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

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Date:   Sat, 12 Aug 2023 21:13:17 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Zheng Yejian <zhengyejian1@...wei.com>
Cc:     <mhiramat@...nel.org>, <laijs@...fujitsu.com>,
        <linux-kernel@...r.kernel.org>,
        <linux-trace-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Jens Axboe <axboe@...nel.dk>, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] tracing: Fix race when concurrently splice_read
 trace_pipe

On Sat, 12 Aug 2023 15:38:12 +0800
Zheng Yejian <zhengyejian1@...wei.com> wrote:

> Reproduction testcase is show as below, it can always reproduce the
> issue in v5.10, and after this patch, the testcase passed.
> 
> In v5.10, when run `cat trace_pipe > /tmp/myfile &`, it call
> sendfile() to transmit data from trace_pipe into /tmp/myfile. And in
> kernel, .splice_read() of trace_pipe is called then the issue is
> reproduced.
> 
> However in the newest v6.5, this reproduction case didn't run into the
> .splice_read() of trace_pipe, because after commit 97ef77c52b78 ("fs:
> check FMODE_LSEEK to control internal pipe splicing"), non-seekable
> trace_pipe cannot be sendfile-ed.

So the test case cannot be run because the "sendfile" on the
trace_pipe now fails?

I'm not sure if this is considered a user space regression or not, but
I figured I'd let the interested parties know.

I don't know what tools out there records trace_pipe via sendfile, but
there might be some.

-- Steve