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SubjectRe: [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@huawei.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@huawei.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)

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