[PATCH v2] 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>
---

Changes since v1:
  - Print partial line to show the broken trace event when overflowed print_trace_line

 kernel/trace/trace.c | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 47a44b055a1d..81c36dc80212 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -6786,7 +6786,32 @@ 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 trace_line of the tracer overflows seq_file
+			 * buffer, trace_seq_to_user returns -EBUSY.
+			 * In this case, we need to consume, otherwise,
+			 * while loop will peek this event next time,
+			 * resulting in an infinite loop.
+			 */
+			if (trace_seq_has_overflowed(&iter->seq)) {
+				/*
+				 * Print the partial line,
+				 * that way we can see the broken trace event.
+				 */
+				char dots[] = "...\n";
+
+				iter->seq.full = 0;
+				if (seq_buf_buffer_left(&iter->seq.seq) < strlen(dots)) {
+					iter->seq.seq.len =
+						seq_buf_used(&iter->seq.seq) - strlen(dots);
+				}
+				trace_seq_puts(&iter->seq, dots);
+
+				trace_consume(iter);
+				break;
+			}
+
+			/* In other cases, don't print partial lines */
 			iter->seq.seq.len = save_len;
 			break;
 		}
-- 
2.30.GIT