libsanitizer/asan/asan_premap_shadow.cpp

   1 //===-- asan_premap_shadow.cpp --------------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file is a part of AddressSanitizer, an address sanity checker.
  10 //
  11 // Reserve shadow memory with an ifunc resolver.
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "asan_mapping.h"
  15
  16 #if ASAN_PREMAP_SHADOW
  17
  18 #include "asan_premap_shadow.h"
  19 #include "sanitizer_common/sanitizer_posix.h"
  20
  21 namespace __asan {
  22
  23 // The code in this file needs to run in an unrelocated binary. It may not
  24 // access any external symbol, including its own non-hidden globals.
  25
  26 // Conservative upper limit.
  27 uptr PremapShadowSize() {
  28   uptr granularity = GetMmapGranularity();
  29   return RoundUpTo(GetMaxVirtualAddress() >> SHADOW_SCALE, granularity);
  30 }
  31
  32 // Returns an address aligned to 8 pages, such that one page on the left and
  33 // PremapShadowSize() bytes on the right of it are mapped r/o.
  34 uptr PremapShadow() {
  35   uptr granularity = GetMmapGranularity();
  36   uptr alignment = granularity * 8;
  37   uptr left_padding = granularity;
  38   uptr shadow_size = PremapShadowSize();
  39   uptr map_size = shadow_size + left_padding + alignment;
  40
  41   uptr map_start = (uptr)MmapNoAccess(map_size);
  42   CHECK_NE(map_start, ~(uptr)0);
  43
  44   uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
  45   uptr shadow_end = shadow_start + shadow_size;
  46   internal_munmap(reinterpret_cast<void *>(map_start),
  47                   shadow_start - left_padding - map_start);
  48   internal_munmap(reinterpret_cast<void *>(shadow_end),
  49                   map_start + map_size - shadow_end);
  50   return shadow_start;
  51 }
  52
  53 bool PremapShadowFailed() {
  54   uptr shadow = reinterpret_cast<uptr>(&__asan_shadow);
  55   uptr resolver = reinterpret_cast<uptr>(&__asan_premap_shadow);
  56   // shadow == resolver is how Android KitKat and older handles ifunc.
  57   // shadow == 0 just in case.
  58   if (shadow == 0 || shadow == resolver)
  59     return true;
  60   return false;
  61 }
  62 } // namespace __asan
  63
  64 extern "C" {
  65 decltype(__asan_shadow)* __asan_premap_shadow() {
  66   // The resolver may be called multiple times. Map the shadow just once.
  67   static uptr premapped_shadow = 0;
  68   if (!premapped_shadow) premapped_shadow = __asan::PremapShadow();
  69   return reinterpret_cast<decltype(__asan_shadow)*>(premapped_shadow);
  70 }
  71
  72 // __asan_shadow is a "function" that has the same address as the first byte of
  73 // the shadow mapping.
  74 INTERFACE_ATTRIBUTE __attribute__((ifunc("__asan_premap_shadow"))) void
  75 __asan_shadow();
  76 }
  77
  78 #endif // ASAN_PREMAP_SHADOW