Add automaton parser (#1426)

* have compilable program

* enable read in file

* add hashmap usage

* add build hashmap; WIP; test if constructed correctly tomorrow

* add testcase to test hashmap

* add sorted symbols list

* build symbols dictionary

* clean up DEBUG

* successfully find automaton path

* fix all memory leaks

* test if automaton same with example

* able to iterate through files in a folder

* finish testing on one random queue
wip
- change macro values
- add bound checking

* add bound checking to program length

* add bound checking to program walk length

* add boundary check to terminal number, terminal lengths and program length

* commit test makefile

* add makefile

* able to add seeds to gramatron

* remove useless argument in automaton_parser

* add automaton parser to gramfuzz

* change build

* revert test.c to original state

* add makefile to test.c for testing

7 files changed, 486 insertions, 9 deletions

diff --git a/custom_mutators/gramatron/automaton-parser.c b/custom_mutators/gramatron/automaton-parser.c
new file mode 100644
index 00000000..3265e0cf
--- /dev/null
+++ b/custom_mutators/gramatron/automaton-parser.c
@@ -0,0 +1,367 @@
+#include "afl-fuzz.h"
+#include "automaton-parser.h"
+
+int free_terminal_arr(any_t placeholder, any_t item) {
+  struct terminal_arr* tmp = item;
+  free(tmp->start);
+  free(tmp);
+  return MAP_OK;
+}
+
+int compare_two_symbols(const void * a, const void * b) {
+    char* a_char = *(char **)a;
+    char* b_char = *(char **)b;
+    size_t fa = strlen(a_char);
+    size_t fb = strlen(b_char);
+    if (fa > fb) return -1;
+    else if (fa == fb) return 0;
+    else return 1;
+
+}
+
+// TODO: create a map
+// key: first character of a symbol, value: a list of symbols that starts with key, the list is sorted in descending order of the symbol lengths
+map_t create_first_char_to_symbols_hashmap(struct symbols_arr *symbols, struct symbols_arr *first_chars) {
+  map_t char_to_symbols = hashmap_new();
+  // TODO: free the allocated map
+  // sort the symbol_dict in descending order of the symbol lengths
+  qsort(symbols->symbols_arr, symbols->len, sizeof(char*), compare_two_symbols);
+  #ifdef DEBUG
+  printf("------ print after sort ------\n");
+  print_symbols_arr(symbols);
+  #endif
+  size_t i;
+  int r; // response from hashmap get and put
+  for (i = 0; i < symbols->len; i++) {
+    char* symbol_curr = symbols->symbols_arr[i];
+    // get first character from symbol_curr
+    char first_character[2];
+    first_character[0] = symbol_curr[0];
+    first_character[1] = '\0';
+    #ifdef DEBUG
+    printf("****** Current symbol is %s, its first character is %s ******\n", symbol_curr, first_character);
+    #endif
+    // key would be the first character of symbol_curr
+    // the value would be an array of chars
+    struct symbols_arr* associated_symbols;
+    r = hashmap_get(char_to_symbols, first_character, (any_t*)&associated_symbols);
+    if (!r) {
+      // append current symbol to existing array
+      #ifdef DEBUG
+      printf("****** First character %s is already in hashmap ******\n", first_character);
+      #endif
+      if(!add_element_to_symbols_arr(associated_symbols, symbol_curr, strlen(symbol_curr) + 1)) {
+        free_hashmap(char_to_symbols, &free_array_of_chars);
+        return NULL;
+      }
+    }
+    else {
+      // start a new symbols_arr
+      #ifdef DEBUG
+      printf("****** First character %s is not in hashmap ******\n", first_character);
+      #endif
+      struct symbols_arr* new_associated_symbols = create_array_of_chars();
+      strncpy(first_chars->symbols_arr[first_chars->len], first_character, 2); // 2 because one character plus the NULL byte
+      add_element_to_symbols_arr(new_associated_symbols, symbol_curr, strlen(symbol_curr) + 1);
+      r = hashmap_put(char_to_symbols, first_chars->symbols_arr[first_chars->len], new_associated_symbols);
+      first_chars->len++;
+      #ifdef DEBUG
+      if (r) {
+        printf("hashmap put failed\n");
+      }
+      else {
+        printf("hashmap put succeeded\n");
+      }
+      #endif
+    }
+  }
+  printf("****** Testing ******\n");
+  struct symbols_arr* tmp_arr;
+  char str[] = "i";
