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Fix integer overflow in analysis around gas cost of undefined instruction #93

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Jul 16, 2019
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Fix integer overflow in analysis around gas cost of undefined instruction #93

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27ef5b0
test: Add test for block cost becoming negative
chfast Jul 10, 2019
5db7b35
Test for incorrect analysis result
gumb0 Jul 16, 2019
b769597
Prevent integer overflow in analysis
gumb0 Jul 16, 2019
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3 changes: 2 additions & 1 deletion lib/evmone/analysis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -84,7 +84,8 @@ code_analysis analyze(
auto& instr = jumpdest ? analysis.instrs.back() : analysis.instrs.emplace_back(fns[c]);

auto metrics = instr_table[c];
block->gas_cost += metrics.gas_cost;
if (metrics.gas_cost > 0) // can be -1 for undefined instruction
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If we know this instruction will be hit, shouldn't the cost be set to max instead (e.g. out of gas)?

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I think then it would stop with EVMC_OUT_OF_GAS at the beginning of the block, instead of expected EVMC_UNDEFINED_INSTRUCTION

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I believe we should set it to 0 as this is real gas cost of executing it. Setting it to "max" is a nice hack, but I don't waste 64-bits in the table just for it.

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ethereum/evmc#356.

block->gas_cost += metrics.gas_cost;
auto stack_req = metrics.num_stack_arguments - block->stack_diff;
block->stack_diff += (metrics.num_stack_returned_items - metrics.num_stack_arguments);
block->stack_req = std::max(block->stack_req, stack_req);
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31 changes: 31 additions & 0 deletions test/unittests/evm_test.cpp
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Expand Up @@ -962,6 +962,37 @@ TEST_F(evm, undefined_instructions)
}
}

TEST_F(evm, undefined_instruction_analysis_overflow)
{
rev = EVMC_PETERSBURG;

auto undefined_opcode = evmc_opcode(0x0c);
auto code = bytecode{undefined_opcode};

execute(code);
EXPECT_EQ(result.status_code, EVMC_UNDEFINED_INSTRUCTION);
}

TEST_F(evm, undefined_instruction_block_cost_negative)
{
// For undefined instructions EVMC instruction tables have cost -1.
// If naively counted block costs can become negative.

const auto max_gas = std::numeric_limits<int64_t>::max();

const auto code1 = bytecode{} + "0f"; // Block cost -1.
execute(max_gas, code1);
EXPECT_STATUS(EVMC_UNDEFINED_INSTRUCTION);

const auto code2 = bytecode{} + OP_JUMPDEST + "c6" + "4b" + OP_STOP; // Block cost -1.
execute(max_gas, code2);
EXPECT_STATUS(EVMC_UNDEFINED_INSTRUCTION);

const auto code3 = bytecode{} + OP_ADDRESS + "2a" + "2b" + "2c" + "2d"; // Block cost -2.
execute(max_gas, code3);
EXPECT_STATUS(EVMC_UNDEFINED_INSTRUCTION);
}

TEST_F(evm, abort)
{
for (auto r = 0; r <= EVMC_MAX_REVISION; ++r)
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