#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/capability.h>
#include <sys/prctl.h>

struct caph {
	int version;
	int pid;
};

struct capd {
	__u32 effective;
	__u32 permitted;
	__u32 inheritable;
};

gid_t gid = 1000;
uid_t uid = 1000;


void printcap(char *str)
{
	int ret;
	struct caph caph;
	struct capd capd;
	
	memset(&capd, 0, sizeof(capd));	
	
	caph.version = 0x19980330;
	caph.pid = getpid();

	ret = capget(&caph, &capd);
	if (ret < 0)
		perror("capget");
	printf("%s: ", str);
	printf("eff:%u perm:%u inh:%u\n", capd.effective, capd.permitted, capd.inheritable);
}

void lower_uid()
{
	int ret;
	gid_t gidlist[1];
	
	gidlist[0] = gid;
	ret = setgroups(1, gidlist);
	if (ret < 0)
		perror("setgroups");
	if (setgid(gid))
		perror("setgid");
	if (setuid(uid))
		perror("setuid");
}

const char *bash[] = { "bash", NULL };

int main(int argc, char **argv)
{
	int ret;
	struct caph caph;
	struct capd capd;

	if (argc == 3) {
		uid = strtoul(argv[1], NULL, 0);
		gid = strtoul(argv[2], NULL, 0);
	} else {
		printf("Usage: \"./program uid gid\" where uid & gid\n");
		printf("are the ids you want to change to (then run bash)\n");
		exit(1);
	}

	printcap("caps when started");

	memset(&capd, 0, sizeof(capd));	

	caph.version = 0x19980330;
	caph.pid = getpid();

	ret = capget(&caph, &capd);
	if (ret < 0)
		perror("capget");
	
	capd.inheritable |= (1 << CAP_CHOWN);
	
	ret = capset(&caph, &capd);
	if (ret < 0)
		perror("capset");

	printf("new inheritable mask: %u\n", capd.inheritable);
	
	ret = prctl(PR_SET_KEEPCAPS, 1);
	if (ret)
		perror("prctl");
	
	lower_uid();

	printcap("after setuid");
        
	ret = execv("/bin/bash", bash);
	if (ret < 0)
		perror("execve");
		
	return 0;
}