1 //===--- SuspiciousMemsetUsageCheck.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 "SuspiciousMemsetUsageCheck.h"
10 #include "clang/AST/ASTContext.h"
11 #include "clang/ASTMatchers/ASTMatchFinder.h"
12 #include "clang/ASTMatchers/ASTMatchers.h"
13 #include "clang/Lex/Lexer.h"
14 #include "clang/Tooling/FixIt.h"
15
16 using namespace clang::ast_matchers;
17
18 namespace clang {
19 namespace tidy {
20 namespace bugprone {
21
registerMatchers(MatchFinder * Finder)22 void SuspiciousMemsetUsageCheck::registerMatchers(MatchFinder *Finder) {
23 // Match the standard memset:
24 // void *memset(void *buffer, int fill_char, size_t byte_count);
25 auto MemsetDecl =
26 functionDecl(hasName("::memset"),
27 parameterCountIs(3),
28 hasParameter(0, hasType(pointerType(pointee(voidType())))),
29 hasParameter(1, hasType(isInteger())),
30 hasParameter(2, hasType(isInteger())));
31
32 // Look for memset(x, '0', z). Probably memset(x, 0, z) was intended.
33 Finder->addMatcher(
34 callExpr(
35 callee(MemsetDecl), argumentCountIs(3),
36 hasArgument(1, characterLiteral(equals(static_cast<unsigned>('0')))
37 .bind("char-zero-fill")),
38 unless(hasArgument(
39 0, anyOf(hasType(pointsTo(isAnyCharacter())),
40 hasType(arrayType(hasElementType(isAnyCharacter()))))))),
41 this);
42
43 // Look for memset with an integer literal in its fill_char argument.
44 // Will check if it gets truncated.
45 Finder->addMatcher(
46 callExpr(callee(MemsetDecl), argumentCountIs(3),
47 hasArgument(1, integerLiteral().bind("num-fill"))),
48 this);
49
50 // Look for memset(x, y, 0) as that is most likely an argument swap.
51 Finder->addMatcher(
52 callExpr(callee(MemsetDecl), argumentCountIs(3),
53 unless(hasArgument(1, anyOf(characterLiteral(equals(
54 static_cast<unsigned>('0'))),
55 integerLiteral()))))
56 .bind("call"),
57 this);
58 }
59
check(const MatchFinder::MatchResult & Result)60 void SuspiciousMemsetUsageCheck::check(const MatchFinder::MatchResult &Result) {
61 if (const auto *CharZeroFill =
62 Result.Nodes.getNodeAs<CharacterLiteral>("char-zero-fill")) {
63 // Case 1: fill_char of memset() is a character '0'. Probably an
64 // integer zero was intended.
65
66 SourceRange CharRange = CharZeroFill->getSourceRange();
67 auto Diag =
68 diag(CharZeroFill->getBeginLoc(), "memset fill value is char '0', "
69 "potentially mistaken for int 0");
70
71 // Only suggest a fix if no macros are involved.
72 if (CharRange.getBegin().isMacroID())
73 return;
74 Diag << FixItHint::CreateReplacement(
75 CharSourceRange::getTokenRange(CharRange), "0");
76 }
77
78 else if (const auto *NumFill =
79 Result.Nodes.getNodeAs<IntegerLiteral>("num-fill")) {
80 // Case 2: fill_char of memset() is larger in size than an unsigned char
81 // so it gets truncated during conversion.
82
83 const auto UCharMax = (1 << Result.Context->getCharWidth()) - 1;
84 Expr::EvalResult EVResult;
85 if (!NumFill->EvaluateAsInt(EVResult, *Result.Context))
86 return;
87
88 llvm::APSInt NumValue = EVResult.Val.getInt();
89 if (NumValue >= 0 && NumValue <= UCharMax)
90 return;
91
92 diag(NumFill->getBeginLoc(), "memset fill value is out of unsigned "
93 "character range, gets truncated");
94 }
95
96 else if (const auto *Call = Result.Nodes.getNodeAs<CallExpr>("call")) {
97 // Case 3: byte_count of memset() is zero. This is most likely an
98 // argument swap.
99
100 const Expr *FillChar = Call->getArg(1);
101 const Expr *ByteCount = Call->getArg(2);
102
103 // Return if `byte_count` is not zero at compile time.
104 Expr::EvalResult Value2;
105 if (ByteCount->isValueDependent() ||
106 !ByteCount->EvaluateAsInt(Value2, *Result.Context) ||
107 Value2.Val.getInt() != 0)
108 return;
109
110 // Return if `fill_char` is known to be zero or negative at compile
111 // time. In these cases, swapping the args would be a nop, or
112 // introduce a definite bug. The code is likely correct.
113 Expr::EvalResult EVResult;
114 if (!FillChar->isValueDependent() &&
115 FillChar->EvaluateAsInt(EVResult, *Result.Context)) {
116 llvm::APSInt Value1 = EVResult.Val.getInt();
117 if (Value1 == 0 || Value1.isNegative())
118 return;
119 }
120
121 // `byte_count` is known to be zero at compile time, and `fill_char` is
122 // either not known or known to be a positive integer. Emit a warning
123 // and fix-its to swap the arguments.
124 auto D = diag(Call->getBeginLoc(),
125 "memset of size zero, potentially swapped arguments");
126 StringRef RHSString = tooling::fixit::getText(*ByteCount, *Result.Context);
127 StringRef LHSString = tooling::fixit::getText(*FillChar, *Result.Context);
128 if (LHSString.empty() || RHSString.empty())
129 return;
130
131 D << tooling::fixit::createReplacement(*FillChar, RHSString)
132 << tooling::fixit::createReplacement(*ByteCount, LHSString);
133 }
134 }
135
136 } // namespace bugprone
137 } // namespace tidy
138 } // namespace clang
139