[PATCH] tracing: Fix race when concurrently splice_read trace_pipe

[Zheng Yejian]

When concurrently splice_read file trace_pipe and per_cpu/cpu*/trace_pipe,
there are more data being read out than expected.

The root cause is that in tracing_splice_read_pipe(), an entry is found
outside locks, it may be read by multiple readers or consumed by other
reader as starting printing it.

To fix it, change to find entry after holding locks.

Fixes: 7e53bd42d14c ("tracing: Consolidate protection of reader access to the ring buffer")
Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
---
 kernel/trace/trace.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b8870078ef58..f169d33b948f 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -7054,14 +7054,16 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
 	if (ret <= 0)
 		goto out_err;
 
-	if (!iter->ent && !trace_find_next_entry_inc(iter)) {
+	trace_event_read_lock();
+	trace_access_lock(iter->cpu_file);
+
+	if (!trace_find_next_entry_inc(iter)) {
+		trace_access_unlock(iter->cpu_file);
+		trace_event_read_unlock();
 		ret = -EFAULT;
 		goto out_err;
 	}
 
-	trace_event_read_lock();
-	trace_access_lock(iter->cpu_file);
-
 	/* Fill as many pages as possible. */
 	for (i = 0, rem = len; i < spd.nr_pages_max && rem; i++) {
 		spd.pages[i] = alloc_page(GFP_KERNEL);
-- 
2.25.1