[Windows] Fix UnmapOrDie and MmapAlignedOrDie

Now ASan can return virtual memory to the underlying OS. Portable
sanitizer runtime code needs to be aware that UnmapOrDie cannot unmap
part of previous mapping.

In particular, this required changing how we implement MmapAlignedOrDie
on Windows, which is what Allocator32 uses.

The new code first attempts to allocate memory of the given size, and if
it is appropriately aligned, returns early. If not, it frees the memory
and attempts to reserve size + alignment bytes. In this region there
must be an aligned address. We then free the oversized mapping and
request a new mapping at the aligned address immediately after. However,
a thread could allocate that virtual address in between our free and
allocation, so we have to retry if that allocation fails. The existing
thread creation stress test managed to trigger this condition, so the
code isn't totally untested.

Reviewers: samsonov

Differential Revision: http://reviews.llvm.org/D17431

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@263160 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 20 insertions, 0 deletions

diff --git a/lib/sanitizer_common/sanitizer_posix.cc b/lib/sanitizer_common/sanitizer_posix.cc
index a383b2158..5bcc86b3b 100644
--- a/lib/sanitizer_common/sanitizer_posix.cc
+++ b/lib/sanitizer_common/sanitizer_posix.cc
@@ -139,6 +139,26 @@ void UnmapOrDie(void *addr, uptr size) {
   DecreaseTotalMmap(size);
 }
 
+// We want to map a chunk of address space aligned to 'alignment'.
+// We do it by maping a bit more and then unmaping redundant pieces.
+// We probably can do it with fewer syscalls in some OS-dependent way.
+void *MmapAlignedOrDie(uptr size, uptr alignment, const char *mem_type) {
+  CHECK(IsPowerOfTwo(size));
+  CHECK(IsPowerOfTwo(alignment));
+  uptr map_size = size + alignment;
+  uptr map_res = (uptr)MmapOrDie(map_size, mem_type);
+  uptr map_end = map_res + map_size;
+  uptr res = map_res;
+  if (res & (alignment - 1))  // Not aligned.
+    res = (map_res + alignment) & ~(alignment - 1);
+  uptr end = res + size;
+  if (res != map_res)
+    UnmapOrDie((void*)map_res, res - map_res);
+  if (end != map_end)
+    UnmapOrDie((void*)end, map_end - end);
+  return (void*)res;
+}
+
 void *MmapNoReserveOrDie(uptr size, const char *mem_type) {
   uptr PageSize = GetPageSizeCached();
   uptr p = internal_mmap(nullptr,