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

[Yang Jihong]

Hello,

On 2022/11/18 5:40, Steven Rostedt wrote:
> On Mon, 14 Nov 2022 10:29:46 +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 | 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.
Ok, will change in next version.(Because iter->seq.seq.len may be 
smaller than strlen(dots), direct subtraction here may not be appropriate.)

Thanks,
Yang