Btrfs: Add readonly snapshots support

Usage:

Set BTRFS_SUBVOL_RDONLY of btrfs_ioctl_vol_arg_v2->flags, and call
ioctl(BTRFS_I0CTL_SNAP_CREATE_V2).

Implementation:

- Set readonly bit of btrfs_root_item->flags.
- Add readonly checks in btrfs_permission (inode_permission),
btrfs_setattr, btrfs_set/remove_xattr and some ioctls.

Changelog for v3:

- Eliminate btrfs_root->readonly, but check btrfs_root->root_item.flags.
- Rename BTRFS_ROOT_SNAP_RDONLY to BTRFS_ROOT_SUBVOL_RDONLY.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/fs/btrfs/xattr.c b/fs/btrfs/xattr.c
index 698fdd2c739c..a5776531dc2b 100644
--- a/fs/btrfs/xattr.c
+++ b/fs/btrfs/xattr.c
@@ -316,6 +316,15 @@ ssize_t btrfs_getxattr(struct dentry *dentry, const char *name,
 int btrfs_setxattr(struct dentry *dentry, const char *name, const void *value,
                    size_t size, int flags)
 {
+        struct btrfs_root *root = BTRFS_I(dentry->d_inode)->root;
+
+        /*
+         * The permission on security.* and system.* is not checked
+         * in permission().
+         */
+        if (btrfs_root_readonly(root))
+                return -EROFS;
+
         /*
          * If this is a request for a synthetic attribute in the system.*
          * namespace use the generic infrastructure to resolve a handler
@@ -336,6 +345,15 @@ int btrfs_setxattr(struct dentry *dentry, const char *name, const void *value,
 
 int btrfs_removexattr(struct dentry *dentry, const char *name)
 {
+        struct btrfs_root *root = BTRFS_I(dentry->d_inode)->root;
+
+        /*
+         * The permission on security.* and system.* is not checked
+         * in permission().
+         */
+        if (btrfs_root_readonly(root))
+                return -EROFS;
+
         /*
          * If this is a request for a synthetic attribute in the system.*
          * namespace use the generic infrastructure to resolve a handler