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

[Alexander Potapenko]

I've checked that IsAccessibleMemoryRange works on Linux and OSX on the following examples:


  char *mem = (char*)mmap(0, 4096 * 2, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
  mprotect(mem + 4096, 4096, PROT_NONE);
  printf("mem: %p\n", mem);
  IsAccessibleMemoryRange(mem, 4095);  // 1
  IsAccessibleMemoryRange(mem, 4096);  // 1
  IsAccessibleMemoryRange(mem, 4097);  // 0
  IsAccessibleMemoryRange(mem + 4095, 1);  // 1
  IsAccessibleMemoryRange(mem + 4095, 2);  // 0
  IsAccessibleMemoryRange(0, 2);  // 0

Care to add a unittest?

================
Comment at: lib/sanitizer_common/sanitizer_posix_libcdep.cc:169
@@ -168,1 +168,3 @@
 
+bool IsAccessibleMemoryRange(uptr beg, uptr size) {
+  int sock_pair[2];
----------------
Any limits on |size|?
I think it must be an sptr greater than -1 (you're going to compare bytes_written to it) and less than kPageSize (otherwise it'll take too much time to check chunks of too big size)

================
Comment at: lib/sanitizer_common/sanitizer_posix_libcdep.cc:173
@@ +172,3 @@
+    return false;
+  uptr bytes_written =
+      internal_write(sock_pair[1], reinterpret_cast<void *>(beg), size);
----------------
internal_write() returns sptr, not uptr (at least it must).
Also, when bytes_written == size (assuming size != -1), internal_iserror() is always false.

http://reviews.llvm.org/D5253