From mboxrd@z Thu Jan 1 00:00:00 1970 From: Waiman Long Subject: Re: [PATCH] dcache: Translating dentry into pathname without taking rename_lock Date: Wed, 04 Sep 2013 15:33:00 -0400 Message-ID: <52278AEC.2020307@hp.com> References: <1378321523-40893-1-git-send-email-Waiman.Long@hp.com> <20130904191104.GK13318@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, "Chandramouleeswaran, Aswin" , "Norton, Scott J" To: Al Viro Return-path: In-Reply-To: <20130904191104.GK13318@ZenIV.linux.org.uk> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On 09/04/2013 03:11 PM, Al Viro wrote: > On Wed, Sep 04, 2013 at 03:05:23PM -0400, Waiman Long wrote: >> >> static int prepend_name(char **buffer, int *buflen, struct qstr *name) >> { >> - return prepend(buffer, buflen, name->name, name->len); >> + /* >> + * With RCU path tracing, it may race with rename. Use >> + * ACCESS_ONCE() to make sure that it is either the old or >> + * the new name pointer. The length does not really matter as >> + * the sequence number check will eventually catch any ongoing >> + * rename operation. >> + */ >> + const char *dname = ACCESS_ONCE(name->name); >> + int dlen = name->len; >> + >> + if (unlikely(!dname || !dlen)) >> + return -EINVAL; >> + return prepend(buffer, buflen, dname, dlen); > NAK. A race with d_move() can very well leave you with dname pointing into > an object of length smaller than dlen. You *can* copy it byte-by-byte > and rely on NUL-termination, but you can't rely on length being accurate - > not without having excluded d_move(). I have thought about that. But if a d_move() is going on, the string in the buffer will be discarded as the sequence number will change. So whether or not it have embedded null byte shouldn't matter. That is why I didn't add code to do byte-by-byte copy at this first patch. I can add code to do that if you think it is safer to do so. Regards, Longman