On Thu, 2011-12-01 at 02:12 +0000, Al Viro wrote:
> On Thu, Dec 01, 2011 at 12:49:22PM +1100, NeilBrown wrote:
>
> > If the path was "/some/long/path/.", then the final component ("path" in
> > this case) has already been revalidated and there is no particular
> > need to do it again.
> >
> > If we change nd->last_type to refer to "the last component looked at"
> > rather than just "the last component", then these cases can be
> > detected by "nd->last_type != LAST_NORM".
>
> This is just plain wrong. Let's *not* bring more dependencies on
> nameidata into ->d_revalidate(). The goal is to get rid of it there...
>
> FWIW, if you want a really nasty bug in that area, consider this:
>
> mkdir /tmp/a
> mkdir /tmp/b
> echo "local file" >/tmp/x
> mount -t nfs4 $SOMETHING /tmp/a
> mount -t nfs4 $SOMETHING /tmp/b
> echo "NFS file" >/tmp/a/x
> mount --bind /tmp/x /tmp/a/x
>
> now try opening /tmp/b/x. And watch the NFS traffic; there won't be OPEN
> request for x on server. Why? Because NFS sees that x is a mountpoint in
> *some* instance of that filesystem. And decides that opening it would be
> wrong. And so it would, if we were asked to open /tmp/a/x. Alas, in this
> case, while dentry is the same, it does *not* have anything mounted on it.
> What we get is ->d_revalidate() returning without issuing OPEN and ->open()
> being called - again, without issuing OPEN, since it assumes that ->lookup()
> or ->d_revalidate() had done it for us.
>
> Plain IO on resulting descriptor will work and work correcly (you'll get
> "NFS file\n" read from it), but try to do F_SETLK on it and it'll fail
> since that requires the server to have seen an OPEN.
We can possibly fix this for the NFSv4.1 case since that adds support
for open-by-filehandle. However, I agree that NFSv4.0 is unfixable: all
OPENs are required to do the equivalent of a lookup, which isn't
possible in the bind mount case.
> As far as I can tell, the idea of open done in ->d_revalidate() is
> unsalvagable. It's simply the wrong place for that. Note that NFS
> is the only filesystem trying to do atomic open stuff in its ->d_revalidate()
> and it's not succeeding.
Not doing an open there is prohibitively expensive, though: you are
likely to see your cached inode flushed down the toilet if you just drop
the dentry...
--
Trond Myklebust
Linux NFS client maintainer
NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com
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