linux-kernel - Re: [RFC][PATCH] vfs: In mntput run deactivate

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Message-ID: <87ppkl1xb7.fsf@x220.int.ebiederm.org>
Date:	Sun, 13 Apr 2014 00:51:56 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Miklos Szeredi <miklos@...redi.hu>,
	Christoph Hellwig <hch@...radead.org>,
	Karel Zak <kzak@...hat.com>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [RFC][PATCH] vfs: In mntput run deactivate_super on a shallow stack.

ebiederm@...ssion.com (Eric W. Biederman) writes:

> Al Viro <viro@...IV.linux.org.uk> writes:
>
>> On Sat, Apr 12, 2014 at 03:15:39PM -0700, Eric W. Biederman wrote:
>>
>>> Can you explain which scenario you are thinking about with respect to a
>>> failed modprobe?
>>
>> Again, it's not impossible to audit (there's not a lot of places where
>> struct file_system_type * is ever stored, there are few instances of
>> struct file_system_type, all statically allocated, etc.), but it's
>> a non-trivial amount of work.  And I honestly don't know if we have
>> any such places right now.  Moreover, unless you feel like repeating
>> that kind of audit every merge window, we'll need a some way of dealing
>> with such situations.  Something like flush_pending_mntput(fs_type), for
>> example, documented as barrier to be used in such places might do, but
>> if you can think of something more fool-proof...
>
> I performed a quick audit and I don't see that case happening in the
> current code.  

Sigh.  I was wrong.  Almost this exact case happens in
btrfs_init_test_fs, and of course I was silly when I thought the module
ref count would be useful for something before init_module succeeds.

Still I suspect I was on the right track.  We do have the get_fs_type,
get_filesystem and put_filesystem.  Which ought to be enough to allow
us to convert unregister_filesystem into an appropriate barrier.

Something to look at after I have slept.

Eric


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