+  int t = hashmap_get(char_to_symbols, str, (any_t *)&tmp_arr);
+  if (!t)
+    print_symbols_arr(tmp_arr);
+  return char_to_symbols;
+}
+
+struct symbols_arr* create_array_of_chars() {
+  struct symbols_arr* ret = (struct symbols_arr*)malloc(sizeof(struct symbols_arr));
+  ret->len = 0; 
+  ret->symbols_arr = (char **)malloc(MAX_TERMINAL_NUMS * sizeof(char*));
+  size_t i;
+  for (i = 0; i < MAX_TERMINAL_NUMS; i++) {
+    ret->symbols_arr[i] = (char *)calloc(MAX_TERMINAL_LENGTH, sizeof(char));
+  }
+  return ret;
+}
+
+// map a symbol to a list of (state, trigger_idx)
+map_t create_pda_hashmap(state* pda, struct symbols_arr* symbols_arr) {
+  int state_idx, trigger_idx, r; // r is the return result for hashmap operation 
+  map_t m = hashmap_new();
+  // iterate over pda
+  for (state_idx = 0; state_idx < numstates; state_idx++) {
+    #ifdef DEBUG
+    printf("------ The state idx is %d ------\n", state_idx);
+    #endif
+    if (state_idx == final_state) continue;
+    state* state_curr = pda + state_idx;
+    for (trigger_idx = 0; trigger_idx < state_curr->trigger_len; trigger_idx++) {
+      #ifdef DEBUG
+      printf("------ The trigger idx is %d ------\n", trigger_idx);
+      #endif
+      trigger* trigger_curr = state_curr->ptr + trigger_idx;
+      char* symbol_curr = trigger_curr->term;
+      size_t symbol_len = trigger_curr->term_len;
+      struct terminal_arr* terminal_arr_curr;
+      r = hashmap_get(m, symbol_curr, (any_t*)&terminal_arr_curr);
+      if (r) {
+        // the symbol is not in the map
+        if (!add_element_to_symbols_arr(symbols_arr, symbol_curr, symbol_len+1)) {
+          // the number of symbols exceed maximual number
+          free_hashmap(m, &free_terminal_arr);
+          return NULL;
+        }
+        #ifdef DEBUG
+        printf("Symbol %s is not in map\n", symbol_curr);
+        #endif
+        struct terminal_arr* new_terminal_arr = (struct terminal_arr*)malloc(sizeof(struct terminal_arr));
+        new_terminal_arr->start = (struct terminal_meta*)calloc(numstates, sizeof(struct terminal_meta));
+        #ifdef DEBUG
+        printf("allocate new memory address %p\n", new_terminal_arr->start);
+        #endif
+        new_terminal_arr->start->state_name = state_idx;
+        new_terminal_arr->start->dest = trigger_curr->dest;
+        new_terminal_arr->start->trigger_idx = trigger_idx;
+        new_terminal_arr->len = 1;
+        #ifdef DEBUG
+        printf("Symbol %s is included in %zu edges\n", symbol_curr, new_terminal_arr->len);
+        #endif
+        r = hashmap_put(m, symbol_curr, new_terminal_arr);
+        #ifdef DEBUG
+        if (r) {
+          printf("hashmap put failed\n");
+        }
+        else {
+          printf("hashmap put succeeded\n");
+        }
+        #endif
+        // if symbol not already in map, it's not in symbol_dict, simply add the symbol to the array
+        // TODO: need to initialize symbol dict (calloc)
+      }
+      else {
+        // the symbol is already in map
+        // append to terminal array
+        // no need to touch start
+        #ifdef DEBUG
+        printf("Symbol %s is in map\n", symbol_curr);
+        #endif
+        struct terminal_meta* modify = terminal_arr_curr->start + terminal_arr_curr->len;
+        modify->state_name = state_idx;
+        modify->trigger_idx = trigger_idx;
+        modify->dest = trigger_curr->dest;
+        terminal_arr_curr->len++;
+        #ifdef DEBUG
+        printf("Symbol %s is included in %zu edges\n", symbol_curr, terminal_arr_curr->len);
+        #endif
+        // if symbol already in map, it's already in symbol_dict as well, no work needs to be done
+      }
+
+    }
+  }
+  return m;
+}
+
+void print_symbols_arr(struct symbols_arr* arr) {
+  size_t i;
+  printf("(");
+  for (i = 0; i < arr->len; i++) {
+    printf("%s", arr->symbols_arr[i]);
+    if (i != arr->len - 1) printf(",");
+  }
+  printf(")\n");
+}
+
+void free_hashmap(map_t m, int (*f)(any_t, any_t)) {
+  if (!m) {
+    printf("m map is empty\n");
+    return;
+  }
+  int r = hashmap_iterate(m, f, NULL);
+  #ifdef DEBUG
+  if (!r) printf("free hashmap items successfully!\n");
+  else printf("free hashmap items failed");
+  #endif
+  hashmap_free(m);
