io_uring: allow iov_len = 0 for recvmsg and buffer select

When using BUFFER_SELECT there is no technical requirement that the user
actually provides iov, and this removes one copy_from_user call.

So allow iov_len to be 0.

Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220630091231.1456789-4-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/io_uring/net.c b/io_uring/net.c
index b77bfbf..06eaef9 100644 (file)
--- a/io_uring/net.c
+++ b/io_uring/net.c
@@ -300,12 +300,18 @@ static int __io_recvmsg_copy_hdr(struct io_kiocb *req,
                return ret;
 
        if (req->flags & REQ_F_BUFFER_SELECT) {
-               if (iov_len > 1)
+               if (iov_len == 0) {
+                       sr->len = iomsg->fast_iov[0].iov_len = 0;
+                       iomsg->fast_iov[0].iov_base = NULL;
+                       iomsg->free_iov = NULL;
+               } else if (iov_len > 1) {
                        return -EINVAL;
-               if (copy_from_user(iomsg->fast_iov, uiov, sizeof(*uiov)))
-                       return -EFAULT;
-               sr->len = iomsg->fast_iov[0].iov_len;
-               iomsg->free_iov = NULL;
+               } else {
+                       if (copy_from_user(iomsg->fast_iov, uiov, sizeof(*uiov)))
+                               return -EFAULT;
+                       sr->len = iomsg->fast_iov[0].iov_len;
+                       iomsg->free_iov = NULL;
+               }
        } else {
                iomsg->free_iov = iomsg->fast_iov;
                ret = __import_iovec(READ, uiov, iov_len, UIO_FASTIOV,