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() >> ASAN_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   return MapDynamicShadow(PremapShadowSize(), /*mmap_alignment_scale*/ 3,
36                           /*min_shadow_base_alignment*/ 0, kHighMemEnd);
37 }
38 
39 bool PremapShadowFailed() {
40   uptr shadow = reinterpret_cast<uptr>(&__asan_shadow);
41   uptr resolver = reinterpret_cast<uptr>(&__asan_premap_shadow);
42   // shadow == resolver is how Android KitKat and older handles ifunc.
43   // shadow == 0 just in case.
44   if (shadow == 0 || shadow == resolver)
45     return true;
46   return false;
47 }
48 } // namespace __asan
49 
50 extern "C" {
51 decltype(__asan_shadow)* __asan_premap_shadow() {
52   // The resolver may be called multiple times. Map the shadow just once.
53   static uptr premapped_shadow = 0;
54   if (!premapped_shadow) premapped_shadow = __asan::PremapShadow();
55   return reinterpret_cast<decltype(__asan_shadow)*>(premapped_shadow);
56 }
57 
58 // __asan_shadow is a "function" that has the same address as the first byte of
59 // the shadow mapping.
60 INTERFACE_ATTRIBUTE __attribute__((ifunc("__asan_premap_shadow"))) void
61 __asan_shadow();
62 }
63 
64 #endif // ASAN_PREMAP_SHADOW
65