+}
+
+int free_array_of_chars(any_t placeholder, any_t item) {
+  if (!item) {
+    printf("item is empty\n");
+    return MAP_MISSING;
+  }
+  struct symbols_arr* arr = item;
+  size_t i;
+  for (i = 0; i < MAX_TERMINAL_NUMS; i++) {
+    free(arr->symbols_arr[i]);
+  }
+  free(arr->symbols_arr);
+  free(arr);
+  return MAP_OK;
+}
+
+void free_pda(state* pda) {
+  if (!pda) {
+    printf("pda is null\n");
+    return;
+  }
+  size_t i, j;
+  for (i = 0; i < numstates; i++) {
+    state* state_curr = pda + i;
+    for (j = 0; j < state_curr->trigger_len; j++) {
+      trigger* trigger_curr = state_curr->ptr + j;
+      free(trigger_curr->id);
+      free(trigger_curr->term);
+    }
+    free(state_curr->ptr);
+  }
+  free(pda);
+}
+
+int dfs(struct terminal_arr** tmp, const char* program, const size_t program_length, struct terminal_arr** res, size_t idx, int curr_state) {
+  if (*res) return 1; // 1 means successfully found a path
+  if (idx == program_length) {
+    // test if the last terminal points to the final state
+    if (curr_state != final_state) return 0;
+    *res = *tmp;
+    return 1;
+  }
+  if ((*tmp)->len == MAX_PROGRAM_WALK_LENGTH) {
+    printf("Reached maximum program walk length\n");
+    return 0;
+  }
+  char first_char[2];
+  first_char[0] = program[idx]; // first character of program
+  first_char[1] = '\0';
+  int r;
+  struct symbols_arr* matching_symbols;
+  r = hashmap_get(first_char_to_symbols_map, first_char, (any_t *)&matching_symbols);
+  if (r) {
+    printf("No symbols match the current character, abort!"); // hopefully won't reach this state
+    return 0;
+  }
+  size_t i;
+  bool matched = false;
+  for (i = 0; i < matching_symbols->len; i++) {
+    if (matched) break;
+    char *matching_symbol = matching_symbols->symbols_arr[i];
+    if (!strncmp(matching_symbol, program + idx, strlen(matching_symbol))) {
+      // there is a match
+      matched = true;
+      // find the possible paths of that symbol
+      struct terminal_arr* ta;
+      int r2 = hashmap_get(pda_map, matching_symbol, (any_t *)&ta);
+      if (!r2) {
+        // the terminal is found in the dictionary
+        size_t j;
+        for (j = 0; j < ta->len; j++) {
+          int state_name = (ta->start + j)->state_name;
+          if (state_name != curr_state) continue;
+          size_t trigger_idx = (ta->start + j)->trigger_idx;
+          int dest = (ta->start + j)->dest;
+          (*tmp)->start[(*tmp)->len].state_name = state_name;
+          (*tmp)->start[(*tmp)->len].trigger_idx = trigger_idx;
+          (*tmp)->start[(*tmp)->len].dest = dest;
+          (*tmp)->len++;
+          if (dfs(tmp, program, program_length, res, idx + strlen(matching_symbol), dest)) return 1;
+          (*tmp)->len--;
+        }
+      }
+      else {
+        printf("No path goes out of this symbol, abort!"); // hopefully won't reach this state
+        return 0;
+      }
+    }
+  }
+  return 0;
+  /*
+  1. First extract the first character of the current program
+  2. Match the possible symbols of that program
+  3. Find the possible paths of that symbol
+  4. Add to temporary terminal array
+  5. Recursion
+  6. Pop the path from the terminal array
+  7. - If idx reaches end of program, set tmp to res
+     - If idx is not at the end and nothing matches, the current path is not working, simply return 0
+  */
+}
+
+Array* constructArray(struct terminal_arr* terminal_arr, state* pda) {
+  Array * res = (Array *)calloc(1, sizeof(Array));
+  initArray(res, INIT_SIZE);
+  size_t i;
+  for (i = 0; i < terminal_arr->len; i ++) {
+    struct terminal_meta* curr = terminal_arr->start + i;
+    int state_name = curr->state_name;
+    int trigger_idx = curr->trigger_idx;
+    // get the symbol from pda
+    state* state_curr = pda + state_name;
+    trigger* trigger_curr = state_curr->ptr + trigger_idx;
+    char *symbol_curr = trigger_curr->term;
+    size_t symbol_curr_len = trigger_curr->term_len;
+    insertArray(res, state_name, symbol_curr, symbol_curr_len, trigger_idx);
+  }
+  return res;
+}
+
+Array* automaton_parser(const uint8_t *seed_fn) {
+    Array* parsed_res = NULL;
+    FILE* ptr;
+    ptr = fopen(seed_fn, "r");
+    if (ptr == NULL) {
