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
21namespace __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.
27uptr 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.
34uptr 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
53bool 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
64extern "C" {
65decltype(__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.
74INTERFACE_ATTRIBUTE __attribute__((ifunc("__asan_premap_shadow"))) void
75__asan_shadow();
76}
77
78#endif // ASAN_PREMAP_SHADOW
79