[analyzer] Fix assertion crash in CStringChecker

[george.karpenkov]

Summary

An offset might be unknown.

rdar://39054939

[NoQ]

Test?

[NoQ]

More LocAsInteger problems. We need more flexible symbolic expressions.

lib/StaticAnalyzer/Checkers/CStringChecker.cpp

bool CStringChecker::IsFirstBufInBound(CheckerContext &C,
 
   SVal LengthVal = state->getSVal(Size, LCtx);
   Optional<NonLoc> Length = LengthVal.getAs<NonLoc>();
   if (!Length)
     return true; // cf top comment.
 
   // Compute the offset of the last element to be accessed: size-1.
   NonLoc One = svalBuilder.makeIntVal(1, sizeTy).castAs<NonLoc>();
-  NonLoc LastOffset =
-      svalBuilder.evalBinOpNN(state, BO_Sub, *Length, One, sizeTy)
-          .castAs<NonLoc>();
+  SVal Offset = svalBuilder.evalBinOpNN(state, BO_Sub, *Length, One, sizeTy);
+  if (Offset.isUnknown())
+    return true; // cf top comment
+  NonLoc LastOffset = Offset.castAs<NonLoc>();
 
   // Check that the first buffer is sufficiently long.
   SVal BufStart = svalBuilder.evalCast(BufVal, PtrTy, FirstBuf->getType());
   Optional<Loc> BufLoc = BufStart.getAs<Loc>();
   if (!BufLoc)
     return true; // cf top comment.
 
   SVal BufEnd =

test/Analysis/string.c

     return;
 
   char x[4];
   strncpy(x, y, 4); // no-warning
 
   // This time, we know that y fits in x anyway.
   clang_analyzer_eval(strlen(x) <= 3); // expected-warning{{UNKNOWN}}
 }
+
+struct S {
+  char f;
+};
+
+void nocrash_on_locint_offset(void *addr, void* from, struct S s) {
+  int iAdd = (int) addr;
+  memcpy(((void *) &(s.f)), from, iAdd);
+}