Re: [PATCH v2] tracing: Fix infinite loop in tracing_read_pipe on overflowed print_trace_line

From: Steven Rostedt
Date: Sun Nov 20 2022 - 14:39:13 EST


On Fri, 18 Nov 2022 18:25:21 +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>
> ---
>
> Changes since v1:
> - Print partial line to show the broken trace event when overflowed print_trace_line
>
> kernel/trace/trace.c | 27 ++++++++++++++++++++++++++-
> 1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 47a44b055a1d..81c36dc80212 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6786,7 +6786,32 @@ 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, otherwise,
> + * while loop will peek this event next time,
> + * resulting in an infinite loop.
> + */
> + if (trace_seq_has_overflowed(&iter->seq)) {

We need to only do this if save_len is zero. Because the reason that it
returns TRACE_TYPE_PARTIAL_LINE is usually because it overflowed.

This loops until the trace_seq is full, so it's OK to have it overflow.
The case I believe you are fixing, is the case were one
print_trace_line() actually fills the entire trace_seq in one shot. In
which case, it will never print, and in that case, save_len will be zero.

-- Steve


> + /*
> + * Print the partial line,
> + * that way we can see the broken trace event.
> + */
> + char dots[] = "...\n";
> +
> + iter->seq.full = 0;
> + if (seq_buf_buffer_left(&iter->seq.seq) < strlen(dots)) {
> + iter->seq.seq.len =
> + seq_buf_used(&iter->seq.seq) - strlen(dots);
> + }
> + trace_seq_puts(&iter->seq, dots);
> +
> + trace_consume(iter);
> + break;
> + }
> +
> + /* In other cases, don't print partial lines */
> iter->seq.seq.len = save_len;
> break;
> }