Re: [PATCH 12/18] io_uring: add support for pre-mapped user IO buffers

On Tue, Jan 29, 2019 at 11:56 PM Jens Axboe [off-list ref] wrote:

On 1/29/19 3:44 PM, Jann Horn wrote:

quoted

On Tue, Jan 29, 2019 at 8:27 PM Jens Axboe [off-list ref] wrote:

quoted

If we have fixed user buffers, we can map them into the kernel when we
setup the io_context. That avoids the need to do get_user_pages() for
each and every IO.

To utilize this feature, the application must call io_uring_register()
after having setup an io_uring context, passing in
IORING_REGISTER_BUFFERS as the opcode. The argument must be a pointer
to an iovec array, and the nr_args should contain how many iovecs the
application wishes to map.

If successful, these buffers are now mapped into the kernel, eligible
for IO. To use these fixed buffers, the application must use the
IORING_OP_READ_FIXED and IORING_OP_WRITE_FIXED opcodes, and then
set sqe->index to the desired buffer index. sqe->addr..sqe->addr+seq->len
must point to somewhere inside the indexed buffer.

The application may register buffers throughout the lifetime of the
io_uring context. It can call io_uring_register() with
IORING_UNREGISTER_BUFFERS as the opcode to unregister the current set of
buffers, and then register a new set. The application need not
unregister buffers explicitly before shutting down the io_uring context.

[...]

quoted

quoted

+       imu = &ctx->user_bufs[index];
+       buf_addr = READ_ONCE(sqe->addr);
+       if (buf_addr < imu->ubuf || buf_addr + len > imu->ubuf + imu->len)

This can wrap around if `buf_addr` or `len` is very big, right? Then
you e.g. get past the first check because `buf_addr` is sufficiently
big, and get past the second check because `buf_addr + len` wraps
around and becomes small.

Good point. I wonder if we have a verification helper for something like
this?

check_add_overflow() exists, I guess that might help a bit. I don't
think I've seen a more specific helper for this situation.

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