vfs: Add a single-or-reconfig keying to vfs_get_super()

Add an additional keying mode to vfs_get_super() to indicate that only a
single superblock should exist in the system, and that, if it does, further
mounts should invoke reconfiguration upon it.

This allows mount_single() to be replaced.

[Fix by Eric Biggers folded in]

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/super.c b/fs/super.c
index da223b4..beaf076 100644 (file)
--- a/fs/super.c
+++ b/fs/super.c
@@ -1160,9 +1160,11 @@ int vfs_get_super(struct fs_context *fc,
 {
        int (*test)(struct super_block *, struct fs_context *);
        struct super_block *sb;
+       int err;
 
        switch (keying) {
        case vfs_get_single_super:
+       case vfs_get_single_reconf_super:
                test = test_single_super;
                break;
        case vfs_get_keyed_super:
@@ -1180,18 +1182,29 @@ int vfs_get_super(struct fs_context *fc,
                return PTR_ERR(sb);
 
        if (!sb->s_root) {
-               int err = fill_super(sb, fc);
-               if (err) {
-                       deactivate_locked_super(sb);
-                       return err;
-               }
+               err = fill_super(sb, fc);
+               if (err)
+                       goto error;
 
                sb->s_flags |= SB_ACTIVE;
+               fc->root = dget(sb->s_root);
+       } else {
+               fc->root = dget(sb->s_root);
+               if (keying == vfs_get_single_reconf_super) {
+                       err = reconfigure_super(fc);
+                       if (err < 0) {
+                               dput(fc->root);
+                               fc->root = NULL;
+                               goto error;
+                       }
+               }
        }
 
-       BUG_ON(fc->root);
-       fc->root = dget(sb->s_root);
        return 0;
+
+error:
+       deactivate_locked_super(sb);
+       return err;
 }
 EXPORT_SYMBOL(vfs_get_super);
 
@@ -1211,6 +1224,14 @@ int get_tree_single(struct fs_context *fc,
 }
 EXPORT_SYMBOL(get_tree_single);
 
+int get_tree_single_reconf(struct fs_context *fc,
+                 int (*fill_super)(struct super_block *sb,
+                                   struct fs_context *fc))
+{
+       return vfs_get_super(fc, vfs_get_single_reconf_super, fill_super);
+}
+EXPORT_SYMBOL(get_tree_single_reconf);
+
 int get_tree_keyed(struct fs_context *fc,
                  int (*fill_super)(struct super_block *sb,
                                    struct fs_context *fc),

diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index 84a5eaa..0424df7 100644 (file)
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -141,6 +141,7 @@ extern void put_fs_context(struct fs_context *fc);
  */
 enum vfs_get_super_keying {
        vfs_get_single_super,   /* Only one such superblock may exist */
+       vfs_get_single_reconf_super, /* As above, but reconfigure if it exists */
        vfs_get_keyed_super,    /* Superblocks with different s_fs_info keys may exist */
        vfs_get_independent_super, /* Multiple independent superblocks may exist */
 };
@@ -155,6 +156,9 @@ extern int get_tree_nodev(struct fs_context *fc,
 extern int get_tree_single(struct fs_context *fc,
                         int (*fill_super)(struct super_block *sb,
                                           struct fs_context *fc));
+extern int get_tree_single_reconf(struct fs_context *fc,
+                        int (*fill_super)(struct super_block *sb,
+                                          struct fs_context *fc));
 extern int get_tree_keyed(struct fs_context *fc,
                         int (*fill_super)(struct super_block *sb,
                                           struct fs_context *fc),