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

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

Reviewers: morehouse

Differential Revision: https://reviews.llvm.org/D57633

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353100 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 1 deletions

diff --git a/lib/Transforms/Instrumentation/SanitizerCoverage.cpp b/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
index 18c6ceb3331..21ad41c47f6 100644
--- a/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
+++ b/lib/Transforms/Instrumentation/SanitizerCoverage.cpp
@@ -377,7 +377,12 @@ bool SanitizerCoverageModule::runOnModule(Module &M) {
 
   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())