syscall.Exec losing setuid

a_c.c

#include <unistd.h>
#include <stdio.h>

int main() {
    int rv = execl("./b", "./b", NULL);
    printf("WHAAT! %d\n", rv);
    return 0;
}

a_go.go

package main

//
import "C"

import (
	"syscall"
)

func main() {
	syscall.Exec("./b", []string{"./b"}, nil)
}

a_p.c

#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#include <time.h>

void* nothing(void *p) {
    return NULL;
}

void run_do_nothing_thread() {
    pthread_t t;
    if (pthread_create(&t, NULL, nothing, NULL) == 0) {
        pthread_join(t, NULL);
    }
}

void* target(void *p) {
    while (1) {
        run_do_nothing_thread();
    }
    return NULL;
}

void run_spinner_thread() {
    pthread_t t;
    pthread_create(&t, NULL, target, NULL);
}

int main() {
    struct timespec tv;
    for (int i = 0; i < 10; i++) {
        run_spinner_thread();
    }
    tv.tv_sec = 0;
    tv.tv_nsec = 100000;
    nanosleep(&tv, NULL);
    int rv = execl("./b", "./b", NULL);
    printf("WHAAT! %d\n", rv);
    return 0;
}

b_bare.c

#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>

int main() {
    uid_t id = geteuid();
    if (id != 0) {
        printf("GOT %d\n", id);
        return 1;
    }
    return 0;
}

Makefile

ALL=a_c a_p a_go b
all: $(ALL)

CFLAGS=-Wall

a_p : LDFLAGS=-pthread

%: %.go
	go build -o $@ $<

clean:
	rm -vf $(ALL) b_bare

b: b_bare
	sudo cp -v b_bare b
	sudo chmod u+s b