readflag.c

#include <time.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <errno.h>

long long stack[100];
int stack_pointer = 0;

void goterror(const char *t)
{
    perror(t);
}

static int
start_subprocess(char *command[], int *pid, int *infd, int *outfd)
{
    int p1[2], p2[2];

    if (!pid || !infd || !outfd)
        return 0;

    if (pipe(p1) == -1)
        goto err_pipe1;
    if (pipe(p2) == -1)
        goto err_pipe2;
    if ((*pid = fork()) == -1)
        goto err_fork;

    if (*pid)
    {
        /* Parent process. */
        *infd = p1[1];
        *outfd = p2[0];
        close(p1[0]);
        close(p2[1]);
        return 1;
    }
    else
    {
        /* Child process. */
        dup2(p1[0], 0);
        dup2(p2[1], 1);
        close(p1[0]);
        close(p1[1]);
        close(p2[0]);
        close(p2[1]);
        execvp(*command, command);
        /* Error occured. */
        fprintf(stderr, "error running %s: %s", *command, strerror(errno));
        abort();
    }

err_fork:
    close(p2[1]);
    close(p2[0]);
err_pipe2:
    close(p1[1]);
    close(p1[0]);
err_pipe1:
    return 0;
}

int seed;

int exp_rand()
{
    int v0; // eax@1

    ++seed;
    v0 = time(0LL);
    srand(v0 + seed * seed);
    return (rand() % 2);
}

int randval[4];
int signs[4];

void gen_str(char *dst)
{
    randval[0] = exp_rand();
    randval[1] = exp_rand();
    randval[2] = exp_rand();
    randval[3] = exp_rand();
    signs[0] = (randval[0] == 1) ? -1 : 1;
    signs[1] = (randval[1] == 1) ? -1 : 1;
    signs[2] = (randval[2] == 1) ? -1 : 1;
    signs[3] = (randval[3] == 1) ? -1 : 1;
}

long long readnum(int infd)
{
    long long sign = 1;
    char x;
    long long val = 0;
    read(infd, &x, 1);
    if (x == '-')
    {
        sign = -1;
        read(infd, &x, 1);
    }
    while (x != ')')
    {
        val *= 10;
        val += (x - '0');
        read(infd, &x, 1);
    }
    return val * sign;
}

int main()
{
    char fmtstr[1000];
    gen_str(fmtstr);

    int pid, infd, outfd;
    char *cmd[2];
    cmd[0] = "/readflag";
    cmd[1] = 0;
    start_subprocess(cmd, &pid, &outfd, &infd);

    char buf1[1000];
    char buf[1000];

    read(infd, buf1, strlen("Solve the easy challenge first\n"));

    long long a, b, c, d, e;

    read(infd, buf1, strlen("((((("));
    a = readnum(infd);
    read(infd, buf1, strlen("+("));
    b = signs[0] * readnum(infd);
    read(infd, buf1, strlen(")+("));
    c = signs[1] * readnum(infd);
    read(infd, buf1, strlen(")+("));
    d = signs[2] * readnum(infd);
    read(infd, buf1, strlen(")+("));
    e = signs[3] * readnum(infd);

    long long v = a + b + c + d + e;
    char v_str[1000];
    sprintf(v_str, "%lld\n", v);

    write(outfd, v_str, strlen(v_str));
    read(infd, buf1, 1000);
    puts(buf1);
    read(infd, buf1, 1000);
    puts(buf1);
    read(infd, buf1, 1000);
    puts(buf1);

    return 0;
}