linux-kernel - Re: [f2fs-dev] [PATCH v3] f2fs: move ioctl interface definitions to separated file

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Date:   Mon, 2 Nov 2020 19:22:34 -0800
From:   Eric Biggers <ebiggers@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     jaegeuk@...nel.org, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [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