|From:||Al Viro <viro-AT-ZenIV.linux.org.uk>|
|To:||Linus Torvalds <torvalds-AT-linux-foundation.org>|
|Subject:||Re: [PATCH] vfs: fix race in rcu lookup of pruned dentry|
|Date:||Mon, 18 Jul 2011 00:16:10 +0100|
|Cc:||Hugh Dickins <hughd-AT-google.com>, Andrew Morton <akpm-AT-linux-foundation.org>, Nick Piggin <npiggin-AT-kernel.dk>, linux-kernel-AT-vger.kernel.org, linux-fsdevel-AT-vger.kernel.org|
On Sun, Jul 17, 2011 at 03:00:06PM -0700, Linus Torvalds wrote: > Yes. However, looking at it, I'm not very happy with your patch. It > doesn't really make sense to me to special-case the NULL inode and > only do a seq_retry for that case. > > I kind of see why you do it for that particular bug, but at the same > time, it just makes me go "Eww". If that inode isn't NULL yet, you > then return the dentry that can get a NULL d_inode later. So the only > special thing there is that we happen to check for a NULL inode there. > What protects *later* checks for a NULL d_inode? > > So my gut feel is that we should instead > > - either remove the -ENOENT return at that point entirely, and move > it to after we have re-verified the dentry lookup for other reasons. > That looks pretty involved, though, and those paths do end up > accessing inode data structures etc, so it looks less than trivial. > > OR > > - simply just not clear d_inode at all in d_kill(), so that when we > prune a dentry due to memory pressure, it doesn't actually change the > state of the dentry. OR - keep part of the patch from Hugh, treating negative in RCU mode as "need to unlazy". Leaving everything else as-is. Normally we'll confirm that sucker is negative and that'll be it. Or we'll see that it's being evicted and fall back to non-lazy mode, which is what we'll need to do anyway. IOW, this: diff --git a/fs/namei.c b/fs/namei.c index 5c867dd..48a38de 100644 --- a/fs/namei.c +++ b/fs/namei.c @@ -1171,6 +1171,8 @@ static int do_lookup(struct nameidata *nd, struct qstr *name, path->dentry = dentry; if (unlikely(!__follow_mount_rcu(nd, path, inode))) goto unlazy; + if (unlikely(!*inode)) + goto unlazy; if (unlikely(path->dentry->d_flags & DCACHE_NEED_AUTOMOUNT)) goto unlazy; return 0;
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