linux-kernel - Re: [PATCH resend] audit: fix mark refcounting

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Message-ID: <20111215084050.GQ2203@ZenIV.linux.org.uk>
Date:	Thu, 15 Dec 2011 08:40:50 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>, Eric Paris <eparis@...hat.com>,
	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH resend] audit: fix mark refcounting

On Wed, Dec 14, 2011 at 06:15:11PM -0800, Linus Torvalds wrote:
> Looks reasonable, but why doesn't all callers have that "put_mark()" thing?
> 
> And if/when all callers *do* have that put_mark() thing, maybe we
> should make destroy_mark() just do it?
> 
> In particular, a quick grep shows that there are destroy_mark users still in:
> 
>  - fs/notify/fanotify/fanotify_user.c
> 
>  - fs/notify/dnotify/dnotify.c (2 of them)
> 
>  - fs/notify/inotify/inotify_fsnotify.c
> 
> 
> that don't do "put_mark()" after the destroy. Why is it ok there?

Um?  dnotify has fsnotify_put_mark() called in both cases...
 
> I don't know the code, it's probably fine, but I'd like to know why
> the audit code needs it but not the other sites (but my grep didn't
> look at context)
> 
> And I'd like Al to say something. Al?

I don't like it; it's called from ->handle_event() and parent->mark is
exactly the inode_mark argument of that method.  It ought to be pinned
by caller.  In other places we *do* need get/put around that destroy
and we generally do that.

AFAICS, we have the following picture:
	* that place in audit_watch - argument of ->handle_event()
	* audit_remove_watch_rule() - pinned explicitly
	* audit_tree - pinned explicitly
	* dnotify (both callrs) - pinned explicitly, and refcount is
dropped unconditionally while fsnotify_destroy_mark() is *not*; IOW,
that's a very strong argument against folding put_mark into destroy_mark.
	* inotify_fsnotify.c - argument of ->handle_event()
	* fanotify_user.c - pinned and dropped by caller; again, refcount
manipulations are unconditional while destroy_mark is not; it's even
worse than in dnotify case, since here we do put_mark is a place where
we don't *know* whether destroy_mark has happened.  We could move the
calls of fsnotify_put_mark() into the fanotify_mark_remove_from_mask()
(where destroy_mark is done), but then we'll get something like
	if (!(oldmask & ~mask))
                fsnotify_destroy_mark(fsn_mark);
	else
                fsnotify_put_mark(fsn_mark);
in there, which is IMO ugly.

Guys, does anybody have a real demonstration of the breakage cured by
pinning the mark down in audit_watch.c ->handle_event()?  Or is that
a pure theory?

Is ->handle_event() argument held by caller?  Eric?  If that's the case,
we don't need to do anything with audit_watch.c instance; otherwise,
both that one and inotify_handle_event() are in trouble...
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