Re: [PATCH v3] tracing: Fix infinite loop in tracing_read_pipe on overflowed print_trace_line

From: Steven Rostedt
Date: Mon Nov 28 2022 - 11:46:21 EST


On Thu, 24 Nov 2022 20:58:50 +0800
Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:

> 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 | 22 +++++++++++++++++++++-
> 1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index a7fe0e115272..55733224fa88 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6787,7 +6787,27 @@ 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 it, otherwise,
> + * while loop will peek this event next time,
> + * resulting in an infinite loop.
> + */
> + if (trace_seq_has_overflowed(&iter->seq)) {

The only way to get here is if the above is true, and that is not going to
cause the infinite loop. What does is if save_len == 0. In fact, that's
all you need to check for:

if (save_len == 0) {

Should do the trick.

-- Steve



> + /*
> + * Here we only consider the case that one
> + * print_trace_line() fills the entire trace_seq
> + * in one shot, in that case, iter->seq.seq.len is zero,
> + * we simply output a log of too long line to inform the user.
> + */
> + 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;
> }