btrfs: protect snapshots from deleting during send

The patch "Btrfs: fix protection between send and root deletion"
(18f687d538449373c37c) does not actually prevent to delete the snapshot
and just takes care during background cleaning, but this seems rather
user unfriendly, this patch implements the idea presented in

http://www.spinics.net/lists/linux-btrfs/msg30813.html

- add an internal root_item flag to denote a dead root
- check if the send_in_progress is set and refuse to delete, otherwise
  set the flag and proceed
- check the flag in send similar to the btrfs_root_readonly checks, for
  all involved roots

The root lookup in send via btrfs_read_fs_root_no_name will check if the
root is really dead or not. If it is, ENOENT, aborted send. If it's
alive, it's protected by send_in_progress, send can continue.

CC: Miao Xie <miaox@cn.fujitsu.com>
CC: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index 484aacac2c89..c76400dda4df 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -5518,7 +5518,7 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
 
         /*
          * The subvolume must remain read-only during send, protect against
-         * making it RW.
+         * making it RW. This also protects against deletion.
          */
         spin_lock(&send_root->root_item_lock);
         send_root->send_in_progress++;
@@ -5578,6 +5578,15 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
         }
 
         sctx->send_root = send_root;
+        /*
+         * Unlikely but possible, if the subvolume is marked for deletion but
+         * is slow to remove the directory entry, send can still be started
+         */
+        if (btrfs_root_dead(sctx->send_root)) {
+                ret = -EPERM;
+                goto out;
+        }
+
         sctx->clone_roots_cnt = arg->clone_sources_count;
 
         sctx->send_max_size = BTRFS_SEND_BUF_SIZE;
@@ -5667,7 +5676,8 @@ long btrfs_ioctl_send(struct file *mnt_file, void __user *arg_)
 
                 spin_lock(&sctx->parent_root->root_item_lock);
                 sctx->parent_root->send_in_progress++;
-                if (!btrfs_root_readonly(sctx->parent_root)) {
+                if (!btrfs_root_readonly(sctx->parent_root) ||
+                                btrfs_root_dead(sctx->parent_root)) {
                         spin_unlock(&sctx->parent_root->root_item_lock);
                         srcu_read_unlock(&fs_info->subvol_srcu, index);
                         ret = -EPERM;