linux-kernel - Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] : ima-appraisal patches)

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Message-ID: <20120427173417.GK6871@ZenIV.linux.org.uk>
Date:	Fri, 27 Apr 2012 18:34:18 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, linux-fsdevel@...r.kernel.org,
	James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	David Safford <safford@...ux.vnet.ibm.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] :
 ima-appraisal patches)

On Fri, Apr 27, 2012 at 10:35:25AM +0300, Kasatkin, Dmitry wrote:

> But have you seen the proposed patch for __fput()?
> [PATCH v4 10/12] ima: defer calling __fput()
> 
> It defers only of course the last AND mmap_sem is locked AND open for write.
> 
> 	if (current->mm && rwsem_is_locked(&current->mm->mmap_sem)) {
> 		if (ima_defer_fput(file) == 0)
> 			return;
> 	}
> 
> Just 5 out of ~100,000 mmap_sem held fput() calls were deferred.

Let me get it straight.
	a) You still ignore all the problems with that described in the
posting right in the beginning of this thread.
	b) You ignore the problems with semantics changes from user-visible
delays of fput() past the return from syscall (described in Linus' posting
upthread - they apply to this "solution" as well).
	c) You seem to consider the fact that this path will be exercised
very rarely, thus making any races on it damn hard to reproduce and debug
as a good thing.  

And as for the sentiment expressed in the beginning of your posting (that
smaller patch size is worth more than clean locking rules, maintainability
of resulting kernel, etc.)...  I'm sorry, but you guys need to decide
what IMA is.  If it's a first-class part of the kernel, you have your
priorities backwards...
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