Re: [PATCH RFC] btrfs: send: Emit all xattr of an inode if the uid/gid changed

[Filipe Manana <fdmanana@gmail.com>]

On Wed, Mar 25, 2020 at 1:52 AM Marcos Paulo de Souza
<marcos@mpdesouza.com> wrote:
>
> From: Marcos Paulo de Souza <mpdesouza@suse.com>
>
> [PROBLEM]
> When doing incremental send with a file with capabilities, there is a
> situation where the capability can be lost in the receiving side. The
> sequence of actions bellow show the problem:
>
> $ mount /dev/sda fs1
> $ mount /dev/sdb fs2
>
> $ touch fs1/foo.bar
> $ setcap cap_sys_nice+ep fs1/foo.bar
> $ btrfs subvol snap -r fs1 fs1/snap_init
> $ btrfs send fs1/snap_init | btrfs receive fs2
>
> $ chgrp adm fs1/foo.bar
> $ setcap cap_sys_nice+ep fs1/foo.bar
>
> $ btrfs subvol snap -r fs1 fs1/snap_complete
> $ btrfs subvol snap -r fs1 fs1/snap_incremental
>
> $ btrfs send fs1/snap_complete | btrfs receive fs2
> $ btrfs send -p fs1/snap_init fs1/snap_incremental | btrfs receive fs2
>
> At this point fs/snap_increment/foo.bar lost the capability, since a
> chgrp was emitted by "btrfs send". The current code only checks for the
> items that changed, and as the XATTR kept the value only the chgrp change
> is emitted.

So, the explanation could be a bit more clear.

First the send stream emits a "chown" command (and not a "chgrp"
command), that's what is used to change both users and groups.

Then mentioning that changing the group of a file causes the
capability xattr to be deleted is crucial - that's why the receiving
side ends up losing the capability, because we send an operation to
change the group but we don't send an operation to set the capability
xattr, since the value of the xattr is the same in both snapshots.

>
> [FIX]
> In order to fix this issue, check if the uid/gid of the inode change,
> and if yes, emit all XATTR again, including the capability.
>
> Fixes: https://github.com/kdave/btrfs-progs/issues/202

The Fixes: tag is used to identify commits, in the kernel, that
introduced some problem.
A "Link:" tag is more appropriate to point to the btrfs-progs github issue.

>
> Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
> ---
>  I'm posting this patch as a RFC because I had some questions
>  * Is this the correct place to fix?

Nop, see below.

>  * Also, emitting all XATTR of the inode seems overkill...

Yes, but I wouldn't worry much - first it's not common for files to
have many xattrs, second they are small values and are layed out
sequentially in the btree, and above all, uids/gids are mostly static
and don't change often.
But that can be avoided, see below.

>  * Should it be fixed in userspace?

No.

Send should emit a sequence of operations that produces correct
results in the receiving side. It should never result in any data or
metadata loss, crashes, etc.

This is no different from rename dependencies for example, where we
make send change the order of rename and other operations so that the
receiving side doesn't fail - otherwise we would have to add a lot of
intelligence and complicated code to btrfs receive in progs - which
brings us to another point - any consumer of a send stream would have
to be changed - btrfs receive from btrfs-progs, is the most well
known, and very few people will use anything else, but there may be
other consumers of send streams out there.

>
>  fs/btrfs/send.c | 29 +++++++++++++++++++++++++----
>  1 file changed, 25 insertions(+), 4 deletions(-)
>
> diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
> index c5f41bd86765..5cffe5da91cf 100644
> --- a/fs/btrfs/send.c
> +++ b/fs/btrfs/send.c
> @@ -6187,6 +6187,14 @@ static int changed_inode(struct send_ctx *sctx,
>                 sctx->cur_inode_mode = btrfs_inode_mode(
>                                 sctx->right_path->nodes[0], right_ii);
>         } else if (result == BTRFS_COMPARE_TREE_CHANGED) {
> +               u64 left_uid = btrfs_inode_uid(sctx->left_path->nodes[0],
> +                                       left_ii);
> +               u64 left_gid = btrfs_inode_gid(sctx->left_path->nodes[0],
> +                                       left_ii);
> +               u64 right_uid = btrfs_inode_uid(sctx->right_path->nodes[0],
> +                                       right_ii);
> +               u64 right_gid = btrfs_inode_gid(sctx->right_path->nodes[0],
> +                                       right_ii);
>                 /*
>                  * We need to do some special handling in case the inode was
>                  * reported as changed with a changed generation number. This
> @@ -6236,15 +6244,12 @@ static int changed_inode(struct send_ctx *sctx,
>                         sctx->send_progress = sctx->cur_ino + 1;
>
>                         /*
> -                        * Now process all extents and xattrs of the inode as if
> +                        * Now process all extents of the inode as if
>                          * they were all new.
>                          */
>                         ret = process_all_extents(sctx);
>                         if (ret < 0)
>                                 goto out;
> -                       ret = process_all_new_xattrs(sctx);
> -                       if (ret < 0)
> -                               goto out;
>                 } else {
>                         sctx->cur_inode_gen = left_gen;
>                         sctx->cur_inode_new = 0;
> @@ -6255,6 +6260,22 @@ static int changed_inode(struct send_ctx *sctx,
>                         sctx->cur_inode_mode = btrfs_inode_mode(
>                                         sctx->left_path->nodes[0], left_ii);
>                 }
> +
> +               /*
> +                * Process all XATTR of the inode if the generation or owner
> +                * changed.
> +                *
> +                * If the inode changed it's uid/gid, but kept a
> +                * security.capability xattr, only the uid/gid will be emitted,
> +                * causing the related xattr to deleted. For this reason always
> +                * emit the XATTR when an inode has changed.
> +                */
> +               if (sctx->cur_inode_new_gen || left_uid != right_uid ||
> +                   left_gid != right_gid) {
> +                       ret = process_all_new_xattrs(sctx);

So the correct place for fixing this issue is at
send.c:finish_inode_if_needed(), in the following place:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/btrfs/send.c?h=v5.6-rc7#n6000

If a chown operation is sent, then just send the xattr - and instead
if sending all xattrs, just send the xattr with the name
"security.capability" - check first if there are any other
capabilities that use other xattr names - if there are, just emit
set_xattr operations for all xattrs with a "security." prefix in their
name.

Thanks.


> +                       if (ret < 0)
> +                               goto out;
> +               }
>         }
>
>  out:
> --
> 2.25.1
>


-- 
Filipe David Manana,

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