+      printf("file can't be opened \n");
+      fclose(ptr);
+      return NULL;
+    }
+    char ch;
+    char program[MAX_PROGRAM_LENGTH];
+    int i = 0;
+    bool program_too_long = false;
+    do {
+      if (i == MAX_PROGRAM_LENGTH) {
+        // the maximum program length is reached
+        printf("maximum program length is reached, give up the current seed\n");
+        program_too_long = true;
+        break;
+      }
+      ch = fgetc(ptr);
+      program[i] = ch;
+      i ++;
+    } while (ch != EOF);
+    program[i-1] = '\0';
+    fclose(ptr);
+    if ((i == 1 && program[0] == '\0') || program_too_long) return NULL;
+    struct terminal_arr* arr_holder;
+    struct terminal_arr* dfs_res = NULL;
+    arr_holder = (struct terminal_arr*)calloc(1, sizeof(struct terminal_arr));
+    arr_holder->start = (struct terminal_meta*)calloc(MAX_PROGRAM_WALK_LENGTH, sizeof(struct terminal_meta));
+    int dfs_success = dfs(&arr_holder, program, strlen(program), &dfs_res, 0, init_state);
+    // printf("*** return value %d *** \n", dfs_success);
+    if (dfs_success) {
+      parsed_res = constructArray(dfs_res, pda);
+    }
+    free(arr_holder->start);
+    free(arr_holder);
+    return parsed_res;
+}
+
+// return 0 if fails
+// return 1 if succeeds
+int add_element_to_symbols_arr(struct symbols_arr* symbols_arr, char* symbol, size_t symbol_len) {
+  if (symbols_arr->len >= MAX_TERMINAL_NUMS || symbol_len >= MAX_TERMINAL_LENGTH) {
+    return 0;
+  }
+  strncpy(symbols_arr->symbols_arr[symbols_arr->len], symbol, symbol_len);
+  symbols_arr->len++;
+  return 1;
+}
\ No newline at end of file
diff --git a/custom_mutators/gramatron/automaton-parser.h b/custom_mutators/gramatron/automaton-parser.h
new file mode 100644
index 00000000..d67a1679
--- /dev/null
+++ b/custom_mutators/gramatron/automaton-parser.h
@@ -0,0 +1,74 @@
+#ifndef _AUTOMATON_PARSER_H
+#define _AUTOMATON_PARSER_H
+
+#define NUMINPUTS 500
+#define MAX_PROGRAM_LENGTH 20000
+#define MAX_PROGRAM_WALK_LENGTH 5000
+#define MAX_TERMINAL_NUMS 5000
+#define MAX_TERMINAL_LENGTH 1000
+#define MAX_PROGRAM_NAME_LENGTH 5000
+
+#include "gramfuzz.h"
+
+// represents an edge in the FSA
+struct terminal_meta {
+
+  int state_name;
+  int trigger_idx;
+  int dest;
+
+} ;
+
+// represents a set of edges 
+struct terminal_arr {
+
+  struct terminal_meta* start;
+  size_t len;
+
+} ;
+
+// essentially a string array
+struct symbols_arr {
+  char** symbols_arr;
+  size_t len;
+} ;
+
+struct symbols_arr* symbols; // symbols contain all the symbols in the language
+map_t pda_map; // a map that maps each symbol in the language to a set of edges 
+struct symbols_arr* first_chars; // an array of first characters, only temperary array
+map_t first_char_to_symbols_map; // a map that maps each first character to a set of symbols (the symbols are sorted in descending order)
+
+
+
+// freeing terminal arrays
+int free_terminal_arr(any_t placeholder, any_t item);
+
+// return a map that maps each symbol in the language to a set of edges 
+// populate symbols_arr with all the symbols in the language
+map_t create_pda_hashmap(state* pda, struct symbols_arr* symbols_arr);
+
+// print the string array
+void print_symbols_arr(struct symbols_arr* arr);
+
+// free hashmap
+// the function pointer contains function to free the values in the hashmap
+void free_hashmap(map_t m, int (*f)(any_t, any_t));
+
+// free string array
+int free_array_of_chars(any_t placeholder, any_t item);
+
+// free the pda
+void free_pda(state* pda);
+
+// create a string array
+struct symbols_arr* create_array_of_chars();
+
+map_t create_first_char_to_symbols_hashmap(struct symbols_arr *symbols, struct symbols_arr *first_chars);
+
+// return the automaton represented by the seed
+Array* automaton_parser(const uint8_t *seed_fn);
+
+int add_element_to_symbols_arr(struct symbols_arr* symbols_arr, char* symbol, size_t symbol_len);
+
+
+#endif
\ No newline at end of file
diff --git a/custom_mutators/gramatron/build_gramatron_mutator.sh b/custom_mutators/gramatron/build_gramatron_mutator.sh
index 9952e7f5..0638e3b2 100755
--- a/custom_mutators/gramatron/build_gramatron_mutator.sh
+++ b/custom_mutators/gramatron/build_gramatron_mutator.sh
@@ -125,7 +125,7 @@ else
   }
 fi
 
