ovl: fix redirect traversal on metacopy dentries

Amir pointed me to metacopy test cases in unionmount-testsuite and I
decided to run "./run --ov=10 --meta" and it failed while running test
"rename-mass-5.py".

Problem is w.r.t absolute redirect traversal on intermediate metacopy
dentry.  We do not store intermediate metacopy dentries and also skip
current loop/layer and move onto lookup in next layer.  But at the end of
loop, we have logic to reset "poe" and layer index if currnently looked up
dentry has absolute redirect.  We skip all that and that means lookup in
next layer will fail.

Following is simple test case to reproduce this.

- mkdir -p lower upper work merged lower/a lower/b
- touch lower/a/foo.txt
- mount -t overlay -o lowerdir=lower,upperdir=upper,workdir=work,metacopy=on none merged

# Following will create absolute redirect "/a/foo.txt" on upper/b/bar.txt.
- mv merged/a/foo.txt merged/b/bar.txt

# unmount overlay and use upper as lower layer (lower2) for next mount.
- umount merged
- mv upper lower2
- rm -rf work; mkdir -p upper work
- mount -t overlay -o lowerdir=lower2:lower,upperdir=upper,workdir=work,metacopy=on none merged

# Force a metacopy copy-up
- chown bin:bin merged/b/bar.txt

# unmount overlay and use upper as lower layer (lower3) for next mount.
- umount merged
- mv upper lower3
- rm -rf work; mkdir -p upper work
- mount -t overlay -o lowerdir=lower3:lower2:lower,upperdir=upper,workdir=work,metacopy=on none merged

# ls merged/b/bar.txt
ls: cannot access 'bar.txt': Input/output error

Intermediate lower layer (lower2) has metacopy dentry b/bar.txt with
absolute redirect "/a/foo.txt".  We skipped redirect processing at the end
of loop which sets poe to roe and sets the appropriate next lower layer
index.  And that means lookup failed in next layer.

Fix this by continuing the loop for any intermediate dentries.  We still do
not save these at lower stack.  With this fix applied unionmount-testsuite,
"./run --ov-10 --meta" now passes.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/namei.c

@@ -901,15 +901,6 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 			goto out_put;
 		}
 
-		/*
-		 * Do not store intermediate metacopy dentries in chain,
-		 * except top most lower metacopy dentry
-		 */
-		if (d.metacopy && ctr) {
-			dput(this);
-			continue;
-		}
-
 		/*
 		 * If no origin fh is stored in upper of a merge dir, store fh
 		 * of lower dir and set upper parent "impure".
@@ -944,9 +935,20 @@ struct dentry *ovl_lookup(struct inode *dir, struct dentry *dentry,
 			origin = this;
 		}
 
-		stack[ctr].dentry = this;
-		stack[ctr].layer = lower.layer;
-		ctr++;
+		if (d.metacopy && ctr) {
+			/*
+			 * Do not store intermediate metacopy dentries in
+			 * lower chain, except top most lower metacopy dentry.
+			 * Continue the loop so that if there is an absolute
+			 * redirect on this dentry, poe can be reset to roe.
+			 */
+			dput(this);
+			this = NULL;
+		} else {
+			stack[ctr].dentry = this;
+			stack[ctr].layer = lower.layer;
+			ctr++;
+		}
 
 		/*
 		 * Following redirects can have security consequences: it's like