[Bug] io_uring_register_files_update broken

[Victor Stewart <v@nametag.social>]

i'm fairly confident there is something broken with
io_uring_register_files_update,
especially the offset parameter.

when trying to update a single fd, and getting a successful result of
1, proceeding
operations with IOSQE_FIXED_FILE fail with -9. but if i update all of
the fds with
then my recv operations succeed, but close still fails with -9.

on Clear LInux 5.12.13-1050.native

here's a diff for liburing send_recv test, to demonstrate this.

diff --git a/test/send_recv.c b/test/send_recv.c
index 19adbdd..492b591 100644
--- a/test/send_recv.c
+++ b/test/send_recv.c
@@ -27,6 +27,8 @@ static char str[] = "This is a test of send and recv
over io_uring!";
 #      define io_uring_prep_recv io_uring_prep_read
 #endif

+static int *fds;
+
 static int recv_prep(struct io_uring *ring, struct iovec *iov, int *sock,
                     int registerfiles)
 {
@@ -54,17 +56,28 @@ static int recv_prep(struct io_uring *ring, struct
iovec *iov, int *sock,
                goto err;
        }

+       fds = malloc(100 * sizeof(int));
+       memset(fds, 0xff, sizeof(int) * 100);
+
        if (registerfiles) {
-               ret = io_uring_register_files(ring, &sockfd, 1);
+               ret = io_uring_register_files(ring, fds, 100);
                if (ret) {
                        fprintf(stderr, "file reg failed\n");
                        goto err;
                }
-               use_fd = 0;
-       } else {
-               use_fd = sockfd;
+
+               fds[sockfd] = sockfd;
+               int result = io_uring_register_files_update(ring,
sockfd, fds, 1);
+
+               if (result != 1)
+               {
+                       fprintf(stderr, "file update failed\n");
+                       goto err;
+               }
        }

+       use_fd = sockfd;
+
        sqe = io_uring_get_sqe(ring);
        io_uring_prep_recv(sqe, use_fd, iov->iov_base, iov->iov_len, 0);
        if (registerfiles)