[SanitizerCoverage] Clang crashes if user declares `__sancov_lowest_stack` variable

[yln]

Summary

If the user declares or defines __sancov_lowest_stack with an
unexpected type, then getOrInsertGlobal inserts a bitcast and the
following cast fails:

Constant *SanCovLowestStackConstant =
       M.getOrInsertGlobal(SanCovLowestStackName, IntptrTy);
SanCovLowestStack = cast<GlobalVariable>(SanCovLowestStackConstant);

This variable is a SanitizerCoverage implementation detail and the user
should generally never have a need to access it, so we emit an error
now.

rdar://problem/44143130

[morehouse]

LGTM

llvm/trunk/lib/Transforms/Instrumentation/SanitizerCoverage.cpp

SanCovTraceDivFunction[1] =
       M.getOrInsertFunction(SanCovTraceDiv8, VoidTy, Int64Ty);
   SanCovTraceGepFunction =
       M.getOrInsertFunction(SanCovTraceGep, VoidTy, IntptrTy);
   SanCovTraceSwitchFunction =
       M.getOrInsertFunction(SanCovTraceSwitchName, VoidTy, Int64Ty, Int64PtrTy);
 
   Constant *SanCovLowestStackConstant =
       M.getOrInsertGlobal(SanCovLowestStackName, IntptrTy);
-  SanCovLowestStack = cast<GlobalVariable>(SanCovLowestStackConstant);
+  SanCovLowestStack = dyn_cast<GlobalVariable>(SanCovLowestStackConstant);
+  if (!SanCovLowestStack) {
+    C->emitError(StringRef("'") + SanCovLowestStackName +
+                 "' should not be declared by the user");
+    return true;
+  }
   SanCovLowestStack->setThreadLocalMode(
       GlobalValue::ThreadLocalMode::InitialExecTLSModel);
   if (Options.StackDepth && !SanCovLowestStack->isDeclaration())
     SanCovLowestStack->setInitializer(Constant::getAllOnesValue(IntptrTy));
 
   // We insert an empty inline asm after cov callbacks to avoid callback merge.
   EmptyAsm = InlineAsm::get(FunctionType::get(IRB.getVoidTy(), false),
                             StringRef(""), StringRef(""),

llvm/trunk/test/Instrumentation/SanitizerCoverage/stack-depth-variable-declared-by-user.ll

+; Ensure that we terminate with a useful error message (instead of crash) if the
+; user declares `__sancov_lowest_stack` with an unexpected type.
+; RUN: not opt < %s -sancov -sanitizer-coverage-level=1 \
+; RUN:         -sanitizer-coverage-stack-depth -S 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Wrong type: i32 instead of expected i64
+@__sancov_lowest_stack = thread_local global i32 0
+
+; CHECK: error: '__sancov_lowest_stack' should not be declared by the user