Trampoline.h

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#pragma once
 
#if defined(SKSE_SUPPORT_XBYAK)
namespace Xbyak
{
    class CodeGenerator;
}
#endif
 
namespace SKSE
{
    namespace detail
    {
        [[nodiscard]] constexpr std::size_t roundup(std::size_t a_number, std::size_t a_multiple) noexcept
        {
            if (a_multiple == 0) {
                return 0;
            }
 
            const auto remainder = a_number % a_multiple;
            return remainder == 0 ?
                       a_number :
                       a_number + a_multiple - remainder;
        }
 
        [[nodiscard]] constexpr std::size_t rounddown(std::size_t a_number, std::size_t a_multiple) noexcept
        {
            if (a_multiple == 0) {
                return 0;
            }
 
            const auto remainder = a_number % a_multiple;
            return remainder == 0 ?
                       a_number :
                       a_number - remainder;
        }
    }
 
    class Trampoline
    {
    public:
        using deleter_type = std::function<void(void* a_mem, std::size_t a_size)>;
 
        Trampoline() = default;
        Trampoline(const Trampoline&) = delete;
 
        Trampoline(Trampoline&& a_rhs) { move_from(std::move(a_rhs)); }
 
        explicit Trampoline(std::string_view a_name) :
            _name(a_name)
        {}
 
        ~Trampoline() { release(); }
 
        Trampoline& operator=(const Trampoline&) = delete;
 
        Trampoline& operator=(Trampoline&& a_rhs)
        {
            if (this != std::addressof(a_rhs)) {
                move_from(std::move(a_rhs));
            }
            return *this;
        }
 
        void create(std::size_t a_size) { return create(a_size, nullptr); }
 
        void create(std::size_t a_size, void* a_module)
        {
            if (a_size == 0) {
                stl::report_and_fail("cannot create a trampoline with a zero size"sv);
            }
 
            if (!a_module) {
                const auto text = REL::Module::get().segment(REL::Segment::textx);
                a_module = text.pointer<std::byte>() + text.size();
            }
 
            auto mem = do_create(a_size, reinterpret_cast<std::uintptr_t>(a_module));
            if (!mem) {
                stl::report_and_fail("failed to create trampoline"sv);
            }
 
            set_trampoline(mem, a_size,
                [](void* a_mem, std::size_t) {
                    WinAPI::VirtualFree(a_mem, 0, WinAPI::MEM_RELEASE);
                });
        }
 
        void set_trampoline(void* a_trampoline, std::size_t a_size) { set_trampoline(a_trampoline, a_size, {}); }
 
        void set_trampoline(void* a_trampoline, std::size_t a_size, deleter_type a_deleter)
        {
            auto trampoline = static_cast<std::byte*>(a_trampoline);
            if (trampoline) {
                constexpr auto INT3 = static_cast<int>(0xCC);
                std::memset(trampoline, INT3, a_size);
            }
 
            release();
 
            _deleter = std::move(a_deleter);
            _data = trampoline;
            _capacity = a_size;
            _size = 0;
 
            log_stats();
        }
 
        [[nodiscard]] void* allocate(std::size_t a_size)
        {
            auto result = do_allocate(a_size);
            log_stats();
            return result;
        }
 
#ifdef SKSE_SUPPORT_XBYAK
        [[nodiscard]] void* allocate(Xbyak::CodeGenerator& a_code);
#endif
 
        template <class T>
        [[nodiscard]] T* allocate()
        {
            return static_cast<T*>(allocate(sizeof(T)));
        }
 
