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

fusl/src/unistd/dup3.c

+#define _GNU_SOURCE
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include "syscall.h"
+#include "libc.h"
+
+int __dup3(int old, int new, int flags)
+{
+        int r;
+#ifdef SYS_dup2
+        if (old==new) return __syscall_ret(-EINVAL);
+        if (flags & O_CLOEXEC) {
+                while ((r=__syscall(SYS_dup3, old, new, flags))==-EBUSY);
+                if (r!=-ENOSYS) return __syscall_ret(r);
+        }
+        while ((r=__syscall(SYS_dup2, old, new))==-EBUSY);
+        if (flags & O_CLOEXEC) __syscall(SYS_fcntl, new, F_SETFD, FD_CLOEXEC);
+#else
+        while ((r=__syscall(SYS_dup3, old, new, flags))==-EBUSY);
+#endif
+        return __syscall_ret(r);
+}
+
+weak_alias(__dup3, dup3);