Re: [PATCH v2 3/4] btrfs: send: fix invalid commands for inodes with changed rdev but same gen

[Filipe Manana <fdmanana@gmail.com>]

On Mon, Jan 25, 2021 at 7:51 PM Roman Anasal <roman.anasal@bdsu.de> wrote:
>
> This is analogous to the preceding patch ("btrfs: send: fix invalid
> commands for inodes with changed type but same gen") but for changed
> rdev:
>
> When doing an incremental send, if a new inode has the same number as an
> inode in the parent subvolume, was created with the same generation but
> has differing rdev it will not be detected as changed and thus not
> recreated. This will lead to incorrect results on the receiver where the
> inode will keep the rdev of the inode in the parent subvolume or even
> fail when also the ref is unchanged.
>
> This case does not happen when doing incremental sends with snapshots
> that are kept read-only by the user all the time, but it may happen if
> - a snapshot was modified in the same transaction as its parent after it
>   was created
> - the subvol used as parent was created independently from the sent subvol
>
> Example reproducers:
>
>   # case 1: same ino at same path
>   btrfs subvolume create subvol1
>   btrfs subvolume create subvol2
>   mknod subvol1/a c 1 3
>   mknod subvol2/a c 1 5
>   btrfs property set subvol1 ro true
>   btrfs property set subvol2 ro true
>   btrfs send -p subvol1 subvol2 | btrfs receive --dump
>
> The produced tree state here is:
>   |-- subvol1
>   |   `-- a         (ino 257, c 1,3)
>   |
>   `-- subvol2
>       `-- a         (ino 257, c 1,5)
>
> Where subvol1/a and subvol2/a are character devices with differing minor
> numbers but same inode number and same generation.
>
> Example output of the receive command:
>   At subvol subvol2
>   snapshot        ./subvol2                       uuid=7513941c-4ef7-f847-b05e-4fdfe003af7b transid=9 parent_uuid=b66f015b-c226-2548-9e39-048c7fdbec99 parent_transid=9
>   utimes          ./subvol2/                      atime=2021-01-25T17:14:36+0000 mtime=2021-01-25T17:14:36+0000 ctime=2021-01-25T17:14:36+0000
>   link            ./subvol2/a                     dest=a
>   unlink          ./subvol2/a
>   utimes          ./subvol2/                      atime=2021-01-25T17:14:36+0000 mtime=2021-01-25T17:14:36+0000 ctime=2021-01-25T17:14:36+0000
>   utimes          ./subvol2/a                     atime=2021-01-25T17:14:36+0000 mtime=2021-01-25T17:14:36+0000 ctime=2021-01-25T17:14:36+0000
>
> => the `link` command causes the receiver to fail with:
>    ERROR: link a -> a failed: File exists
>
> Second example:
>   # case 2: same ino at different path
>   btrfs subvolume create subvol1
>   btrfs subvolume create subvol2
>   mknod subvol1/a c 1 3
>   mknod subvol2/b c 1 5
>   btrfs property set subvol1 ro true
>   btrfs property set subvol2 ro true
>   btrfs send -p subvol1 subvol2 | btrfs receive --dump

As I've told you before for the v1 patchset from a week or two ago,
this is not a supported scenario for incremental sends.
Incremental sends are meant to be used on RO snapshots of the same
subvolume, and those snapshots must never be changed after they were
created.

Incremental sends were simply not designed for these cases, and can
never be guaranteed to work with such cases.

The bug is not having incremental sends fail right away, with an
explicit error message, when the send and parent roots aren't RO
snapshots of the same subvolume.