-test -d json-c/.git || { echo "[-] not checked out, please install git or check your internet connection." ; exit 1 ; }
+test -f json-c/.git || { echo "[-] not checked out, please install git or check your internet connection." ; exit 1 ; }
 echo "[+] Got json-c."
 
 test -e json-c/.libs/libjson-c.a || {
@@ -144,6 +144,6 @@ echo
 echo
 echo "[+] Json-c successfully prepared!"
 echo "[+] Builing gramatron now."
-$CC -O3 -g -fPIC -Wno-unused-result -Wl,--allow-multiple-definition -I../../include -o gramatron.so -shared -I. -I/prg/dev/include gramfuzz.c gramfuzz-helpers.c gramfuzz-mutators.c gramfuzz-util.c hashmap.c ../../src/afl-performance.o json-c/.libs/libjson-c.a || exit 1
+$CC -O3 -g -fPIC -Wno-unused-result -Wl,--allow-multiple-definition -I../../include -o gramatron.so -shared -I. -I/prg/dev/include gramfuzz.c gramfuzz-helpers.c gramfuzz-mutators.c gramfuzz-util.c hashmap.c automaton-parser.c ../../src/afl-performance.o json-c/.libs/libjson-c.a || exit 1
 echo
 echo "[+] gramatron successfully built!"
diff --git a/custom_mutators/gramatron/gramfuzz.c b/custom_mutators/gramatron/gramfuzz.c
index 9c9dbb43..ccdbbe60 100644
--- a/custom_mutators/gramatron/gramfuzz.c
+++ b/custom_mutators/gramatron/gramfuzz.c
@@ -9,6 +9,7 @@
 
 #include "afl-fuzz.h"
 #include "gramfuzz.h"
+#include "automaton-parser.h"
 
 #define MUTATORS 4  // Specify the total number of mutators
 
@@ -163,6 +164,11 @@ my_mutator_t *afl_custom_init(afl_state_t *afl, unsigned int seed) {
   if (automaton_file) {
 
     pda = create_pda(automaton_file);
+    symbols = create_array_of_chars();
+    pda_map = create_pda_hashmap((struct state*)pda, symbols);
+    print_symbols_arr(symbols);
+    first_chars = create_array_of_chars();
+    first_char_to_symbols_map = create_first_char_to_symbols_hashmap(symbols, first_chars);
 
   } else {
 
@@ -281,12 +287,25 @@ u8 afl_custom_queue_new_entry(my_mutator_t * data,
   // filename_new_queue,filename_orig_queue,automaton_fn);
 
   if (filename_orig_queue) {
-
-    write_input(data->mutated_walk, automaton_fn);
+    if (data->mutated_walk) {
+      write_input(data->mutated_walk, automaton_fn);
+    }
+    else {
+      Array* parsed_walk = automaton_parser(filename_new_queue);
+      if (!parsed_walk) PFATAL("Parser unsuccessful on %s", filename_new_queue);
+      write_input(parsed_walk, automaton_fn);
+      free(parsed_walk->start);
+      free(parsed_walk);
+    }
 
