[PATCH] D46415: [analyzer] pr36458: Fix retrieved value cast for symbolic void pointers.

[Artem Dergachev via Phabricator via cfe-commits]

NoQ created this revision.
NoQ added reviewers: dcoughlin, xazax.hun, a.sidorin, george.karpenkov, szepet, rnkovacs.
Herald added subscribers: cfe-commits, baloghadamsoftware.

Through C cast magic it's possible to put a raw void pointer into a variable of non-void pointer type. It is fine - generally, it's possible to put anything into anywhere by temporarily re-interpreting the anywhere. We cast it back to the correct type during load - with the help of `StoreManager::CastRetrievedVal()`.

The attached example demonstrates that we're not casting the retrieved value pedantically enough when it comes to retrieving pointer values. In fact, we don't cast pointers-to-pointers at all because `SValBuilder` doesn't know how to do that or even that it needs to do that at all.

In this patch i perform the pointer cast manually for the situation that causes the crash. Performing the cast more carefully in other cases is possible, but i didn't manage to produce an observable effect (the second test passes just fine without it, and works correctly).

I cannot put the `castRegion()` call into `SValBuilder` because `SValBuilder` doesn't have access to `StoreManager` when doing casts. We really need to decouple this stuff and make a single good function for handling casts, because understanding how current code for pointer casts works is a nightmare.


Repository:
  rC Clang

https://reviews.llvm.org/D46415

Files:
  lib/StaticAnalyzer/Core/Store.cpp
  test/Analysis/casts.c


Index: test/Analysis/casts.c
===================================================================
--- test/Analysis/casts.c
+++ test/Analysis/casts.c
@@ -149,3 +149,19 @@
 
   clang_analyzer_eval(*((char *)y1) == *((char *) y3)); // expected-warning{{TRUE}}
 }
+
+void *getVoidPtr();
+
+void testCastVoidPtrToIntPtrThroughIntTypedAssignment() {
+  int *x;
+  (*((int *)(&x))) = (int)getVoidPtr();
+  *x = 1; // no-crash
+}
+
+void testCastCharPtrToIntPtrThroughIntTypedAssignment() {
+  unsigned c;
+  int *x;
+  (*((int *)(&x))) = (int)&c;
+  *x = 1;
+  clang_analyzer_eval(c == 1); // expected-warning{{TRUE}}
+}
Index: lib/StaticAnalyzer/Core/Store.cpp
===================================================================
--- lib/StaticAnalyzer/Core/Store.cpp
+++ lib/StaticAnalyzer/Core/Store.cpp
@@ -378,6 +378,20 @@
   if (castTy.isNull() || V.isUnknownOrUndef())
     return V;
 
+  // When retrieving symbolic pointer and expecting a non-void pointer,
+  // wrap them into element regions of the expected type if necessary.
+  // SValBuilder::dispatchCast() doesn't do that, but it is necessary to
+  // make sure that the retrieved value makes sense, because there's no other
+  // cast in the AST that would tell us to cast it to the correct pointer type.
+  // We might need to do that for non-void pointers as well.
+  // FIXME: We really need a single good function to perform casts for us
+  // correctly every time we need it.
+  if (castTy->isPointerType() && !castTy->isVoidPointerType())
+    if (const auto *SR = dyn_cast_or_null<SymbolicRegion>(V.getAsRegion()))
+      if (SR->getSymbol()->getType().getCanonicalType() !=
+          castTy.getCanonicalType())
+        return loc::MemRegionVal(castRegion(SR, castTy));
+
   return svalBuilder.dispatchCast(V, castTy);
 }
 


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