>
> The produced tree state here is:
>   |-- subvol1
>   |   `-- a         (ino 257, c 1,3)
>   |
>   `-- subvol2
>       `-- b         (ino 257, c 1,5)
>
> Where subvol1/a and subvol2/b are character devices with differing minor
> numbers but same inode number and same generation.
>
> Example output of the receive command:
>   At subvol subvol2
>   snapshot        ./subvol2                       uuid=1c175819-8b97-0046-a20e-5f95e37cbd40 transid=13 parent_uuid=bad4a908-21b4-6f40-9a08-6b0768346725 parent_transid=13
>   utimes          ./subvol2/                      atime=2021-01-25T17:18:46+0000 mtime=2021-01-25T17:18:46+0000 ctime=2021-01-25T17:18:46+0000
>   link            ./subvol2/b                     dest=a
>   unlink          ./subvol2/a
>   utimes          ./subvol2/                      atime=2021-01-25T17:18:46+0000 mtime=2021-01-25T17:18:46+0000 ctime=2021-01-25T17:18:46+0000
>   utimes          ./subvol2/b                     atime=2021-01-25T17:18:46+0000 mtime=2021-01-25T17:18:46+0000 ctime=2021-01-25T17:18:46+0000
>
> => subvol1/a is renamed to subvol2/b instead of recreated to updated
>    rdev which results in received subvol2/b having the wrong minor
>    number:
>
>   257 crw-r--r--. 1 root root 1, 3 Jan 25 17:18 subvol2/b
>
> Signed-off-by: Roman Anasal <roman.anasal@bdsu.de>
> ---
> v2:
>   - add this patch to also handle changed rdev
> ---
>  fs/btrfs/send.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
>
> diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
> index c8b1f441f..ef544525f 100644
> --- a/fs/btrfs/send.c
> +++ b/fs/btrfs/send.c
> @@ -6263,6 +6263,7 @@ static int changed_inode(struct send_ctx *sctx,
>         struct btrfs_inode_item *right_ii = NULL;
>         u64 left_gen = 0;
>         u64 right_gen = 0;
> +       u64 left_rdev, right_rdev;
>         u64 left_type, right_type;
>
>         sctx->cur_ino = key->objectid;
> @@ -6285,6 +6286,8 @@ static int changed_inode(struct send_ctx *sctx,
>                                 struct btrfs_inode_item);
>                 left_gen = btrfs_inode_generation(sctx->left_path->nodes[0],
>                                 left_ii);
> +               left_rdev = btrfs_inode_rdev(sctx->left_path->nodes[0],
> +                               left_ii);
>         } else {
>                 right_ii = btrfs_item_ptr(sctx->right_path->nodes[0],
>                                 sctx->right_path->slots[0],
> @@ -6300,6 +6303,9 @@ static int changed_inode(struct send_ctx *sctx,
>                 right_gen = btrfs_inode_generation(sctx->right_path->nodes[0],
>                                 right_ii);
>
> +               right_rdev = btrfs_inode_rdev(sctx->right_path->nodes[0],
> +                               right_ii);
> +
>                 left_type = S_IFMT & btrfs_inode_mode(
>                                 sctx->left_path->nodes[0], left_ii);
>                 right_type = S_IFMT & btrfs_inode_mode(
> @@ -6310,7 +6316,8 @@ static int changed_inode(struct send_ctx *sctx,
>                  * the inode as deleted+reused because it would generate a
>                  * stream that tries to delete/mkdir the root dir.
>                  */
> -               if ((left_gen != right_gen || left_type != right_type) &&
> +               if ((left_gen != right_gen || left_type != right_type ||
> +                   left_rdev != right_rdev) &&
>                     sctx->cur_ino != BTRFS_FIRST_FREE_OBJECTID)
>                         sctx->cur_inode_recreated = 1;
>         }
> @@ -6350,8 +6357,7 @@ static int changed_inode(struct send_ctx *sctx,
>                                 sctx->left_path->nodes[0], left_ii);
>                 sctx->cur_inode_mode = btrfs_inode_mode(
>                                 sctx->left_path->nodes[0], left_ii);
> -               sctx->cur_inode_rdev = btrfs_inode_rdev(
> -                               sctx->left_path->nodes[0], left_ii);
> +               sctx->cur_inode_rdev = left_rdev;
>                 if (sctx->cur_ino != BTRFS_FIRST_FREE_OBJECTID)
>                         ret = send_create_inode_if_needed(sctx);
>         } else if (result == BTRFS_COMPARE_TREE_DELETED) {
> @@ -6396,8 +6402,7 @@ static int changed_inode(struct send_ctx *sctx,
>                                         sctx->left_path->nodes[0], left_ii);
>                         sctx->cur_inode_mode = btrfs_inode_mode(
>                                         sctx->left_path->nodes[0], left_ii);
> -                       sctx->cur_inode_rdev = btrfs_inode_rdev(
> -                                       sctx->left_path->nodes[0], left_ii);
> +                       sctx->cur_inode_rdev = left_rdev;
>                         ret = send_create_inode_if_needed(sctx);
>                         if (ret < 0)
>                                 goto out;
> --
> 2.26.2
>


-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”