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Date:   Thu, 23 Jan 2020 17:23:21 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Pengcheng Yang <yangpc@wangsu.com>
Cc:     gregkh@linuxfoundation.org, jannh@google.com,
        viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org
Subject: Re: [PATCH RESEND] kernel/relay.c: fix read_pos error when multiple
 readers
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On Wed, 22 Jan 2020 19:06:15 +0800 Pengcheng Yang <yangpc@wangsu.com> wrote:

> When reading, read_pos should start with bytes_consumed,
> not file->f_pos. Because when there is more than one reader,
> the read_pos corresponding to file->f_pos may have been consumed,
> which will cause the data that has been consumed to be read
> and the bytes_consumed update error.

That sounds fairly serious.  Are you able to describe a userspace setup
which will trigger this?  Do you have any test code which is able to
demonstrate the bug?

We really should have a relay testcase in tools/testing, but relay came
along before we became diligent about this.