linux-kernel - Re: [PATCH] fs: Assert on module file

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Date:   Fri, 7 Oct 2016 21:48:36 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Calvin Owens <calvinowens@...com>
Cc:     Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Rusty Russell <rusty@...tcorp.com.au>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] fs: Assert on module file_operations without an owner

On Fri, Oct 07, 2016 at 01:35:52PM -0700, Calvin Owens wrote:
> Omitting the owner field in file_operations declared in modules is an
> easy mistake to make, and can result in crashes when the module is
> unloaded while userspace is poking the file.
> 
> This patch modifies fops_get() to WARN when it encounters a NULL owner,
> since in this case it cannot take a reference on the containing module.

NAK.  This is complete crap - we do *NOT* need ->owner on a lot of
file_operations.
	* we do not need that on file_operations of a regular file or
directory on a normal filesystem, since that filesystem is not going
away until the file has been closed - ->f_path.mnt is holding a reference
to vfsmount, which is holding a reference to superblock, which is holding
a reference to file_system_type, which is holding a reference to _its_
->owner.
	* we do not need that on anything on procfs - module removal is
legal while a procfs file is opened; its cleanup will be blocked for the
duration of ->read(), ->write(), etc. calls.

If anything, we would be better off with modifications that would get
rid of ->owner on file_operations.  It's not trivial to do, but it might
be not impossible.