Add a "fork" of musl as //fusl.

fusl/src/stat/fchmodat.c

+#include <sys/stat.h>
+#include <fcntl.h>
+#include <errno.h>
+#include "syscall.h"
+
+void __procfdname(char *, unsigned);
+
+int fchmodat(int fd, const char *path, mode_t mode, int flag)
+{
+        if (!flag) return syscall(SYS_fchmodat, fd, path, mode, flag);
+
+        if (flag != AT_SYMLINK_NOFOLLOW)
+                return __syscall_ret(-EINVAL);
+
+        struct stat st;
+        int ret, fd2;
+        char proc[15+3*sizeof(int)];
+
+        if ((ret = __syscall(SYS_fstatat, fd, path, &st, flag)))
+                return __syscall_ret(ret);
+        if (S_ISLNK(st.st_mode))
+                return __syscall_ret(-EOPNOTSUPP);
+
+        if ((fd2 = __syscall(SYS_openat, fd, path, O_RDONLY|O_PATH|O_NOFOLLOW|O_NOCTTY|O_CLOEXEC)) < 0) {
+                if (fd2 == -ELOOP)
+                        return __syscall_ret(-EOPNOTSUPP);
+                return __syscall_ret(fd2);
+        }
+
+        __procfdname(proc, fd2);
+        ret = __syscall(SYS_fstatat, AT_FDCWD, proc, &st, 0);
+        if (!ret) {
+                if (S_ISLNK(st.st_mode)) ret = -EOPNOTSUPP;
+                else ret = __syscall(SYS_fchmodat, AT_FDCWD, proc, mode);
+        }
+
+        __syscall(SYS_close, fd2);
+        return __syscall_ret(ret);
+}