9pfs: fix qemu_mknodat(S_IFSOCK) on macOS

mknod() on macOS does not support creating sockets, so divert to
call sequence socket(), bind() and fchmodat() respectively if S_IFSOCK
was passed with mode argument.

Link: https://lore.kernel.org/qemu-devel/17933734.zYzKuhC07K@silver/
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Message-Id: <2e7b5ecd7a6d83a538db4e8a22d8fb03e9e0f06e.1651228001.git.qemu_oss@crudebyte.com>
[C.S. - Use AT_SYMLINK_NOFOLLOW instead of AT_SYMLINK_NOFOLLOW_ANY. ]
Link: https://lore.kernel.org/qemu-devel/3704033.BMyLRrx2Jx@silver/

hw/9pfs/9p-util-darwin.c

@@ -74,6 +74,42 @@ int fsetxattrat_nofollow(int dirfd, const char *filename, const char *name,
  */
 #if defined CONFIG_PTHREAD_FCHDIR_NP
 
+static int create_socket_file_at_cwd(const char *filename, mode_t mode) {
+    int fd, err;
+    struct sockaddr_un addr = {
+        .sun_family = AF_UNIX
+    };
+
+    err = snprintf(addr.sun_path, sizeof(addr.sun_path), "./%s", filename);
+    if (err < 0 || err >= sizeof(addr.sun_path)) {
+        errno = ENAMETOOLONG;
+        return -1;
+    }
+    fd = socket(PF_UNIX, SOCK_DGRAM, 0);
+    if (fd == -1) {
+        return fd;
+    }
+    err = bind(fd, (struct sockaddr *) &addr, sizeof(addr));
+    if (err == -1) {
+        goto out;
+    }
+    /*
+     * FIXME: Should rather be using descriptor-based fchmod() on the
+     * socket file descriptor above (preferably before bind() call),
+     * instead of path-based fchmodat(), to prevent concurrent transient
+     * state issues between creating the named FIFO file at bind() and
+     * delayed adjustment of permissions at fchmodat(). However currently
+     * macOS (12.x) does not support such operations on socket file
+     * descriptors yet.
+     *
+     * Filed report with Apple: FB9997731
+     */
+    err = fchmodat(AT_FDCWD, filename, mode, AT_SYMLINK_NOFOLLOW);
+out:
+    close_preserve_errno(fd);
+    return err;
+}
+
 int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
 {
     int preserved_errno, err;
@@ -93,7 +129,11 @@ int qemu_mknodat(int dirfd, const char *filename, mode_t mode, dev_t dev)
     if (pthread_fchdir_np(dirfd) < 0) {
         return -1;
     }
-    err = mknod(filename, mode, dev);
+    if (S_ISSOCK(mode)) {
+        err = create_socket_file_at_cwd(filename, mode);
+    } else {
+        err = mknod(filename, mode, dev);
+    }
     preserved_errno = errno;
     /* Stop using the thread-local cwd */
     pthread_fchdir_np(-1);