Re: [PATCH] btrfs: add 32-bit compat ioctl for BTRFS_IOC_GET_SUBVOL_INFO

[David Sterba]

On Thu, May 21, 2026 at 07:51:13AM +0000, Daan De Meyer wrote:
> On 64-bit kernels with 32-bit userspace, > struct btrfs_ioctl_timespec is
> laid out as 16 bytes (8B sec + 4B nsec + 4B trailing padding) instead of
> the 12 bytes a 32-bit userspace expects, because the surrounding struct
> is not packed. As a result, struct btrfs_ioctl_get_subvol_info_args has
> a different size and layout in 32-bit userspace than in the 64-bit
> kernel, and BTRFS_IOC_GET_SUBVOL_INFO returns garbage to 32-bit callers.
> 
> Mirror what was done for BTRFS_IOC_SET_RECEIVED_SUBVOL: add a packed
> btrfs_ioctl_get_subvol_info_args_32 with btrfs_ioctl_timespec_32 fields,
> define BTRFS_IOC_GET_SUBVOL_INFO_32 with that struct as the size
> argument, factor the existing handler into a shared _btrfs_ioctl_get_
> subvol_info() helper, and add btrfs_ioctl_get_subvol_info_32() which
> fills the kernel struct and translates field-by-field into the 32-bit
> struct before copy_to_user().
> 
> Signed-off-by: Daan De Meyer <daan@xxxxxxxxxxxx>

Reviewed-by: David Sterba <dsterba@xxxxxxxx>

Looks like the 64/32 combination is not much used (and not tested during
development) as the ioctl has been there for a long time and nobody
complained. The compat workaround is straightforward though. Thanks.