John Johansen pointed out, that getcwd(2) will give a garbled result if a

bind mount of a non-filesystem-root directory is detached:

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

If it was the root of the filesystem which was detached, it will give a
saner looking result, but it still won't be a valid absolute path by which
the CWD can be reached (assuming the process's root is not also on the
detached mount).

A similar issue happens if the CWD is outside the process's root or in a
different namespace.  These problems are relevant to symlinks under
/proc/<pid>/ and /proc/<pid>/fd/ as well.

This patch addresses all these issues, by prefixing such unreachable paths
with "(unreachable)".  This isn't perfect since the returned path may
still be a valid _relative_ path, and applications may not check the
result of getcwd() for starting with a '/' before using it.

For this reason Andreas Gruenbacher thinks getcwd(2) should return ENOENT
in these cases, but that breaks /bin/pwd and bash in the above cases.
Reported-by: John Johansen <jjohansen@suse.de>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Andreas Gruenbacher <agruen@suse.de>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/dcache.c

@@ -1883,6 +1883,12 @@ static int prepend_name(char **buffer, int *buflen, struct qstr *name)
 	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
@@ -1950,8 +1956,18 @@ out:
 
 global_root:
 	retval += 1;	/* hit the slash */
-	if (prepend_name(&retval, &buflen, &dentry->d_name) != 0)
-		goto Elong;
+
+	if (is_pseudo_root(dentry)) {
+		/* Pseudo filesystem with "foo:" prefix */
+		if (prepend_name(&retval, &buflen, &dentry->d_name) != 0)
+			goto Elong;
+	} else {
+		/*
+		 * Unreachable (detached or outside root or outside namespace)
+		 */
+		if (prepend(&retval, &buflen, "(unreachable)/", 14) != 0)
+			goto Elong;
+	}
 	root->mnt = vfsmnt;
 	root->dentry = dentry;
 	goto out;