Added documentation and a simple example for the custom mutator functionality

custom_mutators/simple_mutator.c

@@ -0,0 +1,40 @@
+/*
+  Simple Custom Mutator for AFL
+
+  Written by Khaled Yakdan <yakdan@code-intelligence.de>
+
+  This a simple mutator that assumes that the generates messages starting with one
+  of the three strings GET, PUT, or DEL followed by a payload. The mutator randomly
+  selects a commend and mutates the payload of the seed provided as input.
+*/
+
+#include <stdint.h>
+#include <stdlib.h>
+#include <string.h>
+
+static const char *commands[] = {
+  "GET",
+  "PUT",
+  "DEL",
+};
+
+static size_t data_size = 100;
+
+size_t afl_custom_mutator (uint8_t *data, size_t size, uint8_t* mutated_out, size_t max_size, unsigned int seed) {
+
+  // Seed the PRNG
+  srand(seed);
+
+  // Make sure that the packet size does not exceed the maximum size expected by the fuzzer
+  size_t mutated_size = data_size <= max_size ? data_size : max_size;
+
+  // Randomly select a command string to add as a header to the packet
+  memcpy(mutated_out, commands[rand() % 3], 3);
+
+  // Mutate the payload of the packet
+  for (int i = 3 ; i < mutated_size ; i++) {
+    mutated_out[i] = (data[i] + rand() % 10) & 0xff;
+  }
+
+  return mutated_size;
+}