nilfs2: deny write access to inodes in snapshots

Snapshots of nilfs are read-only.

After super block instances (sb) will be unified, nilfs will need to
check write access by a way other than implicit test with
IS_RDONLY(inode).  This is because IS_RDONLY() refers to MS_RDONLY bit
of inode->i_sb->s_flags and it will become inaccurate after the
unification of sb.

To prepare for the issue, this uses i_op->permission to deny write
access to inodes in snapshots.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

1 files changed, 11 insertions, 0 deletions

diff --git a/fs/nilfs2/inode.c b/fs/nilfs2/inode.c
index ca09e4362d66..3efef0ecfa24 100644
--- a/fs/nilfs2/inode.c
+++ b/fs/nilfs2/inode.c
@@ -764,6 +764,17 @@ out_err:
         return err;
 }
 
+int nilfs_permission(struct inode *inode, int mask)
+{
+        struct nilfs_root *root = NILFS_I(inode)->i_root;
+
+        if ((mask & MAY_WRITE) && root &&
+            root->cno != NILFS_CPTREE_CURRENT_CNO)
+                return -EROFS; /* snapshot is not writable */
+
+        return generic_permission(inode, mask, NULL);
+}
+
 int nilfs_load_inode_block(struct nilfs_sb_info *sbi, struct inode *inode,
                            struct buffer_head **pbh)
 {