nebula level11 challenge

level11.c

#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <sys/mman.h>

/*
 * Return a random, non predictable file, and return the file descriptor for it.
 */

int getrand(char **path)
{
	char *tmp;
	int pid;
	int fd;

	srandom(time(NULL));

	tmp = getenv("TEMP");
	pid = getpid();
	
	asprintf(path, "%s/%d.%c%c%c%c%c%c", tmp, pid, 
		'A' + (random() % 26), '0' + (random() % 10), 
		'a' + (random() % 26), 'A' + (random() % 26),
		'0' + (random() % 10), 'a' + (random() % 26));

	fd = open(*path, O_CREAT|O_RDWR, 0600);
	unlink(*path);
	return fd;
}

void process(char *buffer, int length)
{
	unsigned int key;
	int i;

	key = length & 0xff;

	for(i = 0; i < length; i++) {
		buffer[i] ^= key;
		key -= buffer[i];
	}
        setgid(getgid());
        setuid(getuid());
	system(buffer);
}

#define CL "Content-Length: "

int main(int argc, char **argv)
{
	char line[256];
	char buf[1024];
	char *mem;
	int length;
	int fd;
	char *path;

	if(fgets(line, sizeof(line), stdin) == NULL) {
		errx(1, "reading from stdin");
	}

	if(strncmp(line, CL, strlen(CL)) != 0) {
		errx(1, "invalid header");
	}

	length = atoi(line + strlen(CL));
	
	if(length < sizeof(buf)) {
		if(fread(buf, length, 1, stdin) != length) {
			err(1, "fread length");
		}
		process(buf, length);
	} else {
		int blue = length;
		int pink;

		fd = getrand(&path);

		while(blue > 0) {
			printf("blue = %d, length = %d, ", blue, length);

			pink = fread(buf, 1, sizeof(buf), stdin);
			printf("pink = %d\n", pink);

			if(pink <= 0) {
				err(1, "fread fail(blue = %d, length = %d)", blue, length);
			}
			write(fd, buf, pink);

			blue -= pink;
		}	

		mem = mmap(NULL, length, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, 0);
		if(mem == MAP_FAILED) {
			err(1, "mmap");
		}
		process(mem, length);
	}

}