longjmp over different process invocations

gistfile1.txt

#include <setjmp.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

static char encoding_table[] = {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
                                'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
                                'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
                                'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f',
                                'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n',
                                'o', 'p', 'q', 'r', 's', 't', 'u', 'v',
                                'w', 'x', 'y', 'z', '0', '1', '2', '3',
                                '4', '5', '6', '7', '8', '9', '+', '/'};
static char *decoding_table = NULL;
static unsigned int mod_table[] = {0, 2, 1};


char *base64_encode(const unsigned char *    , size_t             , size_t *             );
char *base64_encode(const unsigned char *data, size_t input_length, size_t *output_length) {

    *output_length = 4 * ((input_length + 2) / 3);

    char *encoded_data = malloc(*output_length);
    if (encoded_data == NULL) return NULL;

    for (unsigned int i = 0, j = 0; i < input_length;) {

        uint32_t octet_a = i < input_length ? (unsigned char)data[i++] : 0;
        uint32_t octet_b = i < input_length ? (unsigned char)data[i++] : 0;
        uint32_t octet_c = i < input_length ? (unsigned char)data[i++] : 0;

        uint32_t triple = (octet_a << 0x10) + (octet_b << 0x08) + octet_c;

        encoded_data[j++] = encoding_table[(triple >> 3 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 2 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 1 * 6) & 0x3F];
        encoded_data[j++] = encoding_table[(triple >> 0 * 6) & 0x3F];
    }

    for (unsigned int i = 0; i < mod_table[input_length % 3]; i++)
        encoded_data[*output_length - 1 - i] = '=';

    return encoded_data;
}


void build_decoding_table(void);
void build_decoding_table() {

    decoding_table = malloc(256);

    for (size_t i = 0; i < 64; i++)
        decoding_table[(unsigned char) encoding_table[i]] = (char) i;
}


unsigned char *base64_decode(const char *    , size_t             , size_t *             );
unsigned char *base64_decode(const char *data, size_t input_length, size_t *output_length) {

    if (decoding_table == NULL) build_decoding_table();

    if (input_length % 4 != 0) return NULL;

    *output_length = input_length / 4 * 3;
    if (data[input_length - 1] == '=') (*output_length)--;
    if (data[input_length - 2] == '=') (*output_length)--;

    unsigned char *decoded_data = malloc(*output_length);
    if (decoded_data == NULL) return NULL;

    for (unsigned int i = 0, j = 0; i < input_length;) {

        uint32_t sextet_a = data[i] == '=' ? 0 & i++ : (unsigned) decoding_table[data[i++]];
        uint32_t sextet_b = data[i] == '=' ? 0 & i++ : (unsigned) decoding_table[data[i++]];
        uint32_t sextet_c = data[i] == '=' ? 0 & i++ : (unsigned) decoding_table[data[i++]];
        uint32_t sextet_d = data[i] == '=' ? 0 & i++ : (unsigned) decoding_table[data[i++]];

        uint32_t triple = (sextet_a << 3 * 6)
        + (sextet_b << 2 * 6)
        + (sextet_c << 1 * 6)
        + (sextet_d << 0 * 6);

        if (j < *output_length) decoded_data[j++] = (triple >> 2 * 8) & 0xFF;
        if (j < *output_length) decoded_data[j++] = (triple >> 1 * 8) & 0xFF;
        if (j < *output_length) decoded_data[j++] = (triple >> 0 * 8) & 0xFF;
    }

    return decoded_data;
}


void base64_cleanup(void);
void base64_cleanup() {
    free(decoding_table);
}

int main(void) {
  size_t output_length;
  const char* q = getenv("QUERY_STRING");

  freopen("/dev/stdout", "w", stderr);
  printf("Content-Type: text/html\n\n<div style='white-space:pre'>");
  fflush(stdout);
  if (q && strlen(q)) {
    jmp_buf dest;
    build_decoding_table();
    unsigned char* buf = base64_decode((void*) q, strlen(q), &output_length);

    puts(q);
    fflush(stdout);
    memcpy(dest, buf, output_length);
    longjmp(dest, 1);
  } else {
    jmp_buf link1;
    if (!setjmp(link1)) {
      const char* b64 = base64_encode((unsigned char*) &link1, sizeof(jmp_buf), &output_length);
      printf("<a href='a.out?");
      fwrite(b64, sizeof(char), output_length, stdout);
      //printf("%.*s", output_length, b64);
      printf("'>");
      fwrite(b64, sizeof(char), output_length, stdout);
      //printf("%.*s", output_length, b64);
      printf("</a>");
      free((void*) b64);
    } else {
      printf("you pressed the link\n");
    }
  }
  base64_cleanup();
  return 0;
}