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 cours

Answer 1

Switching between long mode and compatibility mode is done by changing CS. User mode code cannot modify the descriptor table, but it can perform a far jump or far call to a code segment that is already present in the descriptor table. In Linux the required descriptor is present (in my experience; this may not be true for all installations).

Here is sample code for 64-bit Linux (Ubuntu) that starts in 32-bit mode, switches to 64-bit mode, runs a function, and then switches back to 32-bit mode. Build with gcc -m32.

#include 
#include 
#include 

extern bool switch_cs(int cs, bool (*f)());
extern bool check_mode();

int main(int argc, char **argv)
{
    int cs = 0x33;
    if (argc > 1)
        cs = strtoull(argv[1], 0, 16);
    printf("switch to CS=%02x\n", cs);

    bool r = switch_cs(cs, check_mode);

    if (r)
        printf("cs=%02x: 64-bit mode\n", cs);
    else
        printf("cs=%02x: 32-bit mode\n", cs);

    return 0;
}


        .intel_syntax noprefix
        .text

        .code32
        .globl  switch_cs
switch_cs:
        mov     eax, [esp+4]
        mov     edx, [esp+8]
        push    0
        push    edx
        push    eax
        push    offset .L2
        lea     eax, [esp+8]
        lcall   [esp]
        add     esp, 16
        ret

.L2:
        call    [eax]
        lret


        .code64
        .globl check_mode
check_mode:
        xor     eax, eax
        // In 32-bit mode, this instruction is executed as
        // inc eax; test eax, eax
        test    rax, rax
        setz    al
        ret