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>

1 files changed, 28 insertions, 7 deletions

diff --git a/fs/super.c b/fs/super.c
index da223b4cfbca..beaf076d9733 100644
--- 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),