[PATCH] PR20721: Don't let UBSan print inaccessible memory

[Alexey Samsonov]

Hi eugenis,

UBSan needs to check if memory snippet it's going to print resides
in addressable memory. Similar check might be helpful in ASan with
dump_instruction_bytes option (see http://reviews.llvm.org/D5167).

Instead of scanning /proc/self/maps manually, delegate this check to
the OS kernel: try to write this memory in a syscall and assume that
memory is inaccessible if the syscall failed (e.g. with EFAULT).

http://reviews.llvm.org/D5253

Files:
  lib/sanitizer_common/sanitizer_common.h
  lib/sanitizer_common/sanitizer_posix_libcdep.cc
  lib/sanitizer_common/sanitizer_win.cc
  lib/ubsan/ubsan_diag.cc
  test/ubsan/TestCases/TypeCheck/misaligned.cpp
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