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post_library.so.c

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+/*
+  Copyright 2015 Google LLC 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
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+*/
+
+/*
+   american fuzzy lop - postprocessor library example
+   --------------------------------------------------
+
+   Written and maintained by Michal Zalewski <lcamtuf@google.com>
+
+   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_postprocess() function for every mutated output buffer.
+   From there, you have three choices:
+
+   1) If you don't want to modify the test case, simply return the original
+      buffer pointer ('in_buf').
+
+   2) If you want to skip this test case altogether and have AFL generate a
+      new one, return 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. You can update *len if necessary, too.
+
+      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).
+
+      *** DO NOT MODIFY THE ORIGINAL 'in_buf' BUFFER. ***
+
+    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"
+
+/* The actual postprocessor routine called by afl-fuzz: */
+
+const unsigned char* afl_postprocess(const unsigned char* in_buf,
+                                     unsigned int* len) {
+
+  static unsigned char* saved_buf;
+  unsigned char* new_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 NULL;
+
+  /* Do nothing for buffers that already start with the expected header. */
+
+  if (!memcmp(in_buf, HEADER, strlen(HEADER))) return in_buf;
+
+  /* Allocate memory for new buffer, reusing previous allocation if
+     possible. */
+
+  new_buf = realloc(saved_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 (!new_buf) return in_buf;
+  saved_buf = new_buf;
+
+  /* Copy the original data to the new location. */
+
+  memcpy(new_buf, in_buf, *len);
+
+  /* Insert the new header. */
+
+  memcpy(new_buf, HEADER, strlen(HEADER));
+
+  /* Return modified buffer. No need to update *len in this particular case,
+     as we're not changing it. */
+
+  return new_buf;
+
+}