classification/semantic_issues/gitlab_semantic_bextr


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x86 BEXTR semantic bug
Description of problem
The result of instruction BEXTR is different with from the CPU. The value of destination register is different. I think QEMU does not consider the operand size limit.

Steps to reproduce

Compile this code

void main() {
    asm("mov rax, 0x17b3693f77fb6e9");
    asm("mov rbx, 0x8f635a775ad3b9b4");
    asm("mov rcx, 0xb717b75da9983018");
    asm("bextr eax, ebx, ecx");
}

Execute and compare the result with the CPU.

CPU
RAX = 0x5a

QEMU
RAX = 0x635a775a

Additional information
This bug is discovered by research conducted by KAIST SoftSec.