| /* |
| american fuzzy lop - postprocessor for PNG |
| ------------------------------------------ |
| |
| Written and maintained by Michal Zalewski <lcamtuf@google.com> |
| |
| 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; |
| |
| } |