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.
> --
> Jens Axboe
>
