linux-kernel - Re: [PATCH v3 14/15] fs/files: export close

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Date:   Thu, 7 Sep 2017 11:51:47 -0700
From:   Eric Biggers <ebiggers3@...il.com>
To:     Gustavo Padovan <gustavo@...ovan.org>
Cc:     linux-media@...r.kernel.org, Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@....samsung.com>,
        Shuah Khan <shuahkh@....samsung.com>,
        linux-kernel@...r.kernel.org,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org,
        Riley Andrews <riandrews@...roid.com>
Subject: Re: [PATCH v3 14/15] fs/files: export close_fd() symbol

On Thu, Sep 07, 2017 at 03:42:25PM -0300, Gustavo Padovan wrote:
> From: Gustavo Padovan <gustavo.padovan@...labora.com>
> 
> Rename __close_fd() to close_fd() and export it to be able close files
> in modules using file descriptors.
> 
> The usecase that motivates this change happens in V4L2 where we send
> events to userspace with a fd that has file installed in it. But if for
> some reason we have to cancel the video stream we need to close the files
> that haven't been shared with userspace yet. Thus the export of
> close_fd() becomes necessary.
> 
> fd_install() happens when we call an ioctl to queue a buffer, but we only
> share the fd with userspace later, and that may happen in a kernel thread
> instead.

What do you mean?  A file descriptor is shared with userspace as soon as it's
installed in the fdtable by fd_install().  As soon as it's there, another thread
can use it (or close it, duplicate it, etc.), even before the syscall that
installed it returns...

Eric