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

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Message-Id: <20230811204257.99df8ba60d591f5bace38615@kernel.org>
Date:   Fri, 11 Aug 2023 20:42:57 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Zheng Yejian <zhengyejian1@...wei.com>
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 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?

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
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>