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+/*
+   american fuzzy lop++ - postprocessor library example
+   --------------------------------------------------
+
+   Originally written by Michal Zalewski
+   Edited by Dominik Maier, 2020
+
+   Copyright 2015 Google Inc. All rights reserved.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at:
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+   Postprocessor libraries can be passed to afl-fuzz to perform final cleanup
+   of any mutated test cases - for example, to fix up checksums in PNG files.
+
+   Please heed the following warnings:
+
+   1) In almost all cases, it is more productive to comment out checksum logic
+      in the targeted binary (as shown in ../libpng_no_checksum/). One possible
+      exception is the process of fuzzing binary-only software in QEMU mode.
+
+   2) The use of postprocessors for anything other than checksums is
+   questionable and may cause more harm than good. AFL is normally pretty good
+   about dealing with length fields, magic values, etc.
+
+   3) Postprocessors that do anything non-trivial must be extremely robust to
+      gracefully handle malformed data and other error conditions - otherwise,
+      they will crash and take afl-fuzz down with them. Be wary of reading past
+      *len and of integer overflows when calculating file offsets.
+
+   In other words, THIS IS PROBABLY NOT WHAT YOU WANT - unless you really,
+   honestly know what you're doing =)
+
+   With that out of the way: the postprocessor library is passed to afl-fuzz
+   via AFL_POST_LIBRARY. The library must be compiled with:
+
+     gcc -shared -Wall -O3 post_library.so.c -o post_library.so
+
+   AFL will call the afl_custom_post_process() function for every mutated output
+   buffer. From there, you have three choices:
+
+   1) If you don't want to modify the test case, simply set `*out_buf = in_buf`
+      and return the original `len`.
+
+   2) If you want to skip this test case altogether and have AFL generate a
+      new one, return 0 or set `*out_buf = NULL`.
+      Use this sparingly - it's faster than running the target program
+      with patently useless inputs, but still wastes CPU time.
+
+   3) If you want to modify the test case, allocate an appropriately-sized
+      buffer, move the data into that buffer, make the necessary changes, and
+      then return the new pointer as out_buf. Return an appropriate len
+   afterwards.
+
+      Note that the buffer will *not* be freed for you. To avoid memory leaks,
+      you need to free it or reuse it on subsequent calls (as shown below).
+
+      *** Feel free to reuse the original 'in_buf' BUFFER and return it. ***
+
+    Aight. The example below shows a simple postprocessor that tries to make
+    sure that all input files start with "GIF89a".
+
+    PS. If you don't like C, you can try out the unix-based wrapper from
+    Ben Nagy instead: https://github.com/bnagy/aflfix
+
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+/* Header that must be present at the beginning of every test case: */
+
+#define HEADER "GIF89a"
+
+typedef struct post_state {
+
+  unsigned char *buf;
+  size_t         size;
+
+} post_state_t;
+
+void *afl_custom_init(void *afl) {
+
+  post_state_t *state = malloc(sizeof(post_state_t));
+  if (!state) {
+
+    perror("malloc");
+    return NULL;
+
+  }
+
+  state->buf = calloc(sizeof(unsigned char), 4096);
+  if (!state->buf) {
+
+    free(state);
+    perror("calloc");
+    return NULL;
+
+  }
+
+  return state;
+
+}
+
+/* The actual postprocessor routine called by afl-fuzz: */
+
+size_t afl_custom_post_process(post_state_t *data, unsigned char *in_buf,
+                               unsigned int len, unsigned char **out_buf) {
+
+  /* Skip execution altogether for buffers shorter than 6 bytes (just to
+     show how it's done). We can trust len to be sane. */
+
+  if (len < strlen(HEADER)) return 0;
+
+  /* Do nothing for buffers that already start with the expected header. */
+
+  if (!memcmp(in_buf, HEADER, strlen(HEADER))) {
+
+    *out_buf = in_buf;
+    return len;
+
+  }
+
+  /* Allocate memory for new buffer, reusing previous allocation if
+     possible. */
+
+  *out_buf = realloc(data->buf, len);
+
+  /* If we're out of memory, the most graceful thing to do is to return the
+     original buffer and give up on modifying it. Let AFL handle OOM on its
+     own later on. */
+
+  if (!*out_buf) {
+
+    *out_buf = in_buf;
+    return len;
+
+  }
+
+  /* Copy the original data to the new location. */
+
+  memcpy(*out_buf, in_buf, len);
+
+  /* Insert the new header. */
+
+  memcpy(*out_buf, HEADER, strlen(HEADER));
+
+  /* Return the new len. It hasn't changed, so it's just len. */
+
+  return len;
+
+}
+
+/* Gets called afterwards */
+void afl_custom_deinit(post_state_t *data) {
+
+  free(data->buf);
+  free(data);
+
+}
+