tracing: Move locking of trace_cmdline_lock into start/stop seq calls

With the conversion of the saved_cmdlines output to use seq_read, there
is now a race between accessing the values of the saved_cmdlines and
the writing to them. The trace_cmdline_lock needs to be taken at
the start and stop of the seq calls.

A new __trace_find_cmdline() call is created to allow for the look up
to happen without taking the lock.

Fixes: 42584c81c5ad tracing: Have saved_cmdlines use the seq_read infrastructure
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 15 insertions, 4 deletions

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 22a902e2ded9..626dbfde5d56 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1484,7 +1484,7 @@ static int trace_save_cmdline(struct task_struct *tsk)
         return 1;
 }
 
-void trace_find_cmdline(int pid, char comm[])
+static void __trace_find_cmdline(int pid, char comm[])
 {
         unsigned map;
 
@@ -1503,13 +1503,19 @@ void trace_find_cmdline(int pid, char comm[])
                 return;
         }
 
-        preempt_disable();
-        arch_spin_lock(&trace_cmdline_lock);
         map = map_pid_to_cmdline[pid];
         if (map != NO_CMDLINE_MAP)
                 strcpy(comm, saved_cmdlines[map]);
         else
                 strcpy(comm, "<...>");
+}
+
+void trace_find_cmdline(int pid, char comm[])
+{
+        preempt_disable();
+        arch_spin_lock(&trace_cmdline_lock);
+
+        __trace_find_cmdline(pid, comm);
 
         arch_spin_unlock(&trace_cmdline_lock);
         preempt_enable();
@@ -3724,6 +3730,9 @@ static void *saved_cmdlines_start(struct seq_file *m, loff_t *pos)
         void *v;
         loff_t l = 0;
 
+        preempt_disable();
+        arch_spin_lock(&trace_cmdline_lock);
+
         v = &map_cmdline_to_pid[0];
         while (l <= *pos) {
                 v = saved_cmdlines_next(m, v, &l);
@@ -3736,6 +3745,8 @@ static void *saved_cmdlines_start(struct seq_file *m, loff_t *pos)
 
 static void saved_cmdlines_stop(struct seq_file *m, void *v)
 {
+        arch_spin_unlock(&trace_cmdline_lock);
+        preempt_enable();
 }
 
 static int saved_cmdlines_show(struct seq_file *m, void *v)
@@ -3743,7 +3754,7 @@ static int saved_cmdlines_show(struct seq_file *m, void *v)
         char buf[TASK_COMM_LEN];
         unsigned int *pid = v;
 
-        trace_find_cmdline(*pid, buf);
+        __trace_find_cmdline(*pid, buf);
         seq_printf(m, "%d %s\n", *pid, buf);
         return 0;
 }