btrfs: Check read-only status of roots during send

All the subvolues that are involved in send must be read-only during the
whole operation. The ioctl SUBVOL_SETFLAGS could be used to change the
status to read-write and the result of send stream is undefined if the
data change unexpectedly.

Fix that by adding a refcount for all involved roots and verify that
there's no send in progress during SUBVOL_SETFLAGS ioctl call that does
read-only -> read-write transition.

We need refcounts because there are no restrictions on number of send
parallel operations currently run on a single subvolume, be it source,
parent or one of the multiple clone sources.

Kernel is silent when the RO checks fail and returns EPERM. The same set
of checks is done already in userspace before send starts.

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 19 insertions, 3 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 89c2f6169b92..c0dc05467ce8 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1706,12 +1706,28 @@ static noinline int btrfs_ioctl_subvol_setflags(struct file *file,
                 goto out_drop_sem;
 
         root_flags = btrfs_root_flags(&root->root_item);
-        if (flags & BTRFS_SUBVOL_RDONLY)
+        if (flags & BTRFS_SUBVOL_RDONLY) {
                 btrfs_set_root_flags(&root->root_item,
                                      root_flags | BTRFS_ROOT_SUBVOL_RDONLY);
-        else
-                btrfs_set_root_flags(&root->root_item,
+        } else {
+                /*
+                 * Block RO -> RW transition if this subvolume is involved in
+                 * send
+                 */
+                spin_lock(&root->root_item_lock);
+                if (root->send_in_progress == 0) {
+                        btrfs_set_root_flags(&root->root_item,
                                      root_flags & ~BTRFS_ROOT_SUBVOL_RDONLY);
+                        spin_unlock(&root->root_item_lock);
+                } else {
+                        spin_unlock(&root->root_item_lock);
+                        btrfs_warn(root->fs_info,
+                        "Attempt to set subvolume %llu read-write during send",
+                                        root->root_key.objectid);
+                        ret = -EPERM;
+                        goto out_drop_sem;
+                }
+        }
 
         trans = btrfs_start_transaction(root, 1);
         if (IS_ERR(trans)) {