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      1 //===-- asan_premap_shadow.cc ---------------------------------------------===//
      2 //
      3 //                     The LLVM Compiler Infrastructure
      4 //
      5 // This file is distributed under the University of Illinois Open Source
      6 // License. See LICENSE.TXT for details.
      7 //
      8 //===----------------------------------------------------------------------===//
      9 //
     10 // This file is a part of AddressSanitizer, an address sanity checker.
     11 //
     12 // Reserve shadow memory with an ifunc resolver.
     13 //===----------------------------------------------------------------------===//
     14 
     15 #include "asan_mapping.h"
     16 
     17 #if ASAN_PREMAP_SHADOW
     18 
     19 #include "asan_premap_shadow.h"
     20 #include "sanitizer_common/sanitizer_posix.h"
     21 
     22 namespace __asan {
     23 
     24 // The code in this file needs to run in an unrelocated binary. It may not
     25 // access any external symbol, including its own non-hidden globals.
     26 
     27 // Conservative upper limit.
     28 uptr PremapShadowSize() {
     29   uptr granularity = GetMmapGranularity();
     30   return RoundUpTo(GetMaxVirtualAddress() >> SHADOW_SCALE, granularity);
     31 }
     32 
     33 // Returns an address aligned to 8 pages, such that one page on the left and
     34 // PremapShadowSize() bytes on the right of it are mapped r/o.
     35 uptr PremapShadow() {
     36   uptr granularity = GetMmapGranularity();
     37   uptr alignment = granularity * 8;
     38   uptr left_padding = granularity;
     39   uptr shadow_size = PremapShadowSize();
     40   uptr map_size = shadow_size + left_padding + alignment;
     41 
     42   uptr map_start = (uptr)MmapNoAccess(map_size);
     43   CHECK_NE(map_start, ~(uptr)0);
     44 
     45   uptr shadow_start = RoundUpTo(map_start + left_padding, alignment);
     46   uptr shadow_end = shadow_start + shadow_size;
     47   internal_munmap(reinterpret_cast<void *>(map_start),
     48                   shadow_start - left_padding - map_start);
     49   internal_munmap(reinterpret_cast<void *>(shadow_end),
     50                   map_start + map_size - shadow_end);
     51   return shadow_start;
     52 }
     53 
     54 bool PremapShadowFailed() {
     55   uptr shadow = reinterpret_cast<uptr>(&__asan_shadow);
     56   uptr resolver = reinterpret_cast<uptr>(&__asan_premap_shadow);
     57   // shadow == resolver is how Android KitKat and older handles ifunc.
     58   // shadow == 0 just in case.
     59   if (shadow == 0 || shadow == resolver)
     60     return true;
     61   return false;
     62 }
     63 } // namespace __asan
     64 
     65 extern "C" {
     66 decltype(__asan_shadow)* __asan_premap_shadow() {
     67   // The resolver may be called multiple times. Map the shadow just once.
     68   static uptr premapped_shadow = 0;
     69   if (!premapped_shadow) premapped_shadow = __asan::PremapShadow();
     70   return reinterpret_cast<decltype(__asan_shadow)*>(premapped_shadow);
     71 }
     72 
     73 // __asan_shadow is a "function" that has the same address as the first byte of
     74 // the shadow mapping.
     75 INTERFACE_ATTRIBUTE __attribute__((ifunc("__asan_premap_shadow"))) void
     76 __asan_shadow();
     77 }
     78 
     79 #endif // ASAN_PREMAP_SHADOW
     80