[patch v3] vfs: fix sys_getcwd for detached mounts

[Miklos Szeredi]

I tested this one in various situations, and it appears to do what
it's supposed to.

A somewhat analogous issue is if we have cwd (or fd) outside
current->root.  Should we add a prefix for that case as well?  Perhaps
a double slash?

--
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
   > umount -l /mnt/foo
   > /bin/pwd
   etcskel

After the patch the result will be "detached:skel".

Or if the root was mounted:

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

After the patch the result will be "detached:etc".

Thanks to John Johansen for pointing out this bug.

v2: comment from Al Viro: use "detached:" prefix to differentiate it from
attached paths.

v3: fix case when mounted dentry is root of the filesystem.

Reported-by: John Johansen <jjohansen@xxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
---
 fs/dcache.c |   18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

Index: linux-2.6/fs/dcache.c
===================================================================
--- linux-2.6.orig/fs/dcache.c	2008-06-23 19:34:21.000000000 +0200
+++ linux-2.6/fs/dcache.c	2008-06-23 20:15:09.000000000 +0200
@@ -1762,6 +1762,12 @@ static int prepend_name(char **buffer, i
 	return prepend(buffer, buflen, name->name, name->len);
 }
 
+static bool is_pseudo_root(struct dentry *dentry)
+{
+	return IS_ROOT(dentry) &&
+		(dentry->d_name.len != 1 || dentry->d_name.name[0] != '/');
+}
+
 /**
  * __d_path - return the path of a dentry
  * @path: the dentry/vfsmount to report
@@ -1828,8 +1834,16 @@ char *__d_path(const struct path *path, 
 
 global_root:
 	retval += 1;	/* hit the slash */
-	if (prepend_name(&retval, &buflen, &dentry->d_name) != 0)
-		goto Elong;
+
+	if (vfsmnt->mnt_ns != NULL || is_pseudo_root(dentry)) {
+		/* Attached, or pseudo filesystem with "foo:" prefix */
+		if (prepend_name(&retval, &buflen, &dentry->d_name) != 0)
+			goto Elong;
+	} else {
+		/* Detached */
+		if (prepend(&retval, &buflen, "detached:", 9) != 0)
+			goto Elong;
+	}
 	root->mnt = vfsmnt;
 	root->dentry = dentry;
 	return retval;
--
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