Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753465AbbDVSII (ORCPT ); Wed, 22 Apr 2015 14:08:08 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:44681 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751909AbbDVSIF (ORCPT ); Wed, 22 Apr 2015 14:08:05 -0400 Date: Wed, 22 Apr 2015 19:07:59 +0100 From: Al Viro To: NeilBrown Cc: Christoph Hellwig , Linus Torvalds , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [RFC][PATCHSET] non-recursive link_path_walk() and reducing stack footprint Message-ID: <20150422180702.GA15209@ZenIV.linux.org.uk> References: <20150420181222.GK889@ZenIV.linux.org.uk> <20150421144959.GR889@ZenIV.linux.org.uk> <20150421150408.GA29838@infradead.org> <553668C1.8030707@nod.at> <20150421154504.GT889@ZenIV.linux.org.uk> <20150421212007.GU889@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150421212007.GU889@ZenIV.linux.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1821 Lines: 36 On Tue, Apr 21, 2015 at 10:20:07PM +0100, Al Viro wrote: > I agree that unlazy_walk() attempted when walking a symlink ought to fail > with -ECHILD; we can't legitimize the symlink itself, so once we are out > of RCU mode, there's nothing to hold the inode of symlink (and its body) > from getting freed. Solution is wrong though; for example, when > nested symlink occurs in the middle of a trailing one, we should *not* > remove the flag upon leaving the nested symlink. > > Another unpleasant thing is that ->follow_link() saying "can't do that in > RCU mode" ends up with restart from scratch - that actually risks to be > worse than the mainline; there we would attempt unlazy_walk() and normally > it would've succeed. > > AFAICS, the real rule is "can't unlazy if nd->last.name points into a symlink > body and we might still need to access it"... And one more: may_follow_link() is now potentially oopsable. Look: suppose we've reached the link in RCU mode, just as it got unlinked. link->dentry has become negative and may_follow_link() steps into /* Allowed if owner and follower match. */ inode = link->dentry->d_inode; if (uid_eq(current_cred()->fsuid, inode->i_uid)) return 0; Oops... Incidentally, I suspect that your __read_seqcount_retry() in follow_link() might be lacking a barrier; why isn't full read_seqcount_retry() needed? FWIW, I would rather fetch ->d_inode *and* checked ->seq proir to calling get_link(), and passed inode to it as an explicit argument. And passed it to may_follow_link() as well... -- 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/