[analyzer] trackNullOrUndefValue: track last store to non-variables.

When reporting certain kinds of analyzer warnings, we use the
bugreporter::trackNullOrUndefValue mechanism, which is part of public checker
API, to understand where a zero, null-pointer, or garbage value came from,
which would highlight important events with respect to that value in the
diagnostic path notes, and help us suppress various false positives that result
from values appearing from particular sources.

Previously, we've lost track of the value when it was written into a memory
region that is not a plain variable. Now try to resume tracking in this
situation by finding where the last write to this region has occured.

Differential revision: https://reviews.llvm.org/D41253

llvm-svn: 321130

clang/lib/StaticAnalyzer/Core/BugReporterVisitors.cpp

@@ -1142,9 +1142,12 @@ bool bugreporter::trackNullOrUndefValue(const ExplodedNode *N,
     else
       RVal = state->getSVal(L->getRegion());
 
-    const MemRegion *RegionRVal = RVal.getAsRegion();
     report.addVisitor(llvm::make_unique<UndefOrNullArgVisitor>(L->getRegion()));
+    if (Optional<KnownSVal> KV = RVal.getAs<KnownSVal>())
+      report.addVisitor(llvm::make_unique<FindLastStoreBRVisitor>(
+          *KV, L->getRegion(), EnableNullFPSuppression));
 
+    const MemRegion *RegionRVal = RVal.getAsRegion();
     if (RegionRVal && isa<SymbolicRegion>(RegionRVal)) {
       report.markInteresting(RegionRVal);
       report.addVisitor(llvm::make_unique<TrackConstraintBRVisitor>(

clang/test/Analysis/inlining/inline-defensive-checks.c

@@ -190,3 +190,21 @@ void idcTrackZeroValueThroughUnaryPointerOperatorsWithArrayField(struct S2 *s) {
   idc(s);
   *(&(s->a[0])) = 7; // no-warning
 }
+
+void idcTrackConstraintThroughSymbolicRegion(int **x) {
+  idc(*x);
+  // FIXME: Should not warn.
+  **x = 7; // expected-warning{{Dereference of null pointer}}
+}
+
+int *idcPlainNull(int coin) {
+  if (coin)
+    return 0;
+  static int X;
+  return &X;
+}
+
+void idcTrackZeroValueThroughSymbolicRegion(int coin, int **x) {
+  *x = idcPlainNull(coin);
+  **x = 7; // no-warning
+}

clang/test/Analysis/nullptr.cpp

@@ -142,8 +142,9 @@ void shouldNotCrash() {
                       // expected-note@-1{{Passing null pointer value via 1st parameter 'x'}}
   if (getSymbol()) {  // expected-note  {{Assuming the condition is true}}
                       // expected-note@-1{{Taking true branch}}
-    X *x = Type().x; // expected-note{{'x' initialized to a null pointer value}}
-    x->f(); // expected-warning{{Called C++ object pointer is null}}
+    X *xx = Type().x; // expected-note   {{Null pointer value stored to field 'x'}}
+                      // expected-note@-1{{'xx' initialized to a null pointer value}}
+    xx->f(); // expected-warning{{Called C++ object pointer is null}}
             // expected-note@-1{{Called C++ object pointer is null}}
   }
 }