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Message-ID: <188a48b7-f426-6348-086e-22e56bb07206@huawei.com>
Date: Fri, 18 Nov 2022 18:21:12 +0800
From: Yang Jihong <yangjihong1@...wei.com>
To: Steven Rostedt <rostedt@...dmis.org>
CC: <mhiramat@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Fix infinite loop in tracing_read_pipe on
overflowed print_trace_line
Hello,
On 2022/11/18 5:40, Steven Rostedt wrote:
> On Mon, 14 Nov 2022 10:29:46 +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 | 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
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