[PATCH v4] tracing: Fix infinite loop in tracing_read_pipe on overflowed print_trace_line

[Yang Jihong]

print_trace_line may overflow seq_file buffer. If the event is not
consumed, the while loop keeps peeking this event, causing a infinite loop.

Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
---
 kernel/trace/trace.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index a7fe0e115272..de7c99f4b539 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6787,7 +6787,20 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
 
 		ret = print_trace_line(iter);
 		if (ret == TRACE_TYPE_PARTIAL_LINE) {
-			/* don't print partial lines */
+			/*
+			 * If one print_trace_line() fills entire trace_seq in one shot,
+			 * trace_seq_to_user() will returns -EBUSY because save_len == 0,
+			 * In this case, we need to consume it, otherwise, loop will peek
+			 * this event next time, resulting in an infinite loop.
+			 */
+			if (save_len == 0) {
+				iter->seq.full = 0;
+				trace_seq_puts(&iter->seq, "[LINE TOO BIG]\n");
+				trace_consume(iter);
+				break;
+			}
+
+			/* In other cases, don't print partial lines */
 			iter->seq.seq.len = save_len;
 			break;
 		}
-- 
2.30.GIT