linux-kernel - Re: [PATCH] tracing: Fix infinite loop in tracing_read_pipe on overflowed print_trace

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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