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    SubjectRe: [PATCH] io_uring: fix compat for IORING_REGISTER_FILES_UPDATE
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    On 1/15/20 9:50 AM, Eugene Syromiatnikov wrote:
    > On Wed, Jan 15, 2020 at 09:41:58AM -0700, Jens Axboe wrote:
    >> On 1/15/20 9:35 AM, Eugene Syromiatnikov wrote:
    >>> fds field of struct io_uring_files_update is problematic with regards
    >>> to compat user space, as pointer size is different in 32-bit, 32-on-64-bit,
    >>> and 64-bit user space. In order to avoid custom handling of compat in
    >>> the syscall implementation, make fds __u64 and use u64_to_user_ptr in
    >>> order to retrieve it. Also, align the field naturally and check that
    >>> no garbage is passed there.
    >>
    >> Good point, it's an s32 pointer so won't align nicely. But how about
    >> just having it be:
    >>
    >> struct io_uring_files_update {
    >> __u32 offset;
    >> __u32 resv;
    >> __s32 *fds;
    >> };
    >>
    >> which should align nicely on both 32 and 64-bit?
    >
    > The issue is that 32-bit user space would pass a 12-byte structure with
    > a 4-byte pointer in it to the 64-bit kernel, that, in turn, would treat it
    > as a 8-byte value (which might sometimes work on little-endian architectures,
    > if there are happen to be zeroes after the pointer, but will be always broken
    > on big-endian ones). __u64 is used in order to avoid special compat wrapper;
    > see, for example, __u64 usage in btrfs or BPF for similar purposes.

    Ah yes, I'm an idiot, apparently not enough coffee yet. We'd need it in
    a union for this to work. I'll just go with yours, it'll work just fine.
    I will fold it in, I need to make some updates and rebase anyway.

    --
    Jens Axboe

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