Date: Fri, 7 Oct 2016 21:48:36 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Calvin Owens <calvinowens@fb.com>
Cc: Jeff Layton <jlayton@poochiereds.net>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Rusty Russell <rusty@rustcorp.com.au>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH] fs: Assert on module file_operations without an owner
Message-ID: <20161007204836.GR19539@ZenIV.linux.org.uk>
References: <48414ef29337b54e2a842bd841f73f01ab74ebe7.1475872278.git.calvinowens@fb.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <48414ef29337b54e2a842bd841f73f01ab74ebe7.1475872278.git.calvinowens@fb.com>
User-Agent: Mutt/1.7.0 (2016-08-17)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1192
Lines: 23

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.