tracing: Fix infinite loop in tracing_read_pipe on overflowed print_trace_line

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.

Link: https://lkml.kernel.org/r/[email protected]

Cc: Masami Hiramatsu <[email protected]>
Cc: [email protected]
Fixes: 088b1e4 ("ftrace: pipe fixes")
Signed-off-by: Yang Jihong <[email protected]>
Signed-off-by: Steven Rostedt (Google) <[email protected]>

Author: Yang Jihong

kernel/trace/trace.c

@@ -6802,7 +6802,20 @@ 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;
 		}