[llvm-bugs] [Bug 39915] New: LLVMDisasmInstruction returns 1 for invalid instruction (x86 prefix only)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39915

            Bug ID: 39915
           Summary: LLVMDisasmInstruction returns 1 for invalid
                    instruction (x86 prefix only)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: MC
          Assignee: unassignedbugs at nondot.org
          Reporter: p.antoine at catenacyber.fr
                CC: llvm-bugs at lists.llvm.org

I am doing differential fuzzing between llvm-mc and capstone.

The following snippet of code has `LLVMDisasmInstruction` returning 1.
But the documentation http://llvm.org/doxygen/group__LLVMCDisassembler.html
states that it should return 0 if there was no valid instruction.
It looks to me that a x86 prefix without anything else is not a valid
instruction.

```
extern "C" int LLVMFuzzerReturnOneInput() {
    LLVMDisasmContextRef Ctx;
    uint8_t Data[1] = {0xf2};
    size_t Size = 1;
    char AssemblyText[80];
    std::vector<uint8_t> DataCopy(Data, Data + Size);
    uint8_t *p = DataCopy.data();
    int r = 1;

    Ctx = LLVMCreateDisasmCPUFeatures("x86_64", "", "", nullptr, 0, nullptr,
nullptr);
    if (LLVMSetDisasmOptions(Ctx, LLVMDisassembler_Option_AsmPrinterVariant) ==
0) {
        abort();
    }
    assert(Ctx);


    if (LLVMDisasmInstruction(Ctx, p, Size, 0, AssemblyText, 80) > 0) {
        r = 0;
    }
    LLVMDisasmDispose(Ctx);

    return r;
}
```

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