        [[nodiscard]] constexpr std::size_t empty() const noexcept { return _capacity == 0; }
        [[nodiscard]] constexpr std::size_t capacity() const noexcept { return _capacity; }
        [[nodiscard]] constexpr std::size_t allocated_size() const noexcept { return _size; }
        [[nodiscard]] constexpr std::size_t free_size() const noexcept { return _capacity - _size; }
 
        template <std::size_t N>
        std::uintptr_t write_branch(std::uintptr_t a_src, std::uintptr_t a_dst)
        {
            std::uint8_t data = 0;
            if constexpr (N == 5) {
                // E9 cd
                // JMP rel32
                data = 0xE9;
            } else if constexpr (N == 6) {
                // FF /4
                // JMP r/m64
                data = 0x25;
            } else {
                static_assert(false && N, "invalid branch size");
            }
 
            return write_branch<N>(a_src, a_dst, data);
        }
 
        template <std::size_t N, class F>
        std::uintptr_t write_branch(std::uintptr_t a_src, F a_dst)
        {
            return write_branch<N>(a_src, stl::unrestricted_cast<std::uintptr_t>(a_dst));
        }
 
        template <std::size_t N>
        std::uintptr_t write_call(std::uintptr_t a_src, std::uintptr_t a_dst)
        {
            std::uint8_t data = 0;
            if constexpr (N == 5) {
                // E8 cd
                // CALL rel32
                data = 0xE8;
            } else if constexpr (N == 6) {
                // FF /2
                // CALL r/m64
                data = 0x15;
            } else {
                static_assert(false && N, "invalid call size");
            }
 
            return write_branch<N>(a_src, a_dst, data);
        }
 
        template <std::size_t N, class F>
        std::uintptr_t write_call(std::uintptr_t a_src, F a_dst)
        {
            return write_call<N>(a_src, stl::unrestricted_cast<std::uintptr_t>(a_dst));
        }
 
    private:
        [[nodiscard]] void* do_create(std::size_t a_size, std::uintptr_t a_address);
 
        [[nodiscard]] void* do_allocate(std::size_t a_size)
        {
            if (a_size > free_size()) {
                stl::report_and_fail("Failed to handle allocation request"sv);
            }
 
            auto mem = _data + _size;
            _size += a_size;
 
            return mem;
        }
 
        void write_5branch(std::uintptr_t a_src, std::uintptr_t a_dst, std::uint8_t a_opcode)
        {
#pragma pack(push, 1)
            struct SrcAssembly
            {
                // jmp/call [rip + imm32]
                std::uint8_t opcode;  // 0 - 0xE9/0xE8
                std::int32_t disp;    // 1
            };
            static_assert(offsetof(SrcAssembly, opcode) == 0x0);
            static_assert(offsetof(SrcAssembly, disp) == 0x1);
            static_assert(sizeof(SrcAssembly) == 0x5);
 
            // FF /4
            // JMP r/m64
            struct TrampolineAssembly
            {
                // jmp [rip]
                std::uint8_t  jmp;    // 0 - 0xFF
                std::uint8_t  modrm;  // 1 - 0x25
                std::int32_t  disp;   // 2 - 0x00000000
                std::uint64_t addr;   // 6 - [rip]
            };
            static_assert(offsetof(TrampolineAssembly, jmp) == 0x0);
            static_assert(offsetof(TrampolineAssembly, modrm) == 0x1);
            static_assert(offsetof(TrampolineAssembly, disp) == 0x2);
            static_assert(offsetof(TrampolineAssembly, addr) == 0x6);
            static_assert(sizeof(TrampolineAssembly) == 0xE);
#pragma pack(pop)
 
            TrampolineAssembly* mem = nullptr;
            if (const auto it = _5branches.find(a_dst); it != _5branches.end()) {
                mem = reinterpret_cast<TrampolineAssembly*>(it->second);
            } else {
                mem = allocate<TrampolineAssembly>();
                _5branches.emplace(a_dst, reinterpret_cast<std::byte*>(mem));
            }
 
            const auto disp =
                reinterpret_cast<const std::byte*>(mem) -
                reinterpret_cast<const std::byte*>(a_src + sizeof(SrcAssembly));
            if (!in_range(disp)) {  // the trampoline should already be in range, so this should never happen
                stl::report_and_fail("displacement is out of range"sv);
            }
 
