Cyberschool 2021 C

hook.c

#define _GNU_SOURCE
#include <stdio.h>
#include <dlfcn.h>

int strcmp(const char *a, const char *b) {
  printf("strcmp %s %s\n", a, b);
  ((int(*)(const char *, const char *))(dlsym(RTLD_NEXT, "strcmp")))(a, b);
}

main.c

#include <stdio.h>
#include <string.h>

int main() {
  FILE* f = fopen("/tmp/kek", "r");
  if (f == NULL) {
    fprintf(stderr, "fopen failed\n");
    return -1;
  }

  char buf[1024];
  fgets(buf, sizeof(buf), f);
  if (strcmp(buf, "[secret]\n")) {
    fprintf(stderr, "1\n");
    return -1;
  }

  fgets(buf, sizeof(buf), f);
  char *it = strchr(buf, '\n');
  if (it != 0) {
    *it = '\0';
  }
  if (strcmp(buf, "JLJPMzknyLbj39VaaYitC79KmXEYzH4wzdzafzCYdziskECfe4R7bUPUmq7Cqi3c")) {
    fprintf(stderr, "2\n");
    return -1;
  }

  it = buf;
  while (*it) {
    if (*it > '9') {
      *it ^= 0x20;
    }
    it++;
  }
  printf("CS{%s}\n", buf);
  return 0;
}

Makefile

.PHONY: nopie pie static hook clean

nopie:
	gcc -no-pie main.c -o main

pie:
	gcc main.c -o main

static:
	gcc -static main.c -o main

hook:
	gcc -shared hook.c -ldl -o libhook.so

clean:
	rm main libhook.so 2>/dev/null; true