From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752495AbZBVSYp (ORCPT ); Sun, 22 Feb 2009 13:24:45 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751295AbZBVSYg (ORCPT ); Sun, 22 Feb 2009 13:24:36 -0500 Received: from e2.ny.us.ibm.com ([32.97.182.142]:45258 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751356AbZBVSYf (ORCPT ); Sun, 22 Feb 2009 13:24:35 -0500 Date: Sun, 22 Feb 2009 10:24:38 -0800 From: "Paul E. McKenney" To: etienne Cc: Tetsuo Handa , linux-security-module@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH][SMACK] convert smack rule list to linux list Message-ID: <20090222182438.GG6860@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <49A17D90.1080905@numericable.fr> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <49A17D90.1080905@numericable.fr> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Feb 22, 2009 at 05:30:08PM +0100, etienne wrote: > Paul E. McKenney wrote: > > On Sun, Feb 22, 2009 at 10:13:49PM +0900, Tetsuo Handa wrote: > >> Paul, would you review this locking? > >> > >>> static DEFINE_MUTEX(smack_known_lock); > >>> > >>> /** > >>> * smk_import_entry - import a label, return the list entry > >>> * @string: a text string that might be a Smack label > >>> * @len: the maximum size, or zero if it is NULL terminated. > >>> * > >>> * Returns a pointer to the entry in the label list that > >>> * matches the passed string, adding it if necessary. > >>> */ > >>> struct smack_known *smk_import_entry(const char *string, int len) > >>> { > >>> struct smack_known *skp; > >>> char smack[SMK_LABELLEN]; > >>> int found; > >>> int i; > >>> > >>> if (len <= 0 || len > SMK_MAXLEN) > >>> len = SMK_MAXLEN; > >>> > >>> for (i = 0, found = 0; i < SMK_LABELLEN; i++) { > >>> if (found) > >>> smack[i] = '\0'; > >>> else if (i >= len || string[i] > '~' || string[i] <= ' ' || > >>> string[i] == '/') { > >>> smack[i] = '\0'; > >>> found = 1; > >>> } else > >>> smack[i] = string[i]; > >>> } > >>> > >>> if (smack[0] == '\0') > >>> return NULL; > >>> > >>> mutex_lock(&smack_known_lock); > >>> > >>> for (skp = smack_known; skp != NULL; skp = skp->smk_next) > >>> if (strncmp(skp->smk_known, smack, SMK_MAXLEN) == 0) > >>> break; > >>> > >>> if (skp == NULL) { > >>> skp = kzalloc(sizeof(struct smack_known), GFP_KERNEL); > >>> if (skp != NULL) { > >>> skp->smk_next = smack_known; > >>> strncpy(skp->smk_known, smack, SMK_MAXLEN); > >>> skp->smk_secid = smack_next_secid++; > >>> skp->smk_cipso = NULL; > >>> spin_lock_init(&skp->smk_cipsolock); > >>> /* > >>> * Make sure that the entry is actually > >>> * filled before putting it on the list. > >>> */ > >>> smp_mb(); > >>> smack_known = skp; > > > > If the read side is not acquiring smack_known_lock, then the above > > assignment to smack_known needs to be: > > > > rcu_assign_pointer(smack_known, skp); > > > > Otherwise, both CPU and compiler are within their rights to reorder > > the assignment to smack_known ahead of the initialization code. > > > > Alternatively, if you make this list use a standard struct list_head, > > you could just use list_add_rcu(). > > > that's what i was going to do ;) > > > >>> } > >>> } > >>> > >>> mutex_unlock(&smack_known_lock); > >>> > >>> return skp; > >>> } > >>> > >>> /** > >>> * smack_from_secid - find the Smack label associated with a secid > >>> * @secid: an integer that might be associated with a Smack label > >>> * > >>> * Returns a pointer to the appropraite Smack label if there is one, > >>> * otherwise a pointer to the invalid Smack label. > >>> */ > >>> char *smack_from_secid(const u32 secid) > >>> { > >>> struct smack_known *skp; > >>> > >>> for (skp = smack_known; skp != NULL; skp = skp->smk_next) > >>> if (skp->smk_secid == secid) > >>> return skp->smk_known; > >>> > >>> /* > >>> * If we got this far someone asked for the translation > >>> * of a secid that is not on the list. > >>> */ > >>> return smack_known_invalid.smk_known; > >>> } > >> I think this is a case called "dependency ordering". > >> This function needs rcu_dereference(), doesn't it? > > > > Indeed! The "for" loop needs to be: > > > > for (skp = rcu_dereference(smack_known); skp != NULL; skp = rcu_dereference(skp->smk_next)) > > > > Alternatively, if you switch to struct list_head, you could use > > list_for_each_entry_rcu(). > > > > There also need to be rcu_read_lock() and rcu_read_unlock() in here > > somewhere. Where they must be depends on how (or whether) you are > > ever removing any elements. If the string referenced by smk_known > > gets freed up, then the caller will need to surround the call to > > smack_from_secid() and the use of the return value with rcu_read_lock() > > and rcu_read_unlock(). Otherwise, only the smack_known structures are > > ever freed up, then just the "for" loop above needs to be so protected. > > > > If these structure are never freed, then please add a comment. > > > > for the time being there are not freed; but if think it's safer to add the > "rcu_read_lock()/rcu_read_unlock()" anyway (in case someone want to implement a del in the future) > I don't think they are any downside? The overhead of rcu_read_lock() and rcu_read_unlock() is quite low, and they are immune from deadlock (aside from doing something blatantly illegal like putting a synchronize_rcu() under an rcu_read_lock()). So the downside is quite small. > thanks for the explanations! NP, hope it works well. Thanx, Paul