add setuid demo

lib_acl/samples/setuid/Makefile

@@ -0,0 +1,4 @@
+setuid: main.c
+	gcc -o setuid main.c -lpthread
+clean:
+	rm -f setuid

lib_acl/samples/setuid/main.c

@@ -0,0 +1,73 @@
+#include <sys/types.h>
+#include <unistd.h>
+#include <grp.h>
+#include <pwd.h>
+#include <pthread.h>
+#include <string.h>
+#include <errno.h>
+#include <stdio.h>
+
+static void* thread_main(void* ctx)
+{
+	time_t begin, now;
+	const char* user = (const char* ) ctx;
+
+	if (user && *user) {
+		struct passwd* pwd = getpwnam(user);
+		if (pwd == NULL) {
+			printf("getpwnam error=%s, user=$%s\r\n",
+				strerror(errno), user);
+			return NULL;
+		}
+		if (setgid(pwd->pw_gid) < 0) {
+			printf("setgid error=%s\r\n", strerror(errno));
+			return NULL;
+		}
+		if (initgroups(user, pwd->pw_gid) < 0) {
+			printf("initgroups error=%s\r\n", strerror(errno));
+			return NULL;
+		}
+
+		if (setuid(pwd->pw_uid) < 0) {
+			printf("setuid error=%s\r\n", strerror(errno));
+			return NULL;
+		}
+		printf("thread-%lu setuid ok\r\n", pthread_self());
+	}
+
+	printf("thread-%lu: sleep a while\r\n", pthread_self());
+	time(&begin);
+
+	while ((now = time(NULL)) <= begin + 5) {
+		sleep(1);
+	}
+
+	printf("thread-%lu: my uid=%ld, gid=%ld, will exit\r\n",
+		(long) getuid(), (long) getgid(), pthread_self());
+	return NULL;
+}
+
+#define MAX	10
+
+int main(int argc, char* argv[])
+{
+	char* user = "nobody";
+	pthread_t threads[MAX], tid;
+	int   i;
+
+	if (argc >= 2) {
+		user = argv[1];
+	}
+
+	for (i = 0; i < MAX; i++) {
+		pthread_create(&threads[i], NULL, thread_main, NULL);
+	}
+	sleep(1);
+	pthread_create(&tid, NULL, thread_main, user);
+
+	for (i = 0; i < MAX; i++) {
+		pthread_join(threads[i], NULL);
+	}
+	pthread_join(tid, NULL);
+	return 0;
+}