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578eeb702e
It is UB to exceed the bounds of the buffer when doing pointer
arithemetic. That means the following is not a valid bounds check:
if (start + 4 <= limit)
Because if we were at the end of the buffer, we wouldn't be
allowed to add 4 anyway. Instead, this must be written as:
if (limit - start >= 4)
Basic forms of this issue are flagged by UBSan. If building with
-fsanitize=undefined, the following test trips an error:
[ RUN ] HASH.SignedUnsignedIssue
.../leveldb/util/hash.cc:30:15: runtime error: applying non-zero offset 4 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/local/google/home/davidben/leveldb/util/hash.cc:30:15 in
[ OK ] HASH.SignedUnsignedIssue (1 ms)
56 lines
1.4 KiB
C++
56 lines
1.4 KiB
C++
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#include "util/hash.h"
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#include <cstring>
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#include "util/coding.h"
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// The FALLTHROUGH_INTENDED macro can be used to annotate implicit fall-through
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// between switch labels. The real definition should be provided externally.
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// This one is a fallback version for unsupported compilers.
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#ifndef FALLTHROUGH_INTENDED
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#define FALLTHROUGH_INTENDED \
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do { \
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} while (0)
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#endif
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namespace leveldb {
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uint32_t Hash(const char* data, size_t n, uint32_t seed) {
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// Similar to murmur hash
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const uint32_t m = 0xc6a4a793;
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const uint32_t r = 24;
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const char* limit = data + n;
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uint32_t h = seed ^ (n * m);
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// Pick up four bytes at a time
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while (limit - data >= 4) {
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uint32_t w = DecodeFixed32(data);
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data += 4;
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h += w;
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h *= m;
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h ^= (h >> 16);
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}
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// Pick up remaining bytes
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switch (limit - data) {
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case 3:
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h += static_cast<uint8_t>(data[2]) << 16;
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FALLTHROUGH_INTENDED;
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case 2:
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h += static_cast<uint8_t>(data[1]) << 8;
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FALLTHROUGH_INTENDED;
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case 1:
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h += static_cast<uint8_t>(data[0]);
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h *= m;
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h ^= (h >> r);
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break;
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}
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return h;
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}
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} // namespace leveldb
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