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SubjectRe: [f2fs-dev] [PATCH v3] f2fs: move ioctl interface definitions to separated file
On Mon, Nov 02, 2020 at 02:21:31PM +0800, Chao Yu wrote:
> +#define F2FS_IOC_MOVE_RANGE _IOWR(F2FS_IOCTL_MAGIC, 9, \
> + struct f2fs_move_range)
[...]
> +#define F2FS_IOC_GARBAGE_COLLECT_RANGE _IOW(F2FS_IOCTL_MAGIC, 11, \
> + struct f2fs_gc_range)
[...]
> +
> +struct f2fs_gc_range {
> + __u32 sync;
> + __u64 start;
> + __u64 len;
> +};
[...]
> +struct f2fs_move_range {
> + __u32 dst_fd; /* destination fd */
> + __u64 pos_in; /* start position in src_fd */
> + __u64 pos_out; /* start position in dst_fd */
> + __u64 len; /* size to move */
> +};

These two structs are weird because there is implicit padding between the __u32
field and the following __u64 field on some 32-bit architectures (e.g. x86_32)
but not others (e.g. arm32).

But f2fs_compat_ioctl() doesn't handle these two ioctls specially, but rather
just calls through to f2fs_ioctl(). That's wrong, and it means that
F2FS_IOC_MOVE_RANGE and F2FS_IOC_GARBAGE_COLLECT_RANGE won't work when called
from an x86_32 binary on an x86_64 kernel.

So something needs to be fixed. I wonder if it's safe to just explicitly add
the padding field after the fact. If no one is actually using these two ioctls
in a case where both userspace and the kernel lack the implicit padding (e.g.,
x86_32 userspace with x86_32 kernel), it should be fine...

- Eric

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