[analyzer][MallocChecker] When modeling realloc-like functions, don't early return if the argument is symbolic

[Szelethus]

Summary

The very essence of MallocChecker lies in 2 overload sets: the FreeMemAux functions and the MallocMemAux functions. The former houses most of the error checking as well (aside from leaks), such as incorrect deallocation. There, we check whether the argument's [[ https://clang.llvm.org/doxygen/classclang_1_1ento_1_1MemSpaceRegion.html | MemSpaceRegion]] is the heap or unknown, and if it isn't, we know we encountered a bug (aside from a corner case patched by @balazske in D76830), as specified by MEM34-C.

In ReallocMemAux, which really is the combination of FreeMemAux and MallocMemAux, we incorrectly early returned if the memory argument of realloc is non-symbolic. The problem is, one of the cases where this happens when we know precisely what the region is, like an array, as demonstrated in the test file. So, lets get rid of this false negative :^)

Side note, I dislike the warning message and the associated checker name, but I'll address it in a later patch.

[xazax.hun]

LGTM, thanks!

I do agree that the warning message is not the best but it is not horrible either :)

clang/lib/StaticAnalyzer/Checkers/MallocChecker.cpp

if (PrtIsNull && !SizeIsZero) {
     ProgramStateRef stateMalloc =
         MallocMemAux(C, CE, TotalSize, UndefinedVal(), StatePtrIsNull, Family);
     return stateMalloc;
   }
 
   if (PrtIsNull && SizeIsZero)
     return State;
 
-  // Get the from and to pointer symbols as in toPtr = realloc(fromPtr, size).
   assert(!PrtIsNull);
-  SymbolRef FromPtr = arg0Val.getAsSymbol();
-  SVal RetVal = C.getSVal(CE);
-  SymbolRef ToPtr = RetVal.getAsSymbol();
-  if (!FromPtr || !ToPtr)
-    return nullptr;
 
   bool IsKnownToBeAllocated = false;
 
   // If the size is 0, free the memory.
   if (SizeIsZero)
     // The semantics of the return value are:
     // If size was equal to 0, either NULL or a pointer suitable to be passed
     // to free() is returned. We just free the input pointer and do not add
if (!stateRealloc)
       return nullptr;
 
     OwnershipAfterReallocKind Kind = OAR_ToBeFreedAfterFailure;
     if (ShouldFreeOnFail)
       Kind = OAR_FreeOnFailure;
     else if (!IsKnownToBeAllocated)
       Kind = OAR_DoNotTrackAfterFailure;
 
+    // Get the from and to pointer symbols as in toPtr = realloc(fromPtr, size).
+    SymbolRef FromPtr = arg0Val.getAsSymbol();
+    SVal RetVal = C.getSVal(CE);
+    SymbolRef ToPtr = RetVal.getAsSymbol();
+    assert(FromPtr && ToPtr &&
+           "By this point, FreeMemAux and MallocMemAux should have checked "
+           "whether the argument or the return value is symbolic!");
+
     // Record the info about the reallocated symbol so that we could properly
     // process failed reallocation.
     stateRealloc = stateRealloc->set<ReallocPairs>(ToPtr,
                                                    ReallocPair(FromPtr, Kind));
     // The reallocated symbol should stay alive for as long as the new symbol.
     C.getSymbolManager().addSymbolDependency(ToPtr, FromPtr);
     return stateRealloc;
   }

clang/test/Analysis/malloc.c

 
 void list_add(struct ListInfo *list, struct ListInfo *item);
 
 void testCStyleListItems(struct ListInfo *list) {
   struct ConcreteListItem *x = malloc(sizeof(struct ConcreteListItem));
   list_add(list, &x->li); // will free 'x'.
 }
 
+// MEM34-C. Only free memory allocated dynamically
+// Second non-compliant example.
+// https://wiki.sei.cmu.edu/confluence/display/c/MEM34-C.+Only+free+memory+allocated+dynamically
+enum { BUFSIZE = 256 };
+
+void MEM34_C(void) {
+  char buf[BUFSIZE];
+  char *p = (char *)realloc(buf, 2 * BUFSIZE);
+  // expected-warning@-1{{Argument to realloc() is the address of the local \
+variable 'buf', which is not memory allocated by malloc() [unix.Malloc]}}
+  if (p == NULL) {
+    /* Handle error */
+  }
+}
+
 // ----------------------------------------------------------------------------
 // False negatives.
 
 void testMallocWithParam(int **p) {
   *p = (int*) malloc(sizeof(int));
   *p = 0; // FIXME: should warn here
 }