[BUG] jmp far & call far (FF25 00000000 ptr & FF15 02000000 ptr)

[kkptm]

[BUG] jmp far & call far (FF25 00000000 ptr & FF15 02000000 ptr)

[kobalicek]

Can you please elaborate?

I don't see a bug anywhere right now. AsmJit assembles far jumps as other assemblers via ljmp and lcall instructions.

[kkptm]

    Assembler assembler;
    CodeHolder asmcode;
    asmcode.reset();
    asmcode.init(asmjit::Environment(Arch::kX64), 0);
    asmcode.attach(&assembler);
    assembler.addDiagnosticOptions(asmjit::DiagnosticOptions::kValidateAssembler);
    {
	    assembler.jmp(0xA123456789);
	    assembler.call(0xA123456789);
    }
    asmjit::String encodedOpcode;
    asmjit::Section* text = asmcode.textSection();
    text->buffer()[0];
    encodedOpcode.appendHex(text->data(), text->bufferSize());
    printf(encodedOpcode.data());

it outputs:
40 E9 00000000
40 E8 00000000

[kkptm]

the correct absolute address jump in 64-bit programs should be:
FF25 00 00 00 00 [64bit address]
and absolute address call:
FF15 02 00 00 00 [64bit address]

[kobalicek]

FF25/FF15 encodes jmp|call r64/m64 forms - you can have that if you use jmp [mem] form of the instruction. When you specify absolute address as an immediate AsmJit bails to relative encoding and in some cases it would output a relocable code so it can patch it later.

In your case there is REX prefix so it can patch the instruction to FFxx form later when relative addressing would be out of reach.

[kkptm]

so it is

[kobalicek]

I'm going to clarify this.

The instruction sequences proposed are not instructions, but they are instructions or multiple instructions + data. To do a jump with absolute address that is stored next to the jump, you can simply do:

Label L = assembler->newLabel();
assembler->jmp(x86::mem(L));
assembler->bind(L);
assembler->dq(uint64_t(address));

Alternatively, without a label with RIP addressing:

assembler->jmp(x86::ptr(x86::rip));
assembler->dq(uint64_t(address));

The thing is that I don't want to create a macro-assembler from AsmJit as that would be much more confusing than just offering what the target ISA offers. AsmJit still supports some extra functionality (for example the address-table stuff), but that's just for convenience.

[kobalicek]

Closing as the behavior of AsmJit is OK.