was possible that a reader could look up a pid, and get the
wrong comm because it had no locking.

This also required changing trace_find_cmdline to copy the comm cache
and not just send back a pointer to it.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>

1 files changed, 12 insertions, 8 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index efe3202c020..2796bd2b17e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -770,25 +770,29 @@ static void trace_save_cmdline(struct task_struct *tsk)
 	__raw_spin_unlock(&trace_cmdline_lock);
 }
 
-char *trace_find_cmdline(int pid)
+void trace_find_cmdline(int pid, char comm[])
 {
-	char *cmdline = "<...>";
 	unsigned map;
 
-	if (!pid)
-		return "<idle>";
+	if (!pid) {
+		strcpy(comm, "<idle>");
+		return;
+	}
 
-	if (pid > PID_MAX_DEFAULT)
-		goto out;
+	if (pid > PID_MAX_DEFAULT) {
+		strcpy(comm, "<...>");
+		return;
+	}
 
+	__raw_spin_lock(&trace_cmdline_lock);
 	map = map_pid_to_cmdline[pid];
 	if (map >= SAVED_CMDLINES)
 		goto out;
 
-	cmdline = saved_cmdlines[map];
+	strcpy(comm, saved_cmdlines[map]);
 
  out:
-	return cmdline;
+	__raw_spin_unlock(&trace_cmdline_lock);
 }
 
 void tracing_record_cmdline(struct task_struct *tsk)