asan_premap_shadow.cc

//===-- asan_premap_shadow.cc ---------------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file is a part of AddressSanitizer, an address sanity checker.
//
// Reserve shadow memory with an ifunc resolver.
//===----------------------------------------------------------------------===//

#include "asan_mapping.h"

#if ASAN_PREMAP_SHADOW

#include "asan_premap_shadow.h"
#include "sanitizer_common/sanitizer_posix.h"

namespace __asan {

// The code in this file needs to run in an unrelocated binary. It may not
// access any external symbol, including its own non-hidden globals.

// Conservative upper limit.
uptr PremapShadowSize() {
  uptr granularity = GetMmapGranularity();
  return RoundUpTo(GetMaxVirtualAddress() >> SHADOW_SCALE, granularity);
}

// Returns an address aligned to 8 pages, such that one page on the left and
// PremapShadowSize() bytes on the right of it are mapped r/o.
uptr PremapShadow() {
  uptr granularity = GetMmapGranularity();
  uptr alignment = granularity * 8;
  uptr left_padding = granularity;
  uptr shadow_size = PremapShadowSize();
  uptr map_size = shadow_size + left_padding + alignment;

  uptr map_start = (uptr)MmapNoAccess(map_size);
  CHECK_NE(map_start, ~(uptr)0);

  uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
  uptr shadow_end = shadow_start + shadow_size;
  internal_munmap(reinterpret_cast<void *>(map_start),
                  shadow_start - left_padding - map_start);
  internal_munmap(reinterpret_cast<void *>(shadow_end),
                  map_start + map_size - shadow_end);
  return shadow_start;
}

bool PremapShadowFailed() {
  uptr shadow = reinterpret_cast<uptr>(&__asan_shadow);
  uptr resolver = reinterpret_cast<uptr>(&__asan_premap_shadow);
  // shadow == resolver is how Android KitKat and older handles ifunc.
  // shadow == 0 just in case.
  if (shadow == 0 || shadow == resolver)
    return true;
  return false;
}
} // namespace __asan

extern "C" {
decltype(__asan_shadow)* __asan_premap_shadow() {
  // The resolver may be called multiple times. Map the shadow just once.
  static uptr premapped_shadow = 0;
  if (!premapped_shadow) premapped_shadow = __asan::PremapShadow();
  return reinterpret_cast<decltype(__asan_shadow)*>(premapped_shadow);
}

// __asan_shadow is a "function" that has the same address as the first byte of
// the shadow mapping.
INTERFACE_ATTRIBUTE __attribute__((ifunc("__asan_premap_shadow"))) void
__asan_shadow();
}

#endif // ASAN_PREMAP_SHADOW