X86HelperGen.cpp

/*
$info$
tags: glue|x86-guest-code
desc: Guest-side assembly helpers used by the backends
$end_info$
*/

#include "Interface/Core/X86HelperGen.h"
#include "FEXCore/Utils/AllocatorHooks.h"

#include <FEXCore/Config/Config.h>
#include <FEXCore/Utils/Allocator.h>
#include <FEXHeaderUtils/Syscalls.h>

#include <cstdint>
#include <cstring>

namespace FEXCore {
constexpr size_t CODE_SIZE = 0x1000;

X86GeneratedCode::X86GeneratedCode() {
#ifdef _WIN32
  // No need to allocate anything in this config.
#else

  // Allocate a page for our emulated guest
  CodePtr = AllocateGuestCodeSpace(CODE_SIZE);

  constexpr std::array<uint8_t, 2> SignalReturnCode = {
    0x0F, 0x37, // CALLBACKRET FEX Instruction
  };

  // Signal return handlers need to be bit-exact to what the Linux kernel provides in VDSO.
  // GDB and unwinding libraries key off of these instructions to understand if the stack frame is a signal frame or not.
  // This two code sections match exactly what libSegFault expects.
  //
  // Typically this handlers are provided by the 32-bit VDSO thunk library, but that isn't available in all cases.
  // Falling back to this generated code segment still allows a backtrace to work, just might not show
  // the symbol as VDSO since there is no ELF to parse.
  constexpr std::array<uint8_t, 9> sigreturn_32_code = {
    0x58, // pop eax
    0xb8, 0x77, 0x00, 0x00, 0x00, // mov eax, 0x77
    0xcd, 0x80, // int 0x80
    0x90, // nop
  };

  constexpr std::array<uint8_t, 7> rt_sigreturn_32_code = {
    0xb8, 0xad, 0x00, 0x00, 0x00, // mov eax, 0xad
    0xcd, 0x80, // int 0x80
  };

  CallbackReturn = reinterpret_cast<uint64_t>(CodePtr);
  sigreturn_32 = CallbackReturn + SignalReturnCode.size();
  rt_sigreturn_32 = sigreturn_32 + sigreturn_32_code.size();

  memcpy(reinterpret_cast<void*>(CallbackReturn), &SignalReturnCode.at(0), SignalReturnCode.size());
  memcpy(reinterpret_cast<void*>(sigreturn_32), &sigreturn_32_code.at(0), sigreturn_32_code.size());
  memcpy(reinterpret_cast<void*>(rt_sigreturn_32), &rt_sigreturn_32_code.at(0), rt_sigreturn_32_code.size());

  mprotect(CodePtr, CODE_SIZE, PROT_READ);
#endif
}

X86GeneratedCode::~X86GeneratedCode() {
#ifndef _WIN32
  FEXCore::Allocator::VirtualFree(CodePtr, CODE_SIZE);
#endif
}

void* X86GeneratedCode::AllocateGuestCodeSpace(size_t Size) {
#ifndef _WIN32
  FEX_CONFIG_OPT(Is64BitMode, IS64BIT_MODE);

  if (Is64BitMode()) {
    // 64bit mode can have its sigret handler anywhere
    return FEXCore::Allocator::VirtualAlloc(Size);
  }

  // First 64bit page
  constexpr uintptr_t LOCATION_MAX = 0x1'0000'0000;

  // 32bit mode
  // We need to have the sigret handler in the lower 32bits of memory space
  // Scan top down and try to allocate a location
  for (size_t Location = 0xFFFF'E000; Location != 0x0; Location -= 0x1000) {
    void *Ptr = ::mmap(reinterpret_cast<void*>(Location), Size, PROT_READ | PROT_WRITE, MAP_FIXED_NOREPLACE | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);

    if (Ptr != MAP_FAILED &&
        reinterpret_cast<uintptr_t>(Ptr) >= LOCATION_MAX) {
      // Failed to map in the lower 32bits
      // Try again
      // Can happen in the case that host kernel ignores MAP_FIXED_NOREPLACE
      ::munmap(Ptr, Size);
      continue;
    }

    if (Ptr != MAP_FAILED) {
      return Ptr;
    }
  }

  // Can't do anything about this
  // Here's hoping the application doesn't use signals
  return MAP_FAILED;
#else
  return nullptr;
#endif
}

}