linux-kernel - Re: [PATCH] tracing: Fix infinite loop in tracing_read_pipe on overflowed print_trace

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Date:   Thu, 17 Nov 2022 16:40:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yang Jihong <yangjihong1@...wei.com>
Cc:     <mhiramat@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Fix infinite loop in tracing_read_pipe on
 overflowed print_trace_line

On Mon, 14 Nov 2022 10:29:46 +0800
Yang Jihong <yangjihong1@...wei.com> 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@...wei.com>
> ---
>  kernel/trace/trace.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 47a44b055a1d..2a8d5c68c29b 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6788,6 +6788,19 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>  		if (ret == TRACE_TYPE_PARTIAL_LINE) {
>  			/* don't print partial lines */
>  			iter->seq.seq.len = save_len;
> +
> +			/*
> +			 * If one trace_line of the tracer overflows seq_file
> +			 * buffer, trace_seq_to_user returns -EBUSY because
> +			 * nothing in the sequence (iter->seq.seq.len = \
> +			 * iter->seq.seq.readpos = 0).
> +			 * In this case, we need to consume, otherwise,
> +			 * "while" will peek this event next time, resulting
> +			 * in an infinite loop.
> +			 */
> +			if (trace_seq_has_overflowed(&iter->seq))
> +				trace_consume(iter);

Instead of consuming it, I think the right solution is to print the partial
line. Something like:

			if (trace_seq_has_overflowed(&iter->seq)) {
				char dots[] = "...";

				iter->seq.seq.len -= sizeof(dots) + 1;
				iter->seq.seq.full = 0;
				trace_seq_puts(&iter->seq, dots);
				trace_consume(iter);
				break;
			}

			iter->seq.seq.len = save_len;
			break;

That way we can see the broken trace event and not just silently drop it.

-- Steve

> +
>  			break;
>  		}
>  		if (ret != TRACE_TYPE_NO_CONSUME)