Re: [PATCH 1/3] btrfs-progs: unify sizeof(struct btrfs_super_block) and BTRFS_SUPER_INFO_SIZE

[David Sterba <dsterba@suse.cz>]

On Thu, Oct 21, 2021 at 09:40:18AM +0800, Qu Wenruo wrote:
> Just like kernel change, pad struct btrfs_super_block to 4096 bytes.
> 
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  kernel-shared/ctree.h   | 7 +++++++
>  kernel-shared/disk-io.h | 3 ---
>  2 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel-shared/ctree.h b/kernel-shared/ctree.h
> index 563ea50b3587..6451690ce4fa 100644
> --- a/kernel-shared/ctree.h
> +++ b/kernel-shared/ctree.h
> @@ -406,6 +406,9 @@ struct btrfs_root_backup {
>  	u8 unused_8[10];
>  } __attribute__ ((__packed__));
>  
> +#define BTRFS_SUPER_INFO_OFFSET SZ_64K
> +#define BTRFS_SUPER_INFO_SIZE 4096
> +
>  /*
>   * the super block basically lists the main trees of the FS
>   * it currently lacks any block count etc etc
> @@ -456,8 +459,12 @@ struct btrfs_super_block {
>  	__le64 reserved[28];
>  	u8 sys_chunk_array[BTRFS_SYSTEM_CHUNK_ARRAY_SIZE];
>  	struct btrfs_root_backup super_roots[BTRFS_NUM_BACKUP_ROOTS];
> +	/* Padded to 4096 bytes */
> +	u8 padding[565];
>  } __attribute__ ((__packed__));
>  
> +static_assert(sizeof(struct btrfs_super_block) == BTRFS_SUPER_INFO_SIZE);

Using static_assert breaks build on musl (you can verify that by running
ci/ci-build-musl if you have docker installed and set up).

There already is macro BUILD_ASSERT used in ioctl.h, eventually we can
copy the static_assert from kernel or use _Static_assert directly.