fs/cifs: make share unaccessible at root level mountable

if, when mounting //HOST/share/sub/dir/foo we can query /sub/dir/foo but
not any of the path components above:

- store the /sub/dir/foo prefix in the cifs super_block info
- in the superblock, set root dentry to the subpath dentry (instead of
  the share root)
- set a flag in the superblock to remember it
- use prefixpath when building path from a dentry

fixes bso#8950

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
CC: Stable <stable@vger.kernel.org>
Reviewed-by: Pavel Shilovsky <pshilovsky@samba.org>
Signed-off-by: Steve French <smfrench@gmail.com>

1 files changed, 13 insertions, 1 deletions

diff --git a/fs/cifs/cifsfs.c b/fs/cifs/cifsfs.c
index 5d841f39c4b70..6bbec5e784cd4 100644
--- a/fs/cifs/cifsfs.c
+++ b/fs/cifs/cifsfs.c
@@ -689,6 +689,14 @@ cifs_do_mount(struct file_system_type *fs_type,
 		goto out_cifs_sb;
 	}
 
+	if (volume_info->prepath) {
+		cifs_sb->prepath = kstrdup(volume_info->prepath, GFP_KERNEL);
+		if (cifs_sb->prepath == NULL) {
+			root = ERR_PTR(-ENOMEM);
+			goto out_cifs_sb;
+		}
+	}
+
 	cifs_setup_cifs_sb(volume_info, cifs_sb);
 
 	rc = cifs_mount(cifs_sb, volume_info);
@@ -727,7 +735,11 @@ cifs_do_mount(struct file_system_type *fs_type,
 		sb->s_flags |= MS_ACTIVE;
 	}
 
-	root = cifs_get_root(volume_info, sb);
+	if (cifs_sb->mnt_cifs_flags & CIFS_MOUNT_USE_PREFIX_PATH)
+		root = dget(sb->s_root);
+	else
+		root = cifs_get_root(volume_info, sb);
+
 	if (IS_ERR(root))
 		goto out_super;