[clang] [clang][bytecode] Fix incorrect handling of arithmetic on string literals (PR #173212)

[Oliver Hunt via cfe-commits]

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@@ -294,12 +294,14 @@ bool Context::evaluateStrlen(State &Parent, const Expr *E, uint64_t &Result) {
     if (!FieldDesc->isPrimitiveArray())
       return false;
 
-    if (Ptr.isDummy() || Ptr.isUnknownSizeArray())
+    if (Ptr.isDummy() || Ptr.isUnknownSizeArray() || Ptr.isPastEnd())
       return false;
 
     unsigned N = Ptr.getNumElems();
     if (Ptr.elemSize() == 1) {
-      Result = strnlen(reinterpret_cast<const char *>(Ptr.getRawAddress()), N);
+      unsigned Size = N - Ptr.getIndex();
+      Result =
+          strnlen(reinterpret_cast<const char *>(Ptr.getRawAddress()), Size);
       return Result != N;
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ojhunt wrote:

Ok, worked out an additional test case that fails with the `Result != N` check instead of `Result != Size`.

Also discovered that we don't warn when the source string is not null terminated

https://github.com/llvm/llvm-project/pull/173212