Inline 64bit Assembly in 32bit GCC C Program

Question

I'm compiling a 32 bit binary but want to embed some 64 bit assembly in it.

void method() {
   asm("...64 bit assembly...");
}

Of course when I compile I get errors about referring to bad registers because the registers are 64 bit.

evil.c:92: Error: bad register name `%rax'

Is it possible to add some annotations so gcc will process the asm sections using the 64bit assembler instead. I have a workaround which is compile separately, map in a page with PROT_EXEC|PROT_WRITE and copy in my code but this is very awkward.

Answer 1

No, this isn't possible. You can't run 64-bit assembly from a 32-bit binary, as the processor will not be in long mode while running your program.

Copying 64-bit code to an executable page will result in that code being interpreted incorrectly as 32-bit code, which will have unpredictable and undesirable results.