[PATCH RFC] trace, blktrace: remove trace from running list only if trace is running

[Arianna Avanzini]

Currently, blktrace can be started/stopped via its ioctl-based interface
(used by the userspace blktrace tool) or via its ftrace interface. The
function blk_trace_remove_queue(), called each time an "enable" tunable
of the ftrace interface transitions to zero, removes unconditionally the
trace from the running list, even if its state is not Blktrace_running.
In fact, the state of a blk_trace is modified only by the ioctl-based
interface, and a blk_trace is added to the running list only when its
state transitions from Blktrace_setup or Blktrace_stopped to
Blktrace_running. If the ioctl-based interface is not being used, the
state of the blk_trace is undefined.
In this case, using the sysfs tunable to stop a trace would trigger a
removal of a blk_trace from the running list while it is not on such a
list, leading to a null pointer dereference. This commit attempts to fix
the issue by letting the blk_trace_remove_queue() function remove the
blk_trace from the running list only if its state is Blktrace_running.

Signed-off-by: Arianna Avanzini <avanzini.arianna@xxxxxxxxx>
---
 kernel/trace/blktrace.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index c1bd4ad..f58b617 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1493,9 +1493,11 @@ static int blk_trace_remove_queue(struct request_queue *q)
 	if (atomic_dec_and_test(&blk_probes_ref))
 		blk_unregister_tracepoints();
 
-	spin_lock_irq(&running_trace_lock);
-	list_del(&bt->running_list);
-	spin_unlock_irq(&running_trace_lock);
+	if (bt->trace_state == Blktrace_running) {
+		spin_lock_irq(&running_trace_lock);
+		list_del(&bt->running_list);
+		spin_unlock_irq(&running_trace_lock);
+	}
 	blk_trace_free(bt);
 	return 0;
 }
-- 
2.1.2

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