1 //===--- ProperlySeededRandomGeneratorCheck.cpp - clang-tidy---------------===//
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 #include "ProperlySeededRandomGeneratorCheck.h"
10 #include "clang/AST/ASTContext.h"
11 #include "clang/ASTMatchers/ASTMatchFinder.h"
12 #include "llvm/ADT/STLExtras.h"
13 
14 using namespace clang::ast_matchers;
15 
16 namespace clang {
17 namespace tidy {
18 namespace cert {
19 
20 ProperlySeededRandomGeneratorCheck::ProperlySeededRandomGeneratorCheck(
21     StringRef Name, ClangTidyContext *Context)
22     : ClangTidyCheck(Name, Context),
23       RawDisallowedSeedTypes(
24           Options.get("DisallowedSeedTypes", "time_t,std::time_t")) {
25   StringRef(RawDisallowedSeedTypes).split(DisallowedSeedTypes, ',');
26 }
27 
28 void ProperlySeededRandomGeneratorCheck::storeOptions(
29     ClangTidyOptions::OptionMap &Opts) {
30   Options.store(Opts, "DisallowedSeedTypes", RawDisallowedSeedTypes);
31 }
32 
33 void ProperlySeededRandomGeneratorCheck::registerMatchers(MatchFinder *Finder) {
34   auto RandomGeneratorEngineDecl = cxxRecordDecl(hasAnyName(
35       "::std::linear_congruential_engine", "::std::mersenne_twister_engine",
36       "::std::subtract_with_carry_engine", "::std::discard_block_engine",
37       "::std::independent_bits_engine", "::std::shuffle_order_engine"));
38   auto RandomGeneratorEngineTypeMatcher = hasType(hasUnqualifiedDesugaredType(
39       recordType(hasDeclaration(RandomGeneratorEngineDecl))));
40 
41   // std::mt19937 engine;
42   // engine.seed();
43   //        ^
44   // engine.seed(1);
45   //        ^
46   // const int x = 1;
47   // engine.seed(x);
48   //        ^
49   Finder->addMatcher(
50       cxxMemberCallExpr(
51           has(memberExpr(has(declRefExpr(RandomGeneratorEngineTypeMatcher)),
52                          member(hasName("seed")),
53                          unless(hasDescendant(cxxThisExpr())))))
54           .bind("seed"),
55       this);
56 
57   // std::mt19937 engine;
58   //              ^
59   // std::mt19937 engine(1);
60   //              ^
61   // const int x = 1;
62   // std::mt19937 engine(x);
63   //              ^
64   Finder->addMatcher(
65       traverse(ast_type_traits::TK_AsIs,
66                cxxConstructExpr(RandomGeneratorEngineTypeMatcher).bind("ctor")),
67       this);
68 
69   // srand();
70   // ^
71   // const int x = 1;
72   // srand(x);
73   // ^
74   Finder->addMatcher(
75       callExpr(callee(functionDecl(hasAnyName("::srand", "::std::srand"))))
76           .bind("srand"),
77       this);
78 }
79 
80 void ProperlySeededRandomGeneratorCheck::check(
81     const MatchFinder::MatchResult &Result) {
82   const auto *Ctor = Result.Nodes.getNodeAs<CXXConstructExpr>("ctor");
83   if (Ctor)
84     checkSeed(Result, Ctor);
85 
86   const auto *Func = Result.Nodes.getNodeAs<CXXMemberCallExpr>("seed");
87   if (Func)
88     checkSeed(Result, Func);
89 
90   const auto *Srand = Result.Nodes.getNodeAs<CallExpr>("srand");
91   if (Srand)
92     checkSeed(Result, Srand);
93 }
94 
95 template <class T>
96 void ProperlySeededRandomGeneratorCheck::checkSeed(
97     const MatchFinder::MatchResult &Result, const T *Func) {
98   if (Func->getNumArgs() == 0 || Func->getArg(0)->isDefaultArgument()) {
99     diag(Func->getExprLoc(),
100          "random number generator seeded with a default argument will generate "
101          "a predictable sequence of values");
102     return;
103   }
104 
105   Expr::EvalResult EVResult;
106   if (Func->getArg(0)->EvaluateAsInt(EVResult, *Result.Context)) {
107     diag(Func->getExprLoc(),
108          "random number generator seeded with a constant value will generate a "
109          "predictable sequence of values");
110     return;
111   }
112 
113   const std::string SeedType(
114       Func->getArg(0)->IgnoreCasts()->getType().getAsString());
115   if (llvm::find(DisallowedSeedTypes, SeedType) != DisallowedSeedTypes.end()) {
116     diag(Func->getExprLoc(),
117          "random number generator seeded with a disallowed source of seed "
118          "value will generate a predictable sequence of values");
119     return;
120   }
121 }
122 
123 } // namespace cert
124 } // namespace tidy
125 } // namespace clang
126