linux-kernel - Re: [PATCH] tracing: Fix race when concurrently splice_read trace

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Date:   Fri, 11 Aug 2023 20:37:07 +0800
From:   Zheng Yejian <zhengyejian1@...wei.com>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
CC:     <rostedt@...dmis.org>, <laijs@...fujitsu.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing: Fix race when concurrently splice_read
 trace_pipe

On 2023/8/11 19:42, Masami Hiramatsu (Google) wrote:
> On Thu, 10 Aug 2023 20:39:05 +0800
> Zheng Yejian <zhengyejian1@...wei.com> wrote:
> 
>> When concurrently splice_read file trace_pipe and per_cpu/cpu*/trace_pipe,
>> there are more data being read out than expected.
>>
>> The root cause is that in tracing_splice_read_pipe(), an entry is found
>> outside locks, it may be read by multiple readers or consumed by other
>> reader as starting printing it.
>>
>> To fix it, change to find entry after holding locks.
>>
>> Fixes: 7e53bd42d14c ("tracing: Consolidate protection of reader access to the ring buffer")
>> Signed-off-by: Zheng Yejian <zhengyejian1@...wei.com>
>> ---
>>   kernel/trace/trace.c | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
>> index b8870078ef58..f169d33b948f 100644
>> --- a/kernel/trace/trace.c
>> +++ b/kernel/trace/trace.c
>> @@ -7054,14 +7054,16 @@ static ssize_t tracing_splice_read_pipe(struct file *filp,
>>   	if (ret <= 0)
>>   		goto out_err;
>>   
>> -	if (!iter->ent && !trace_find_next_entry_inc(iter)) {
>> +	trace_event_read_lock();
>> +	trace_access_lock(iter->cpu_file);
>> +
>> +	if (!trace_find_next_entry_inc(iter)) {
> 
> It seems you skips '!iter->ent' check. Is there any reason for this change?

IIUC, 'iter->ent' may be the entry that was found but not consumed
in last call tracing_splice_read_pipe(), and in this call, 'iter->ent'
may have being consumed, so we may should find a new 'iter->ent' before
printing it in tracing_fill_pipe_page(), see following reduced codes:

   tracing_splice_read_pipe() {
     if (!iter->ent && !trace_find_next_entry_inc(iter)) {  // 1. find 
entry here
         ... ...
     }
     tracing_fill_pipe_page() {
       for (;;) {
         ... ...
         ret = print_trace_line(iter);  // 2. print entry
         ... ...
         if (!trace_find_next_entry_inc()) {  // 3. find next entry
           ... ...
           break;
         }
       }

--

Thanks,
Zheng Yejian

> 
> Thank you,
> 
>> +		trace_access_unlock(iter->cpu_file);
>> +		trace_event_read_unlock();
>>   		ret = -EFAULT;
>>   		goto out_err;
>>   	}
>>   
>> -	trace_event_read_lock();
>> -	trace_access_lock(iter->cpu_file);
>> -
>>   	/* Fill as many pages as possible. */
>>   	for (i = 0, rem = len; i < spd.nr_pages_max && rem; i++) {
>>   		spd.pages[i] = alloc_page(GFP_KERNEL);
>> -- 
>> 2.25.1
>>
> 
>