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Subject[PATCH 3.16 321/346] tracing: Move mutex to protect against resetting of seq data
3.16.39-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (Red Hat)" <rostedt@goodmis.org>

commit 1245800c0f96eb6ebb368593e251d66c01e61022 upstream.

The iter->seq can be reset outside the protection of the mutex. So can
reading of user data. Move the mutex up to the beginning of the function.

Fixes: d7350c3f45694 ("tracing/core: make the read callbacks reentrants")
Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
kernel/trace/trace.c | 15 ++++++++-------
1 file changed, 8 insertions(+), 7 deletions(-)

--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -4435,13 +4435,6 @@ tracing_read_pipe(struct file *filp, cha
struct trace_array *tr = iter->tr;
ssize_t sret;

- /* return any leftover data */
- sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
- if (sret != -EBUSY)
- return sret;
-
- trace_seq_init(&iter->seq);
-
/* copy the tracer to avoid using a global lock all around */
mutex_lock(&trace_types_lock);
if (unlikely(iter->trace->name != tr->current_trace->name))
@@ -4454,6 +4447,14 @@ tracing_read_pipe(struct file *filp, cha
* is protected.
*/
mutex_lock(&iter->mutex);
+
+ /* return any leftover data */
+ sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
+ if (sret != -EBUSY)
+ goto out;
+
+ trace_seq_init(&iter->seq);
+
if (iter->trace->read) {
sret = iter->trace->read(iter, filp, ubuf, cnt, ppos);
if (sret)
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