mach-detours/src/arm64/detours_arm64.h

// Copyright (c) Microsoft Corporation.  All rights reserved.
// Copyright (c) Lysann Tranvouez.  All rights reserved.

#pragma once
#ifndef MACH_DETOURS_ARM64_H
#define MACH_DETOURS_ARM64_H

#include <stdint.h>

typedef struct detour_align
{
    uint8_t    obTarget;
    uint8_t    obTrampoline;
} detour_align;

typedef struct detour_trampoline
{
    // An ARM64 instruction is 4 bytes long.
    //
    // The overwrite is always composed of 3 instructions (12 bytes) which perform an indirect jump
    // using _DETOUR_TRAMPOLINE::pbDetour as the address holding the target location.
    //
    // Copied instructions can expand.
    //
    // The scheme using MovImmediate can cause an instruction
    // to grow as much as 6 times.
    // That would be Bcc or Tbz with a large address space:
    //   4 instructions to form immediate
    //   inverted tbz/bcc
    //   br
    //
    // An expansion of 4 is not uncommon -- bl/blr and small address space:
    //   3 instructions to form immediate
    //   br or brl
    //
    // A theoretical maximum for rbCode is thefore 4*4*6 + 16 = 112 (another 16 for jmp to pbRemain).
    //
    // With literals, the maximum expansion is 5, including the literals: 4*4*5 + 16 = 96.
    //
    // The number is rounded up to 128. m_rbScratchDst should match this.
    //
    uint8_t            rbCode[128];    // target code + jmp to pbRemain
    uint8_t            cbCode;         // size of moved target code.
    uint8_t            cbCodeBreak[3]; // padding to make debugging easier.
    uint8_t            rbRestore[24];  // original target code.
    uint8_t            cbRestore;      // size of original target code.
    uint8_t            cbRestoreBreak[3]; // padding to make debugging easier.
    detour_align   rAlign[8];      // instruction alignment array.
    uint8_t*          pbRemain;       // first instruction after moved code. [free list]
    uint8_t*          pbDetour;       // first instruction of detour function.
} detour_trampoline;

static_assert(sizeof(detour_trampoline) == 192);

#endif //MACH_DETOURS_ARM64_H