linux-kernel - Re: [PATCH v3] tracing: Fix infinite loop in tracing_read_pipe on overflowed print_trace

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Date:   Mon, 28 Nov 2022 11:46:13 -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 v3] tracing: Fix infinite loop in tracing_read_pipe on
 overflowed print_trace_line

On Thu, 24 Nov 2022 20:58:50 +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 | 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;
>  		}