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

[Yang Jihong]

Hello,

On 2022/11/21 3:38, Steven Rostedt wrote:
> 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.
Yes, I'm fixing the situation you mentioned.
For the sake of discussion, I've replied in the v1 patch to see if it's 
just add "trace_seq_puts(&iter->seq, "[LINE TOO BIG]\n");" or a more 
complex situation.

Thanks,
Yang