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

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Date:   Sat, 12 Aug 2023 10:22:43 +0800
From:   Zheng Yejian <zhengyejian1@...wei.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     "Masami Hiramatsu (Google)" <mhiramat@...nel.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 2023/8/12 03:24, Steven Rostedt wrote:
> On Fri, 11 Aug 2023 20:37:07 +0800
> Zheng Yejian <zhengyejian1@...wei.com> wrote:
> 
>> 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:
> 
> And if it wasn't consumed? We just lost it?

If 'iter->ent' was not consumed, trace_find_next_entry_inc() will find
it again, will it?

-- Zheng Yejian

> 
>>
>>     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
>>           ... ...
> 
> You missed:
> 
>             count = trace_seq_used(&iter->seq) - save_len;
>             if (rem < count) {
>                  rem = 0;
>                  iter->seq.seq.len = save_len;
> 
> Where the above just threw away what was printed in the above
> "print_trace_line()", and it never went to console.
> 
>                  break;
>             }
> 

Thanks for pointing this out!

-- Zheng Yejian

> -- Steve
> 
> 
>>           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
>>>>   
>>>
>>>    
> 
>