Use kMaxStackFrameSize and remove local true_start.

Update test to passing.

Author: thurstond

compiler-rt/lib/asan/asan_fake_stack.cpp

@@ -54,8 +54,9 @@ FakeStack *FakeStack::Create(uptr stack_size_log) {
     stack_size_log = kMinStackSizeLog;
   if (stack_size_log > kMaxStackSizeLog)
     stack_size_log = kMaxStackSizeLog;
+  CHECK_LE(kMaxStackFrameSizeLog, stack_size_log);
   uptr size = RequiredSize(stack_size_log);
-  uptr padded_size = size + (1 << kMaxStackFrameSizeLog);
+  uptr padded_size = size + kMaxStackFrameSize;
   void *true_res = reinterpret_cast<void *>(
       flags()->uar_noreserve ? MmapNoReserveOrDie(padded_size, "FakeStack")
                              : MmapOrDie(padded_size, "FakeStack"));
@@ -64,7 +65,7 @@ FakeStack *FakeStack::Create(uptr stack_size_log) {
   // (1 << kMaxStackFrameSizeLog).
   // We didn't use MmapAlignedOrDieOnFatalError, because it requires that the
   // *size* is a power of 2, which is an overly strong condition.
-  static_assert(alignof(FakeStack) <= (1 << kMaxStackFrameSizeLog));
+  static_assert(alignof(FakeStack) <= kMaxStackFrameSize);
   FakeStack *res = reinterpret_cast<FakeStack *>(
       RoundUpTo(
           (uptr)true_res + kFlagsOffset + SizeRequiredForFlags(stack_size_log),
@@ -94,8 +95,7 @@ void FakeStack::Destroy(int tid) {
     Report("T%d: FakeStack destroyed: %s\n", tid, str.data());
   }
   uptr size = RequiredSize(stack_size_log_);
-  uptr padded_size = size + (1 << kMaxStackFrameSizeLog);
-  void *true_start = this->true_start;
+  uptr padded_size = size + kMaxStackFrameSize;
   FlushUnneededASanShadowMemory(reinterpret_cast<uptr>(true_start),
                                 padded_size);
   UnmapOrDie(true_start, padded_size);

compiler-rt/lib/asan/asan_fake_stack.h

@@ -56,6 +56,9 @@ struct FakeFrame {
 class FakeStack {
   static const uptr kMinStackFrameSizeLog = 6;  // Min frame is 64B.
   static const uptr kMaxStackFrameSizeLog = 16;  // Max stack frame is 64K.
+  static_assert(kMaxStackFrameSizeLog >= kMinStackFrameSizeLog);
+
+  static const u64 kMaxStackFrameSize = 1 << kMaxStackFrameSizeLog;
 
  public:
   static const uptr kNumberOfSizeClasses =

compiler-rt/test/asan/TestCases/fakestack_alignment.cpp

@@ -1,11 +1,11 @@
 // Regression test 1:
-// This deterministically fails: when the stack size is 1<<16, FakeStack's
-// GetFrame() is out of alignment, because SizeRequiredForFlags(16) == 2K.
+// When the stack size is 1<<16, SizeRequiredForFlags(16) == 2KB. This forces
+// FakeStack's GetFrame() out of alignment if the FakeStack isn't padded.
 // RUN: %clangxx_asan -fsanitize-address-use-after-return=always -O0 -DALIGNMENT=4096  -DTHREAD_COUNT=1 -DTHREAD_STACK_SIZE=65536 %s -o %t && %run %t 2>&1
 
 // Regression test 2:
-// The FakeStack frame is not guaranteed to be aligned, but alignment can
-// happen by chance, so try this on many threads.
+// Check that the FakeStack frame is aligned, beyond the typical 4KB page
+// alignment. Alignment can happen by chance, so try this on many threads.
 // RUN: %clangxx_asan -fsanitize-address-use-after-return=always -O0 -DALIGNMENT=8192  -DTHREAD_COUNT=32 -DTHREAD_STACK_SIZE=131072 %s -o %t && %run %t 2>&1
 // RUN: %clangxx_asan -fsanitize-address-use-after-return=always -O0 -DALIGNMENT=16384 -DTHREAD_COUNT=32 -DTHREAD_STACK_SIZE=131072 %s -o %t && %run %t 2>&1
 
@@ -17,8 +17,6 @@
 // RUN: %clangxx_asan -fsanitize-address-use-after-return=always -O0 -DALIGNMENT=8192  -DTHREAD_COUNT=32 -DTHREAD_STACK_SIZE=131072 %s -o %t && %run %t 2>&1
 // RUN: %clangxx_asan -fsanitize-address-use-after-return=always -O0 -DALIGNMENT=16384 -DTHREAD_COUNT=32 -DTHREAD_STACK_SIZE=131072 %s -o %t && %run %t 2>&1
 
-// XFAIL: *
-
 #include <assert.h>
 #include <pthread.h>
 #include <stdio.h>