Re: [PATCH RFC] NFSD: fix cannot umounting mount points under pseudo root

[NeilBrown <neilb@suse.de>]

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On Fri, 1 May 2015 09:29:53 -0400 "J. Bruce Fields" <bfields@fieldses.org>
wrote:

> On Fri, May 01, 2015 at 01:08:26PM +1000, NeilBrown wrote:
> > On Fri, 1 May 2015 03:29:40 +0100 Al Viro <viro@ZenIV.linux.org.uk> wrote:
> > 
> > > On Fri, May 01, 2015 at 12:23:33PM +1000, NeilBrown wrote:
> > > > > What kind of consistency warranties do callers expect, BTW?  You do realize
> > > > > that between iterate_dir() and callbacks an entry might have been removed
> > > > > and/or replaced?
> > > > 
> > > > For READDIR_PLUS, lookup_one_len is called on each name and it requires
> > > > i_mutex, so the code currently holds i_mutex over the whole sequence.
> > > > This is triggering a deadlock.
> > > 
> > > Yes, I've seen the context.  However, you are _not_ holding it between
> > > actual iterate_dir() and those callbacks, which opens a window when
> > > directory might have been changed.
> > > 
> > > Again, what kind of consistency is expected by callers?  Are they ready to
> > > cope with "there's no such entry anymore" or "inumber is nothing like
> > > what we'd put in ->ino, since it's no the same object" or "->d_type is
> > > completely unrelated to what we'd found, since the damn thing had been
> > > removed and created from scratch"?
> > 
> > Ah, sorry.
> > 
> > Yes, the callers are prepared for "there's no such entry anymore".
> > They don't use d_type, so don't care if it might be meaningless.
> > NFSv4 doesn't use ino either, but NFSv3 does and isn't properly cautious
> > about ino changing.
> > 
> > In nfs3xdr, we should probably pass 'ino' to encode_entryplus_baggage() and
> > thence to compose_entry_fh() and it should report failure if
> > dchild->d_inode->i_ino doesn't match.
> 
> Just to make sure I understand the concern..... So it shouldn't really
> be a problem if readdir and lookup find different objects for the same
> name, the problem is just when we mix attributes from the two objects,
> right?  Looks like the v3 code could return an inode number derived from
> the readdir and a filehandle from the lookup, which is a problem.  The
> v4 code will get everything from the result of the lookup, which should
> be OK.

That agrees with my understanding, yes.

I did wonder for a little while about the possibility of a directory
containing both 'a' and 'b', and NFSv4 doing the readdir and the stat of 'a',
and the a "mv a b" happening before the stat of 'b'.

Then the readdir response will show both 'a' and 'b' referring to the same
object with a link count of 1.

I can't quite decide if that is a problem or not.


> 
> > Simply not returning the extra attributes is perfectly acceptable in NFSv3.
> 
> Right, so no big deal anyway.--b.

Not a big deal, but we should really add a patch like the following ("like"
as in "actually compile tested and documented" which this one isn't).

NeilBrown

> 
> > So it looks like we are mostly OK here - we don't really need i_mutex to be
> > held for very long.
> > 
> > NeilBrown
> > 
> 

diff --git a/fs/nfsd/nfs3xdr.c b/fs/nfsd/nfs3xdr.c
index e4b2b4322553..f6e7cbabac5a 100644
--- a/fs/nfsd/nfs3xdr.c
+++ b/fs/nfsd/nfs3xdr.c
@@ -805,7 +805,7 @@ encode_entry_baggage(struct nfsd3_readdirres *cd, __be32 *p, const char *name,
 
 static __be32
 compose_entry_fh(struct nfsd3_readdirres *cd, struct svc_fh *fhp,
-		const char *name, int namlen)
+		 const char *name, int namlen, u64 ino)
 {
 	struct svc_export	*exp;
 	struct dentry		*dparent, *dchild;
@@ -830,19 +830,21 @@ compose_entry_fh(struct nfsd3_readdirres *cd, struct svc_fh *fhp,
 		goto out;
 	if (d_really_is_negative(dchild))
 		goto out;
+	if (dchild->d_inode->i_ino != ino)
+		goto out;
 	rv = fh_compose(fhp, exp, dchild, &cd->fh);
 out:
 	dput(dchild);
 	return rv;
 }
 
-static __be32 *encode_entryplus_baggage(struct nfsd3_readdirres *cd, __be32 *p, const char *name, int namlen)
+static __be32 *encode_entryplus_baggage(struct nfsd3_readdirres *cd, __be32 *p, const char *name, int namlen, u64 ino)
 {
 	struct svc_fh	*fh = &cd->scratch;
 	__be32 err;
 
 	fh_init(fh, NFS3_FHSIZE);
-	err = compose_entry_fh(cd, fh, name, namlen);
+	err = compose_entry_fh(cd, fh, name, namlen, ino);
 	if (err) {
 		*p++ = 0;
 		*p++ = 0;
@@ -927,7 +929,7 @@ encode_entry(struct readdir_cd *ccd, const char *name, int namlen,
 		p = encode_entry_baggage(cd, p, name, namlen, ino);
 
 		if (plus)
-			p = encode_entryplus_baggage(cd, p, name, namlen);
+			p = encode_entryplus_baggage(cd, p, name, namlen, ino);
 		num_entry_words = p - cd->buffer;
 	} else if (*(page+1) != NULL) {
 		/* temporarily encode entry into next page, then move back to
@@ -941,7 +943,7 @@ encode_entry(struct readdir_cd *ccd, const char *name, int namlen,
 		p1 = encode_entry_baggage(cd, p1, name, namlen, ino);
 
 		if (plus)
-			p1 = encode_entryplus_baggage(cd, p1, name, namlen);
+			p1 = encode_entryplus_baggage(cd, p1, name, namlen, ino);
 
 		/* determine entry word length and lengths to go in pages */
 		num_entry_words = p1 - tmp;

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