   } else {
 
-    new_input = gen_input(pda, NULL);
+    // TODO: try to parse the input seeds here, if they can be parsed, then generate the corresponding automaton file
+    // if not, then generate a new input
+    new_input = automaton_parser(filename_new_queue);
+    if (new_input == NULL) {
+      new_input = gen_input(pda, NULL);
+    }
     write_input(new_input, automaton_fn);
 
     // Update the placeholder file
@@ -328,6 +347,16 @@ uint8_t afl_custom_queue_get(my_mutator_t *data, const uint8_t *filename) {
 
   // get the filename
   u8 *        automaton_fn = alloc_printf("%s.aut", filename);
+  // find the automaton file, if the automaton file cannot be found, do not fuzz the current entry on the queue
+  FILE *fp;
+  fp = fopen(automaton_fn, "rb");
+  if (fp == NULL) {
+
+    printf("File '%s' does not exist, exiting. Would not fuzz current entry on the queue\n", automaton_fn);
+    return 0;
+
+  }
+
   IdxMap_new *statemap_ptr;
   terminal *  term_ptr;
   int         state;
@@ -424,6 +453,10 @@ void afl_custom_deinit(my_mutator_t *data) {
 
   free(data->mutator_buf);
   free(data);
-
+  free_hashmap(pda_map, &free_terminal_arr);
+  free_hashmap(first_char_to_symbols_map, &free_array_of_chars);
+  free_pda(pda);
+  free_array_of_chars(NULL, symbols); // free the array of symbols
+  free_array_of_chars(NULL, first_chars);
 }
 
diff --git a/custom_mutators/gramatron/hashmap.c b/custom_mutators/gramatron/hashmap.c
index 09715b87..4f97e085 100644
--- a/custom_mutators/gramatron/hashmap.c
+++ b/custom_mutators/gramatron/hashmap.c
@@ -151,7 +151,7 @@ static unsigned long crc32_tab[] = {
 
 /* Return a 32-bit CRC of the contents of the buffer. */
 
-unsigned long crc32(const unsigned char *s, unsigned int len) {
+unsigned long custom_crc32(const unsigned char *s, unsigned int len) {
 
   unsigned int  i;
   unsigned long crc32val;
@@ -171,8 +171,9 @@ unsigned long crc32(const unsigned char *s, unsigned int len) {
  * Hashing function for a string
  */
 unsigned int hashmap_hash_int(hashmap_map *m, char *keystring) {
+  unsigned int keystring_len = strlen(keystring);
 
-  unsigned long key = crc32((unsigned char *)(keystring), strlen(keystring));
+  unsigned long key = custom_crc32((unsigned char *)(keystring), keystring_len);
 
   /* Robert Jenkins' 32 bit Mix Function */
   key += (key << 12);
diff --git a/custom_mutators/gramatron/testMakefile.mk b/custom_mutators/gramatron/testMakefile.mk
new file mode 100644
index 00000000..0b2c6236
--- /dev/null
+++ b/custom_mutators/gramatron/testMakefile.mk
@@ -0,0 +1,2 @@
+test: test.c
+	gcc -g -fPIC -Wno-unused-result -Wl,--allow-multiple-definition -I../../include -o test -I. -I/prg/dev/include test.c gramfuzz-helpers.c gramfuzz-mutators.c gramfuzz-util.c hashmap.c ../../src/afl-performance.o json-c/.libs/libjson-c.a
\ No newline at end of file
diff --git a/custom_mutators/grammar_mutator/build_grammar_mutator.sh b/custom_mutators/grammar_mutator/build_grammar_mutator.sh
index 15b8b1db..e8594ba3 100755
--- a/custom_mutators/grammar_mutator/build_grammar_mutator.sh
+++ b/custom_mutators/grammar_mutator/build_grammar_mutator.sh
@@ -119,7 +119,7 @@ else
   }
 fi
 
-test -d grammar_mutator/.git || { echo "[-] not checked out, please install git or check your internet connection." ; exit 1 ; }
+test -f grammar_mutator/.git || { echo "[-] not checked out, please install git or check your internet connection." ; exit 1 ; }
 echo "[+] Got grammar mutator."
 
 cd "grammar_mutator" || exit 1