From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id CA7C1C43334 for ; Mon, 4 Jul 2022 02:54:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231263AbiGDCyC (ORCPT ); Sun, 3 Jul 2022 22:54:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:38834 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229473AbiGDCyA (ORCPT ); Sun, 3 Jul 2022 22:54:00 -0400 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [IPv6:2a03:a000:7:0:5054:ff:fe1c:15ff]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 87E8E2C3; Sun, 3 Jul 2022 19:53:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=vO9e4FK09OExUNkGvxoa/0O9s2rDe2YLeW0ZmG6+6Kg=; b=a6Wr3eXalqQ1x7Xzq1DJMB980z QgVgjoWDXyhE1vfAlsv77O6kJ5yU8hRobZz9zroeVNJVJ7czHKf2GMMT0kXnag3lE3S4CZrRV/uLD tB/0kbe8/lARWcluoxdeIDazoZWcbCP6K+DL0RVC8oB+0VhqVRsoOhU/Wvs96MQmvGAYZrShhZJ0Y 5uEwjJo8fghfu6nUn53dtd3+t4yH+d+XtSHXNdR6dz/2JWMhws29rzznPkLJyl4z/oSTFpSwG9Ido 8Hp5aAFLodbmqdfSDYAO9YgZn0IXcWcMyy3FiDGtc0IY2HTsyNYNNQQwYWlRDE5WSjxek13qg6TMR 6XywjS2w==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.95 #2 (Red Hat Linux)) id 1o8CCn-007r8W-Fz; Mon, 04 Jul 2022 02:52:57 +0000 Date: Mon, 4 Jul 2022 03:52:57 +0100 From: Al Viro To: Linus Torvalds Cc: Alexander Potapenko , Alexei Starovoitov , Andrew Morton , Andrey Konovalov , Andy Lutomirski , Arnd Bergmann , Borislav Petkov , Christoph Hellwig , Christoph Lameter , David Rientjes , Dmitry Vyukov , Eric Dumazet , Greg Kroah-Hartman , Herbert Xu , Ilya Leoshkevich , Ingo Molnar , Jens Axboe , Joonsoo Kim , Kees Cook , Marco Elver , Mark Rutland , Matthew Wilcox , "Michael S. Tsirkin" , Pekka Enberg , Peter Zijlstra , Petr Mladek , Steven Rostedt , Thomas Gleixner , Vasily Gorbik , Vegard Nossum , Vlastimil Babka , kasan-dev , Linux-MM , linux-arch , Linux Kernel Mailing List , Evgenii Stepanov , Nathan Chancellor , Nick Desaulniers , Segher Boessenkool , Vitaly Buka , linux-toolchains Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to step_into() Message-ID: References: <20220701142310.2188015-1-glider@google.com> <20220701142310.2188015-44-glider@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jul 02, 2022 at 10:23:16AM -0700, Linus Torvalds wrote: > Al - can you please take a quick look? FWIW, trying to write a coherent documentation had its usual effect... The thing is, we don't really need to fetch the inode that early. All we really care about is that in RCU mode ->d_seq gets sampled before we fetch ->d_inode *and* we don't treat "it looks negative" as hard -ENOENT in case of ->d_seq mismatch. Which can be bloody well left to step_into(). So we don't need to pass it inode argument at all - just dentry and seq. Makes a bunch of functions simpler as well... It does *not* deal with the "uninitialized" seq argument in !RCU case; I'll handle that in the followup, but that's a separate story, IMO (and very clearly a false positive). Cumulative diff follows; splitup is in #work.namei. Comments? diff --git a/fs/namei.c b/fs/namei.c index 1f28d3f463c3..7f4f61ade9e3 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1467,7 +1467,7 @@ EXPORT_SYMBOL(follow_down); * we meet a managed dentry that would need blocking. */ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, - struct inode **inode, unsigned *seqp) + unsigned *seqp) { struct dentry *dentry = path->dentry; unsigned int flags = dentry->d_flags; @@ -1497,13 +1497,6 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, dentry = path->dentry = mounted->mnt.mnt_root; nd->state |= ND_JUMPED; *seqp = read_seqcount_begin(&dentry->d_seq); - *inode = dentry->d_inode; - /* - * We don't need to re-check ->d_seq after this - * ->d_inode read - there will be an RCU delay - * between mount hash removal and ->mnt_root - * becoming unpinned. - */ flags = dentry->d_flags; continue; } @@ -1515,8 +1508,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path, } static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry, - struct path *path, struct inode **inode, - unsigned int *seqp) + struct path *path, unsigned int *seqp) { bool jumped; int ret; @@ -1525,9 +1517,7 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry, path->dentry = dentry; if (nd->flags & LOOKUP_RCU) { unsigned int seq = *seqp; - if (unlikely(!*inode)) - return -ENOENT; - if (likely(__follow_mount_rcu(nd, path, inode, seqp))) + if (likely(__follow_mount_rcu(nd, path, seqp))) return 0; if (!try_to_unlazy_next(nd, dentry, seq)) return -ECHILD; @@ -1547,7 +1537,6 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry, if (path->mnt != nd->path.mnt) mntput(path->mnt); } else { - *inode = d_backing_inode(path->dentry); *seqp = 0; /* out of RCU mode, so the value doesn't matter */ } return ret; @@ -1607,9 +1596,7 @@ static struct dentry *__lookup_hash(const struct qstr *name, return dentry; } -static struct dentry *lookup_fast(struct nameidata *nd, - struct inode **inode, - unsigned *seqp) +static struct dentry *lookup_fast(struct nameidata *nd, unsigned *seqp) { struct dentry *dentry, *parent = nd->path.dentry; int status = 1; @@ -1628,22 +1615,11 @@ static struct dentry *lookup_fast(struct nameidata *nd, return NULL; } - /* - * This sequence count validates that the inode matches - * the dentry name information from lookup. - */ - *inode = d_backing_inode(dentry); - if (unlikely(read_seqcount_retry(&dentry->d_seq, seq))) - return ERR_PTR(-ECHILD); - - /* + /* * This sequence count validates that the parent had no * changes while we did the lookup of the dentry above. - * - * The memory barrier in read_seqcount_begin of child is - * enough, we can use __read_seqcount_retry here. */ - if (unlikely(__read_seqcount_retry(&parent->d_seq, nd->seq))) + if (unlikely(read_seqcount_retry(&parent->d_seq, nd->seq))) return ERR_PTR(-ECHILD); *seqp = seq; @@ -1838,13 +1814,21 @@ static const char *pick_link(struct nameidata *nd, struct path *link, * for the common case. */ static const char *step_into(struct nameidata *nd, int flags, - struct dentry *dentry, struct inode *inode, unsigned seq) + struct dentry *dentry, unsigned seq) { struct path path; - int err = handle_mounts(nd, dentry, &path, &inode, &seq); + struct inode *inode; + int err = handle_mounts(nd, dentry, &path, &seq); if (err < 0) return ERR_PTR(err); + inode = path.dentry->d_inode; + if (unlikely(!inode)) { + if ((nd->flags & LOOKUP_RCU) && + read_seqcount_retry(&path.dentry->d_seq, seq)) + return ERR_PTR(-ECHILD); + return ERR_PTR(-ENOENT); + } if (likely(!d_is_symlink(path.dentry)) || ((flags & WALK_TRAILING) && !(nd->flags & LOOKUP_FOLLOW)) || (flags & WALK_NOFOLLOW)) { @@ -1870,9 +1854,7 @@ static const char *step_into(struct nameidata *nd, int flags, return pick_link(nd, &path, inode, seq, flags); } -static struct dentry *follow_dotdot_rcu(struct nameidata *nd, - struct inode **inodep, - unsigned *seqp) +static struct dentry *follow_dotdot_rcu(struct nameidata *nd, unsigned *seqp) { struct dentry *parent, *old; @@ -1895,7 +1877,6 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd, } old = nd->path.dentry; parent = old->d_parent; - *inodep = parent->d_inode; *seqp = read_seqcount_begin(&parent->d_seq); if (unlikely(read_seqcount_retry(&old->d_seq, nd->seq))) return ERR_PTR(-ECHILD); @@ -1910,9 +1891,7 @@ static struct dentry *follow_dotdot_rcu(struct nameidata *nd, return NULL; } -static struct dentry *follow_dotdot(struct nameidata *nd, - struct inode **inodep, - unsigned *seqp) +static struct dentry *follow_dotdot(struct nameidata *nd, unsigned *seqp) { struct dentry *parent; @@ -1937,7 +1916,6 @@ static struct dentry *follow_dotdot(struct nameidata *nd, return ERR_PTR(-ENOENT); } *seqp = 0; - *inodep = parent->d_inode; return parent; in_root: @@ -1952,7 +1930,6 @@ static const char *handle_dots(struct nameidata *nd, int type) if (type == LAST_DOTDOT) { const char *error = NULL; struct dentry *parent; - struct inode *inode; unsigned seq; if (!nd->root.mnt) { @@ -1961,17 +1938,17 @@ static const char *handle_dots(struct nameidata *nd, int type) return error; } if (nd->flags & LOOKUP_RCU) - parent = follow_dotdot_rcu(nd, &inode, &seq); + parent = follow_dotdot_rcu(nd, &seq); else - parent = follow_dotdot(nd, &inode, &seq); + parent = follow_dotdot(nd, &seq); if (IS_ERR(parent)) return ERR_CAST(parent); if (unlikely(!parent)) error = step_into(nd, WALK_NOFOLLOW, - nd->path.dentry, nd->inode, nd->seq); + nd->path.dentry, nd->seq); else error = step_into(nd, WALK_NOFOLLOW, - parent, inode, seq); + parent, seq); if (unlikely(error)) return error; @@ -1995,7 +1972,6 @@ static const char *handle_dots(struct nameidata *nd, int type) static const char *walk_component(struct nameidata *nd, int flags) { struct dentry *dentry; - struct inode *inode; unsigned seq; /* * "." and ".." are special - ".." especially so because it has @@ -2007,7 +1983,7 @@ static const char *walk_component(struct nameidata *nd, int flags) put_link(nd); return handle_dots(nd, nd->last_type); } - dentry = lookup_fast(nd, &inode, &seq); + dentry = lookup_fast(nd, &seq); if (IS_ERR(dentry)) return ERR_CAST(dentry); if (unlikely(!dentry)) { @@ -2017,7 +1993,7 @@ static const char *walk_component(struct nameidata *nd, int flags) } if (!(flags & WALK_MORE) && nd->depth) put_link(nd); - return step_into(nd, flags, dentry, inode, seq); + return step_into(nd, flags, dentry, seq); } /* @@ -2473,8 +2449,7 @@ static int handle_lookup_down(struct nameidata *nd) { if (!(nd->flags & LOOKUP_RCU)) dget(nd->path.dentry); - return PTR_ERR(step_into(nd, WALK_NOFOLLOW, - nd->path.dentry, nd->inode, nd->seq)); + return PTR_ERR(step_into(nd, WALK_NOFOLLOW, nd->path.dentry, nd->seq)); } /* Returns 0 and nd will be valid on success; Retuns error, otherwise. */ @@ -3394,7 +3369,6 @@ static const char *open_last_lookups(struct nameidata *nd, int open_flag = op->open_flag; bool got_write = false; unsigned seq; - struct inode *inode; struct dentry *dentry; const char *res; @@ -3410,7 +3384,7 @@ static const char *open_last_lookups(struct nameidata *nd, if (nd->last.name[nd->last.len]) nd->flags |= LOOKUP_FOLLOW | LOOKUP_DIRECTORY; /* we _can_ be in RCU mode here */ - dentry = lookup_fast(nd, &inode, &seq); + dentry = lookup_fast(nd, &seq); if (IS_ERR(dentry)) return ERR_CAST(dentry); if (likely(dentry)) @@ -3464,7 +3438,7 @@ static const char *open_last_lookups(struct nameidata *nd, finish_lookup: if (nd->depth) put_link(nd); - res = step_into(nd, WALK_TRAILING, dentry, inode, seq); + res = step_into(nd, WALK_TRAILING, dentry, seq); if (unlikely(res)) nd->flags &= ~(LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL); return res;