On Wed, Jan 16, 2019 at 4:12 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 1/16/19 3:41 AM, Arnd Bergmann wrote:
> > On Tue, Jan 15, 2019 at 3:55 AM Jens Axboe <axboe@xxxxxxxxx> wrote:
> >> diff --git a/arch/x86/entry/syscalls/syscall_32.tbl b/arch/x86/entry/syscalls/syscall_32.tbl
> >> index 3cf7b533b3d1..194e79c0032e 100644
> >> --- a/arch/x86/entry/syscalls/syscall_32.tbl
> >> +++ b/arch/x86/entry/syscalls/syscall_32.tbl
> >> +SYSCALL_DEFINE2(io_uring_setup, u32, entries,
> >> + struct io_uring_params __user *, params)
> >> +{
> >> + return io_uring_setup(entries, params, false);
> >> +}
> >> +
> >> +#ifdef CONFIG_COMPAT
> >> +COMPAT_SYSCALL_DEFINE2(io_uring_setup, u32, entries,
> >> + struct io_uring_params __user *, params)
> >> +{
> >> + return io_uring_setup(entries, params, true);
> >> +}
> >> +#endif
> >
> > The compat syscall has the same calling conventions as the
> > native one here, so I think you can just use that directly.
>
> Not sure I understand what you mean here. I need to know if it's the
> compat one, hence 'true' vs 'false', so I know what the size of the user
> pointers/structs are.
My mistake, I missed the true/false difference between the two
functions.
> >> +/*
> >> + * IO submission data structure (Submission Queue Entry)
> >> + */
> >> +struct io_uring_sqe {
> >> + __u8 opcode; /* type of operation for this sqe */
> >> + __u8 flags; /* as of now unused */
> >> + __u16 ioprio; /* ioprio for the request */
> >> + __s32 fd; /* file descriptor to do IO on */
> >> + __u64 off; /* offset into file */
> >> + union {
> >> + void *addr; /* buffer or iovecs */
> >> + __u64 __pad;
> >> + };
> >
> > It seems a bit unfortunate to keep the pointer field only
> > almost compatible between 32-bit and 64-bit big-endian
> > architectures, as that requires an in_compat_syscall()
> > check whenever we access the pointer from the kernel.
> >
> > Could you use a __u64 field to store the pointer itself
> > instead?
>
> I feel like I'm missing something here, we'll still need the compat code
> on the kernel side for 32-bit app on 64-bit kernel, so what would we
> solve by making this an __u64?
It means you don't have to define a compat_io_uring_sqe
structure with a compat_uptr_t member in it.
Arnd
