Fix ARM64 sign extension helper#380
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detour_sign_extend should build the sign-extension mask from the source bit width, not from the amount used to shift the sign bit into position. Using 64 - bits places the mask at the wrong bit position and produces incorrect results for signed values whose sign bit is set. One affected path is ARM64 import thunk decoding, where negative ADRP page offsets can be decoded incorrectly.
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Fixes #296. I fixed this in my SlimDetours commit fc42391, and now create this PR to Detours.
detour_sign_extendshould build the sign-extension mask from the source bit width, not from the amount used to shift the sign bit into position.Using 64 - bits places the mask at the wrong bit position and produces incorrect results for signed values whose sign bit is set. One affected path is ARM64 import thunk decoding, where negative
ADRPpage offsets can be decoded incorrectly.Using the example from #296, with
value = 0x0ffea2e4andbits = 28, the sign bit is set, so the value should be sign-extended.The current code uses
m1 << left, whereleft = 64 - bits = 36:This is incorrect because the mask starts at bit 36. It should start at the source width, bit 28:
Therefore the sign mask should use
m1 << bits.