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121 lines
3.2 KiB
121 lines
3.2 KiB
/*
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Copyright 2015 Google LLC All rights reserved.
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at:
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License.
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*/
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/*
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american fuzzy lop - postprocessor for PNG
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------------------------------------------
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Written and maintained by Michal Zalewski <lcamtuf@google.com>
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See post_library.so.c for a general discussion of how to implement
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postprocessors. This specific postprocessor attempts to fix up PNG
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checksums, providing a slightly more complicated example than found
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in post_library.so.c.
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Compile with:
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gcc -shared -Wall -O3 post_library_png.so.c -o post_library_png.so -lz
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*/
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#include <stdio.h>
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#include <stdlib.h>
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#include <stdint.h>
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#include <string.h>
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#include <zlib.h>
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#include <arpa/inet.h>
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/* A macro to round an integer up to 4 kB. */
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#define UP4K(_i) ((((_i) >> 12) + 1) << 12)
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const unsigned char* afl_postprocess(const unsigned char* in_buf,
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unsigned int* len) {
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static unsigned char* saved_buf;
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static unsigned int saved_len;
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unsigned char* new_buf = (unsigned char*)in_buf;
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unsigned int pos = 8;
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/* Don't do anything if there's not enough room for the PNG header
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(8 bytes). */
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if (*len < 8) return in_buf;
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/* Minimum size of a zero-length PNG chunk is 12 bytes; if we
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don't have that, we can bail out. */
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while (pos + 12 <= *len) {
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unsigned int chunk_len, real_cksum, file_cksum;
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/* Chunk length is the first big-endian dword in the chunk. */
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chunk_len = ntohl(*(uint32_t*)(in_buf + pos));
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/* Bail out if chunk size is too big or goes past EOF. */
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if (chunk_len > 1024 * 1024 || pos + 12 + chunk_len > *len) break;
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/* Chunk checksum is calculated for chunk ID (dword) and the actual
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payload. */
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real_cksum = htonl(crc32(0, in_buf + pos + 4, chunk_len + 4));
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/* The in-file checksum is the last dword past the chunk data. */
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file_cksum = *(uint32_t*)(in_buf + pos + 8 + chunk_len);
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/* If the checksums do not match, we need to fix the file. */
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if (real_cksum != file_cksum) {
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/* First modification? Make a copy of the input buffer. Round size
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up to 4 kB to minimize the number of reallocs needed. */
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if (new_buf == in_buf) {
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if (*len <= saved_len) {
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new_buf = saved_buf;
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} else {
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new_buf = realloc(saved_buf, UP4K(*len));
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if (!new_buf) return in_buf;
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saved_buf = new_buf;
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saved_len = UP4K(*len);
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memcpy(new_buf, in_buf, *len);
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}
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}
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*(uint32_t*)(new_buf + pos + 8 + chunk_len) = real_cksum;
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}
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/* Skip the entire chunk and move to the next one. */
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pos += 12 + chunk_len;
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}
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return new_buf;
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}
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