#include <fcntl.h>
#include <pwd.h>
#include <stdio.h>
#include <sys/prctl.h>
#include <sys/resource.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>

int main(int argc, char* argv[])
{
	if (argc < 3) {
		fprintf(stderr, "usage: %s USERNAME FILE\n", argv[0]);
		return 1;
	}

	// switch user (requires root or "setcap cap_setuid,cap_setgid+ep")
	char const* username = argv[1];
	struct passwd const* pw = getpwnam(username);
	if (!pw) {
		fprintf(stderr, "no such user: %s\n", username);
		return 1;
	}
	int ret = 0;
	if ((ret = setuid(pw->pw_uid)) != 0)
		fprintf(stderr, "setuid returned %d\n", ret);
	if ((ret = setgid(pw->pw_gid)) != 0)
		fprintf(stderr, "setgid returned %d\n", ret);

	// limit CPU time to 1 second
	struct rlimit const cpu_limit = { .rlim_cur = 1, .rlim_max = 1 };
	if ((ret = setrlimit(RLIMIT_CPU, &cpu_limit)) != 0)
		fprintf(stderr, "setrlimit(CPU) returned %d\n", ret);

	// don't allow writing files of any size
	struct rlimit const fsize_limit = { .rlim_cur = 0, .rlim_max = 0 };
	if ((ret = setrlimit(RLIMIT_FSIZE, &fsize_limit)) != 0)
		fprintf(stderr, "setrlimit(FSIZE) returned %d\n", ret);

	// there will be no fork
	struct rlimit const nproc_limit = { .rlim_cur = 0, .rlim_max = 0 };
	if ((ret = setrlimit(RLIMIT_NPROC, &nproc_limit)) != 0)
		fprintf(stderr, "setrlimit(NPROC) returned %d\n", ret);

	char* const args[] = { argv[2], (char*)0 };
	return execvp(argv[2], args);
}