1 //=== BuiltinFunctionChecker.cpp --------------------------------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This checker evaluates clang builtin functions.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "clang/Basic/Builtins.h"
14 #include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
15 #include "clang/StaticAnalyzer/Core/Checker.h"
16 #include "clang/StaticAnalyzer/Core/CheckerManager.h"
17 #include "clang/StaticAnalyzer/Core/PathSensitive/CallEvent.h"
18 #include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
19 #include "clang/StaticAnalyzer/Core/PathSensitive/DynamicExtent.h"
20 
21 using namespace clang;
22 using namespace ento;
23 
24 namespace {
25 
26 class BuiltinFunctionChecker : public Checker<eval::Call> {
27 public:
28   bool evalCall(const CallEvent &Call, CheckerContext &C) const;
29 };
30 
31 }
32 
33 bool BuiltinFunctionChecker::evalCall(const CallEvent &Call,
34                                       CheckerContext &C) const {
35   ProgramStateRef state = C.getState();
36   const auto *FD = dyn_cast_or_null<FunctionDecl>(Call.getDecl());
37   if (!FD)
38     return false;
39 
40   const LocationContext *LCtx = C.getLocationContext();
41   const Expr *CE = Call.getOriginExpr();
42 
43   switch (FD->getBuiltinID()) {
44   default:
45     return false;
46 
47   case Builtin::BI__builtin_assume: {
48     assert (Call.getNumArgs() > 0);
49     SVal Arg = Call.getArgSVal(0);
50     if (Arg.isUndef())
51       return true; // Return true to model purity.
52 
53     state = state->assume(Arg.castAs<DefinedOrUnknownSVal>(), true);
54     // FIXME: do we want to warn here? Not right now. The most reports might
55     // come from infeasible paths, thus being false positives.
56     if (!state) {
57       C.generateSink(C.getState(), C.getPredecessor());
58       return true;
59     }
60 
61     C.addTransition(state);
62     return true;
63   }
64 
65   case Builtin::BI__builtin_unpredictable:
66   case Builtin::BI__builtin_expect:
67   case Builtin::BI__builtin_expect_with_probability:
68   case Builtin::BI__builtin_assume_aligned:
69   case Builtin::BI__builtin_addressof:
70   case Builtin::BI__builtin_function_start: {
71     // For __builtin_unpredictable, __builtin_expect,
72     // __builtin_expect_with_probability and __builtin_assume_aligned,
73     // just return the value of the subexpression.
74     // __builtin_addressof is going from a reference to a pointer, but those
75     // are represented the same way in the analyzer.
76     assert (Call.getNumArgs() > 0);
77     SVal Arg = Call.getArgSVal(0);
78     C.addTransition(state->BindExpr(CE, LCtx, Arg));
79     return true;
80   }
81 
82   case Builtin::BI__builtin_alloca_with_align:
83   case Builtin::BI__builtin_alloca: {
84     // FIXME: Refactor into StoreManager itself?
85     MemRegionManager& RM = C.getStoreManager().getRegionManager();
86     const AllocaRegion* R =
87       RM.getAllocaRegion(CE, C.blockCount(), C.getLocationContext());
88 
89     // Set the extent of the region in bytes. This enables us to use the
90     // SVal of the argument directly. If we save the extent in bits, we
91     // cannot represent values like symbol*8.
92     auto Size = Call.getArgSVal(0);
93     if (Size.isUndef())
94       return true; // Return true to model purity.
95 
96     state = setDynamicExtent(state, R, Size.castAs<DefinedOrUnknownSVal>(),
97                              C.getSValBuilder());
98 
99     C.addTransition(state->BindExpr(CE, LCtx, loc::MemRegionVal(R)));
100     return true;
101   }
102 
103   case Builtin::BI__builtin_dynamic_object_size:
104   case Builtin::BI__builtin_object_size:
105   case Builtin::BI__builtin_constant_p: {
106     // This must be resolvable at compile time, so we defer to the constant
107     // evaluator for a value.
108     SValBuilder &SVB = C.getSValBuilder();
109     SVal V = UnknownVal();
110     Expr::EvalResult EVResult;
111     if (CE->EvaluateAsInt(EVResult, C.getASTContext(), Expr::SE_NoSideEffects)) {
112       // Make sure the result has the correct type.
113       llvm::APSInt Result = EVResult.Val.getInt();
114       BasicValueFactory &BVF = SVB.getBasicValueFactory();
115       BVF.getAPSIntType(CE->getType()).apply(Result);
116       V = SVB.makeIntVal(Result);
117     }
118 
119     if (FD->getBuiltinID() == Builtin::BI__builtin_constant_p) {
120       // If we didn't manage to figure out if the value is constant or not,
121       // it is safe to assume that it's not constant and unsafe to assume
122       // that it's constant.
123       if (V.isUnknown())
124         V = SVB.makeIntVal(0, CE->getType());
125     }
126 
127     C.addTransition(state->BindExpr(CE, LCtx, V));
128     return true;
129   }
130   }
131 }
132 
133 void ento::registerBuiltinFunctionChecker(CheckerManager &mgr) {
134   mgr.registerChecker<BuiltinFunctionChecker>();
135 }
136 
137 bool ento::shouldRegisterBuiltinFunctionChecker(const CheckerManager &mgr) {
138   return true;
139 }
140