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[C++] Bound var data against the remaining buffer so a uint64 length cannot wrap the position - #1175
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…cannot wrap the position. Compare dataLength with m_bufferLength - pos rather than checking pos + dataLength, which wraps for a uint64 length and lets the position move backwards. Do not go back to the addition.
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With a uint64 var data length, the position + length passed to sbePosition() can wrap, pass the bounds check and move the position backwards, so get(dst, len) copies past the buffer and skip() and () return as if the data were present.
get(), skip() and () now compare the length with bufferLength - position, which cannot wrap. getAsString() and getAsStringView() get the same check in #1160.
Part of #969.
Verified: VarDataUint64LengthTest fails on master (heap-buffer-overflow in getPayload() under ASan) and passes here; sbe-tool checkstyle and test, cppbuild and cppbuild --debug-build --sanitise-build pass.