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

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Date:   Fri, 7 Oct 2016 17:39:37 -0400
From:   Calvin Owens <calvinowens@...com>
To:     Al Viro <viro@...IV.linux.org.uk>
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 Friday 10/07 at 17:18 -0400, Calvin Owens wrote:
> On Friday 10/07 at 21:48 +0100, Al Viro wrote:
> > 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.
> 
> This isn't a theoretical issue: I have a proprietary module that makes this
> mistake and crashes when poking a chrdev it exposes in userspace races with
> unloading the module.
> 
> Of course, the bug is in this silly module. I'm not arguing that it isn't. I
> was hesitant to even mention this because I know waving at something in an OOT
> module is a poor argument for changing anything in the proper kernel.
> 
> But what I'm trying to do here is prevent people from making that mistake in
> the future by yelling at them when they do. The implicit ignoring of a NULL
> owner in try_module_get() in fops_get() is not necessarily obvious.

Let's drop this, I should never have sent the patch in the first place.

> > 	* 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.
> 
> I see why this is true, and it's something I considered. But when there is
> zero cost to being explicit and setting ->owner, why not do it?
> 
> > 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.

I'll look into this, I'm interested.

Thanks,
Calvin

>