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

fusl/src/unistd/setxid.c

Index: fusl/src/unistd/setxid.c
diff --git a/fusl/src/unistd/setxid.c b/fusl/src/unistd/setxid.c
new file mode 100644
index 0000000000000000000000000000000000000000..0239f8afa95019ae9aa6dad5d79fcaf4eade139b
--- /dev/null
+++ b/fusl/src/unistd/setxid.c
@@ -0,0 +1,39 @@
+#include <unistd.h>
+#include <errno.h>
+#include "syscall.h"
+#include "libc.h"
+#include "pthread_impl.h"
+
+struct ctx {
+	int id, eid, sid;
+	int nr, err;
+};
+
+static void do_setxid(void *p)
+{
+	struct ctx *c = p;
+	if (c->err>0) return;
+	int ret = -__syscall(c->nr, c->id, c->eid, c->sid);
+	if (ret && !c->err) {
+		/* If one thread fails to set ids after another has already
+		 * succeeded, forcibly killing the process is the only safe
+		 * thing to do. State is inconsistent and dangerous. Use
+		 * SIGKILL because it is uncatchable. */
+		__block_all_sigs(0);
+		__syscall(SYS_kill, __syscall(SYS_getpid), SIGKILL);
+	}
+	c->err = ret;
+}
+
+int __setxid(int nr, int id, int eid, int sid)
+{
+	/* err is initially nonzero so that failure of the first thread does not
+	 * trigger the safety kill above. */
+	struct ctx c = { .nr = nr, .id = id, .eid = eid, .sid = sid, .err = -1 };
+	__synccall(do_setxid, &c);
+	if (c.err) {
+		if (c.err>0) errno = c.err;
+		return -1;
+	}
+	return 0;
+}