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/*
american fuzzy lop++ - postprocessor for PNG
------------------------------------------
Originally written by Michal Zalewski
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
See post_library.so.c for a general discussion of how to implement
postprocessors. This specific postprocessor attempts to fix up PNG
checksums, providing a slightly more complicated example than found
in post_library.so.c.
Compile with:
gcc -shared -Wall -O3 post_library_png.so.c -o post_library_png.so -lz
*/
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <zlib.h>
#include <arpa/inet.h>
/* A macro to round an integer up to 4 kB. */
#define UP4K(_i) ((((_i) >> 12) + 1) << 12)
const unsigned char* afl_postprocess(const unsigned char* in_buf,
unsigned int* len) {
static unsigned char* saved_buf;
static unsigned int saved_len;
unsigned char* new_buf = (unsigned char*)in_buf;
unsigned int pos = 8;
/* Don't do anything if there's not enough room for the PNG header
(8 bytes). */
if (*len < 8) return in_buf;
/* Minimum size of a zero-length PNG chunk is 12 bytes; if we
don't have that, we can bail out. */
while (pos + 12 <= *len) {
unsigned int chunk_len, real_cksum, file_cksum;
/* Chunk length is the first big-endian dword in the chunk. */
chunk_len = ntohl(*(uint32_t*)(in_buf + pos));
/* Bail out if chunk size is too big or goes past EOF. */
if (chunk_len > 1024 * 1024 || pos + 12 + chunk_len > *len) break;
/* Chunk checksum is calculated for chunk ID (dword) and the actual
payload. */
real_cksum = htonl(crc32(0, in_buf + pos + 4, chunk_len + 4));
/* The in-file checksum is the last dword past the chunk data. */
file_cksum = *(uint32_t*)(in_buf + pos + 8 + chunk_len);
/* If the checksums do not match, we need to fix the file. */
if (real_cksum != file_cksum) {
/* First modification? Make a copy of the input buffer. Round size
up to 4 kB to minimize the number of reallocs needed. */
if (new_buf == in_buf) {
if (*len <= saved_len) {
new_buf = saved_buf;
} else {
new_buf = realloc(saved_buf, UP4K(*len));
if (!new_buf) return in_buf;
saved_buf = new_buf;
saved_len = UP4K(*len);
memcpy(new_buf, in_buf, *len);
}
}
*(uint32_t*)(new_buf + pos + 8 + chunk_len) = real_cksum;
}
/* Skip the entire chunk and move to the next one. */
pos += 12 + chunk_len;
}
return new_buf;
}
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