Date: Tue, 21 Oct 2008 16:46:19 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: hooanon05@yahoo.co.jp
Cc: linux-kernel@vger.kernel.org
Subject: Re: __vm_enough_memory(), OVERCOMMIT_NEVER, current->mm, kernel
 thread
Message-ID: <20081021164619.341f01d9@lxorguk.ukuu.org.uk>
In-Reply-To: <7123.1224602071@jrobl>
References: <7123.1224602071@jrobl>
Organization: Red Hat UK Cyf., Amberley Place, 107-111 Peascod Street,
 Windsor, Berkshire, SL4 1TE, Y Deyrnas Gyfunol. Cofrestrwyd yng Nghymru a
 Lloegr o'r rhif cofrestru 3798903
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1068
Lines: 27

> In this case, NFSD (kernel thread) calls shmem_file_write() or
> shmem_write_begin() and __vm_enough_memory() is called. But current->mm
> is NULL and the kernel crashes.
> If a user have to set OVERCOMMIT_NEVER, where should we fix?

Calling into the file system code assuming that current->mm is
NULL isn't safe and hasn't been for a very long time since someone added
the 3% hack.

The shmem case is actually a bit special so my thoughts are:

Make security_vm_enough_memory() WARN() if current->mm = NULL
Make security_vm_enough_memory_mm() WARN() if the passed mm = NULL
Add security_vm_enough_memory_fs() which does not do the warning test

All would still call security->ops->vm_enough_memory and then
__vm_enough_memory() would skip the 3% adjustment when the passed mm was
NULL

Does that sound sensible ?

Alan
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/