Re: [PATCH V3 03/10] xfs: Check for extent overflow when deleting an extent

Darrick J. Wong
2020-08-31T16:34:55Z

On Thu, Aug 20, 2020 at 11:13:42AM +0530, Chandan Babu R wrote:
> Deleting a file range from the middle of an existing extent can cause
> the per-inode extent count to increase by 1. This commit checks for
> extent count overflow in such cases.
>
> Signed-off-by: Chandan Babu R <chandanrlinux@gmail.com>
> ---
> fs/xfs/libxfs/xfs_inode_fork.h | 6 ++++++
> fs/xfs/xfs_bmap_item.c | 4 ++++
> fs/xfs/xfs_bmap_util.c | 5 +++++
> 3 files changed, 15 insertions(+)
>
> diff --git a/fs/xfs/libxfs/xfs_inode_fork.h b/fs/xfs/libxfs/xfs_inode_fork.h
> index 7fc2b129a2e7..2642e4847ee0 100644
> --- a/fs/xfs/libxfs/xfs_inode_fork.h
> +++ b/fs/xfs/libxfs/xfs_inode_fork.h
> @@ -39,6 +39,12 @@ struct xfs_ifork {
> * extent to a fork and there's no possibility of splitting an existing mapping.
> */
> #define XFS_IEXT_ADD_NOSPLIT_CNT (1)
> +/*
> + * Removing an extent from the middle of an existing extent can cause the extent
> + * count to increase by 1.
> + * i.e. | Old extent | Hole | Old extent |
> + */
> +#define XFS_IEXT_REMOVE_CNT (1)

The first thought that popped into my head after reading the subject
line was "UH-oh, is this going to result in undeletable files when the
extent counts hit max and the user tries to rm?"

Then I realized that "when deleting an extent" actually refers to
punching holes in the middle of files, not truncating them.

So I think at the very least the subject line should be changed to
say that we're talking about hole punching, not general file deletion;
and the constant probably ought to be called XFS_IEXT_PUNCH_CNT to make
that clearer.

Aside from that the logic seems ok to me.

(Also PS I'm not reviewing these patches in order...)

--D

>
> /*
> * Fork handling.
> diff --git a/fs/xfs/xfs_bmap_item.c b/fs/xfs/xfs_bmap_item.c
> index ec3691372e7c..b9c35fb10de4 100644
> --- a/fs/xfs/xfs_bmap_item.c
> +++ b/fs/xfs/xfs_bmap_item.c
> @@ -519,6 +519,10 @@ xfs_bui_item_recover(
> }
> xfs_trans_ijoin(tp, ip, 0);
>
> + error = xfs_iext_count_may_overflow(ip, whichfork, XFS_IEXT_REMOVE_CNT);
> + if (error)
> + goto err_inode;
> +
> count = bmap->me_len;
> error = xfs_trans_log_finish_bmap_update(tp, budp, type, ip, whichfork,
> bmap->me_startoff, bmap->me_startblock, &count, state);
> diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
> index 7b76a48b0885..59d4da38aadf 100644
> --- a/fs/xfs/xfs_bmap_util.c
> +++ b/fs/xfs/xfs_bmap_util.c
> @@ -891,6 +891,11 @@ xfs_unmap_extent(
>
> xfs_trans_ijoin(tp, ip, 0);
>
> + error = xfs_iext_count_may_overflow(ip, XFS_DATA_FORK,
> + XFS_IEXT_REMOVE_CNT);
> + if (error)
> + goto out_trans_cancel;
> +
> error = xfs_bunmapi(tp, ip, startoffset_fsb, len_fsb, 0, 2, done);
> if (error)
> goto out_trans_cancel;
> --
> 2.28.0
>