[patch 2/3] vfs: fix sys_getcwd for detached mounts

[Miklos Szeredi]

From: Miklos Szeredi <mszeredi@xxxxxxx>

Currently getcwd(2) on a detached mount will give a garbled result:

   > mkdir /mnt/foo
   > mount --bind /etc /mnt/foo
   > cd /mnt/foo/skel
   > /bin/pwd
   /mnt/foo/skel
   > umount -l /mnt/foo
   > /bin/pwd
   etcskel

After the patch it will give a much saner "/skel" result.

Thanks to John Johansen for pointing out this bug.

Reported-by: John Johansen <jjohansen@xxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
Acked-by: Christoph Hellwig <hch@xxxxxxxxxxxxx>
---
 fs/dcache.c |   18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c	2008-06-13 13:12:57.000000000 +0200
+++ linux-2.6/fs/dcache.c	2008-06-13 13:13:01.000000000 +0200
@@ -1825,10 +1825,20 @@ char *__d_path(const struct path *path, 
 	return retval;
 
 global_root:
-	retval += 1;	/* hit the slash */
-	name = &dentry->d_name;
-	if (prepend(&retval, &buflen, name->name, name->len) != 0)
-		goto Elong;
+	/*
+	 * If this is a root dentry, then overwrite the slash.  This
+	 * will also DTRT with pseudo filesystems which have root
+	 * dentries named "foo:".
+	 *
+	 * Otherwise this is the root of a detached mount, so don't do
+	 * anything.
+	 */
+	if (IS_ROOT(dentry)) {
+		retval += 1;
+		name = &dentry->d_name;
+		if (prepend(&retval, &buflen, name->name, name->len) != 0)
+			goto Elong;
+	}
 	root->mnt = vfsmnt;
 	root->dentry = dentry;
 	return retval;

--
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