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Avoid potential overflow in num_words #945

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Avoid potential overflow in num_words #945

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eff49e6
Avoid potential overflow in num_words
yperbasis Jul 3, 2024
21b14a4
fix lint
yperbasis Jul 3, 2024
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3 changes: 2 additions & 1 deletion lib/evmone/instructions.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -60,7 +60,8 @@ constexpr auto word_size = 32;
/// i.e. it rounds up the number bytes to number of words.
inline constexpr int64_t num_words(uint64_t size_in_bytes) noexcept
{
return static_cast<int64_t>((size_in_bytes + (word_size - 1)) / word_size);
return static_cast<int64_t>(
size_in_bytes / word_size + static_cast<uint64_t>(size_in_bytes % word_size != 0));
}

/// Computes gas cost of copying the given amount of bytes to/from EVM memory.
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2 changes: 1 addition & 1 deletion test/state/precompiles.cpp
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Expand Up @@ -32,7 +32,7 @@ constexpr auto GasCostMax = std::numeric_limits<int64_t>::max();

inline constexpr int64_t num_words(size_t size_in_bytes) noexcept
{
return static_cast<int64_t>((size_in_bytes + 31) / 32);
return static_cast<int64_t>(size_in_bytes / 32 + static_cast<size_t>(size_in_bytes % 32 != 0));
}

template <int BaseCost, int WordCost>
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2 changes: 1 addition & 1 deletion test/state/state.cpp
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Expand Up @@ -20,7 +20,7 @@ constexpr auto MAX_INITCODE_COUNT = 256;

inline constexpr int64_t num_words(size_t size_in_bytes) noexcept
{
return static_cast<int64_t>((size_in_bytes + 31) / 32);
return static_cast<int64_t>(size_in_bytes / 32 + static_cast<size_t>(size_in_bytes % 32 != 0));
}

int64_t compute_tx_data_cost(evmc_revision rev, bytes_view data) noexcept
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15 changes: 15 additions & 0 deletions test/unittests/instructions_test.cpp
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Expand Up @@ -204,3 +204,18 @@ TEST(instructions, compare_with_evmc_instruction_names)
EXPECT_STREQ(instr::traits[i].name, evmc_tbl[i]);
}
}


TEST(instructions, num_words)
{
EXPECT_EQ(num_words(0), 0);
EXPECT_EQ(num_words(1), 1);
EXPECT_EQ(num_words(31), 1);
EXPECT_EQ(num_words(32), 1);
EXPECT_EQ(num_words(33), 2);
EXPECT_EQ(num_words(0xFFFFFFFFFFFFFFDF), 0x7FFFFFFFFFFFFFF);
EXPECT_EQ(num_words(0xFFFFFFFFFFFFFFE0), 0x7FFFFFFFFFFFFFF);
EXPECT_EQ(num_words(0xFFFFFFFFFFFFFFE1), 0x800000000000000);
EXPECT_EQ(num_words(0xFFFFFFFFFFFFFFFE), 0x800000000000000);
EXPECT_EQ(num_words(0xFFFFFFFFFFFFFFFF), 0x800000000000000);
}