On Mon, Jul 13, 2020 at 11:42:51AM +0200, David Sterba wrote:
> On Fri, Jul 10, 2020 at 11:05:09PM +0900, Johannes Thumshirn wrote:
> > @@ -261,7 +264,8 @@ struct btrfs_ioctl_fs_info_args {
> > __u32 flags; /* in/out */
> > __u16 csum_type; /* out */
> > __u16 csum_size; /* out */
> > - __u8 reserved[972]; /* pad to 1k */
> > + __u32 generation; /* out */
> > + __u8 reserved[968]; /* pad to 1k */
>
> I've tested the static assert by switching just the type but not the
> remaining reserved bytes
>
> ./include/linux/build_bug.h:78:41: error: static assertion failed: "sizeof(struct btrfs_ioctl_fs_info_args) == 1024"
> 78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
> | ^~~~~~~~~~~~~~
> ./include/linux/build_bug.h:77:34: note: in expansion of macro ‘__static_assert’
> 77 | #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
> | ^~~~~~~~~~~~~~~
> ./include/uapi/linux/btrfs.h:270:1: note: in expansion of macro ‘static_assert’
> 270 | static_assert(sizeof(struct btrfs_ioctl_fs_info_args) == 1024);
> | ^~~~~~~~~~~~~
> make[2]: *** [scripts/Makefile.build:281: fs/btrfs/super.o] Error 1
> make[1]: *** [scripts/Makefile.build:497: fs/btrfs] Error 2
> make: *** [Makefile:1756: fs] Error 2
>
> Good.
There's extra padding now required for u64:
struct btrfs_ioctl_fs_info_args {
__u64 max_id; /* 0 8 */
__u64 num_devices; /* 8 8 */
__u8 fsid[16]; /* 16 16 */
__u32 nodesize; /* 32 4 */
__u32 sectorsize; /* 36 4 */
__u32 clone_alignment; /* 40 4 */
__u32 flags; /* 44 4 */
__u16 csum_type; /* 48 2 */
__u16 csum_size; /* 50 2 */
/* XXX 4 bytes hole, try to pack */
__u64 generation; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
__u8 reserved[964]; /* 64 964 */
/* size: 1032, cachelines: 17, members: 11 */
/* sum members: 1024, holes: 1, sum holes: 4 */
/* padding: 4 */
/* last cacheline: 8 bytes */
};
What if, instead of inserting a padding/reserved field we switch the
flags to u64 too. This unfortunatelly requires swapping order for flags
and csum_type/_size but the result
struct btrfs_ioctl_fs_info_args {
__u64 max_id; /* 0 8 */
__u64 num_devices; /* 8 8 */
__u8 fsid[16]; /* 16 16 */
__u32 nodesize; /* 32 4 */
__u32 sectorsize; /* 36 4 */
__u32 clone_alignment; /* 40 4 */
__u16 csum_type; /* 44 2 */
__u16 csum_size; /* 46 2 */
__u64 flags; /* 48 8 */
__u64 generation; /* 56 8 */
/* --- cacheline 1 boundary (64 bytes) --- */
__u8 reserved[960]; /* 64 960 */
/* size: 1024, cachelines: 16, members: 11 */
};
does not require any padding and leaves the end member with 8 byte
alignment.