            SrcAssembly assembly;
            assembly.opcode = a_opcode;
            assembly.disp = static_cast<std::int32_t>(disp);
            REL::safe_write(a_src, &assembly, sizeof(assembly));
 
            mem->jmp = static_cast<std::uint8_t>(0xFF);
            mem->modrm = static_cast<std::uint8_t>(0x25);
            mem->disp = static_cast<std::int32_t>(0);
            mem->addr = static_cast<std::uint64_t>(a_dst);
        }
 
        void write_6branch(std::uintptr_t a_src, std::uintptr_t a_dst, std::uint8_t a_modrm)
        {
#pragma pack(push, 1)
            struct Assembly
            {
                // jmp/call [rip + imm32]
                std::uint8_t opcode;  // 0 - 0xFF
                std::uint8_t modrm;   // 1 - 0x25/0x15
                std::int32_t disp;    // 2
            };
            static_assert(offsetof(Assembly, opcode) == 0x0);
            static_assert(offsetof(Assembly, modrm) == 0x1);
            static_assert(offsetof(Assembly, disp) == 0x2);
            static_assert(sizeof(Assembly) == 0x6);
#pragma pack(pop)
 
            std::uintptr_t* mem = nullptr;
            if (const auto it = _6branches.find(a_dst); it != _6branches.end()) {
                mem = reinterpret_cast<std::uintptr_t*>(it->second);
            } else {
                mem = allocate<std::uintptr_t>();
                _6branches.emplace(a_dst, reinterpret_cast<std::byte*>(mem));
            }
 
            const auto disp =
                reinterpret_cast<const std::byte*>(mem) -
                reinterpret_cast<const std::byte*>(a_src + sizeof(Assembly));
            if (!in_range(disp)) {  // the trampoline should already be in range, so this should never happen
                stl::report_and_fail("displacement is out of range"sv);
            }
 
            Assembly assembly;
            assembly.opcode = static_cast<std::uint8_t>(0xFF);
            assembly.modrm = a_modrm;
            assembly.disp = static_cast<std::int32_t>(disp);
            REL::safe_write(a_src, &assembly, sizeof(assembly));
 
            *mem = a_dst;
        }
 
        template <std::size_t N>
        [[nodiscard]] std::uintptr_t write_branch(std::uintptr_t a_src, std::uintptr_t a_dst, std::uint8_t a_data)
        {
            const auto disp = reinterpret_cast<std::int32_t*>(a_src + N - 4);
            const auto nextOp = a_src + N;
            const auto func = nextOp + *disp;
 
            if constexpr (N == 5) {
                write_5branch(a_src, a_dst, a_data);
            } else if constexpr (N == 6) {
                write_6branch(a_src, a_dst, a_data);
            } else {
                static_assert(false && N, "invalid branch size");
            }
 
            return func;
        }
 
        void move_from(Trampoline&& a_rhs)
        {
            _5branches = std::move(a_rhs._5branches);
            _6branches = std::move(a_rhs._6branches);
            _name = std::move(a_rhs._name);
 
            _deleter = std::move(a_rhs._deleter);
 
            _data = a_rhs._data;
            a_rhs._data = nullptr;
 
            _capacity = a_rhs._capacity;
            a_rhs._capacity = 0;
 
            _size = a_rhs._size;
            a_rhs._size = 0;
        }
 
        void log_stats() const;
 
        [[nodiscard]] bool in_range(std::ptrdiff_t a_disp) const
        {
            constexpr auto min = std::numeric_limits<std::int32_t>::min();
            constexpr auto max = std::numeric_limits<std::int32_t>::max();
 
            return min <= a_disp && a_disp <= max;
        }
 
        void release()
        {
            if (_data && _deleter) {
                _deleter(_data, _capacity);
            }
 
            _5branches.clear();
            _6branches.clear();
            _data = nullptr;
            _capacity = 0;
            _size = 0;
        }
 
        std::map<std::uintptr_t, std::byte*> _5branches;
        std::map<std::uintptr_t, std::byte*> _6branches;
        std::string                          _name{ "Default Trampoline"sv };
        deleter_type                         _deleter;
        std::byte*                           _data{ nullptr };
        std::size_t                          _capacity{ 0 };
        std::size_t                          _size{ 0 };
    };
}