[clang-tidy] New checker to detect suspicious string constructor.

[etienneb]

Summary

Checker to validate string constructor parameters.

A common mistake is to swap parameter for the fill-constructor.

std::string str('x', 4);
std::string str('4', x);

[alexfh]

I wonder whether misc-swapped-arguments can be tuned to handle these cases in a more generic way? The only missing part so far seems to be the lack of cxxConstructExpr support.

clang-tidy/misc/StringConstructorCheck.cpp

105

It seems to be wasteful to have several matchers sharing a bunch checks (the LiteralStringConstructor part). I'd suggest to merge the matchers and use anyOf to handle different sub-cases:

cxxConstructExpr(hasDeclaration(cxxMethodDecl(hasName("basic_string"))),
                 hasArgument(0, hasType(CharPtrType)),
                 hasArgument(1, hasType(isInteger())),
                 anyOf(cxxConstructExpr(hasArgument(1, ZeroExpr)).bind("empty-string"),
                       cxxConstructExpr(...).bind("case2"), ...)

docs/clang-tidy/checks/misc-string-constructor.rst

16

nit: add a semicolon

[alexfh]

In D19146#402410, @alexfh wrote:

I wonder whether misc-swapped-arguments can be tuned to handle these cases in a more generic way? The only missing part so far seems to be the lack of cxxConstructExpr support.

And by "these cases" I mean the swapped arguments mistake, not the more string-specific ones.

[etienneb]

alexfh comments.

[etienneb]

fix unittests

[etienneb]

add missing unittest for large length parameter

[alexfh]

In D19146#402414, @alexfh wrote:

In D19146#402410, @alexfh wrote:

I wonder whether misc-swapped-arguments can be tuned to handle these cases in a more generic way? The only missing part so far seems to be the lack of cxxConstructExpr support.

And by "these cases" I mean the swapped arguments mistake, not the more string-specific ones.

What about this comment?

[alexfh]

clang-tidy/misc/StringConstructorCheck.cpp

105

We usually add some description to asserts (assert(X && "X should not be nullptr");).

108

Might be nicer to pull E->getLocStart() to a variable.

[etienneb]

alexfh nits.

[alexfh]

Looks good with a nit.

In D19146#403234, @alexfh wrote:

In D19146#402414, @alexfh wrote:

In D19146#402410, @alexfh wrote:

I wonder whether misc-swapped-arguments can be tuned to handle these cases in a more generic way? The only missing part so far seems to be the lack of cxxConstructExpr support.

And by "these cases" I mean the swapped arguments mistake, not the more string-specific ones.

What about this comment?

As per offline discussion, there might be value in having the check for swapped std::string ctor arguments here, since this specialized check is likely to have much lower false positive rate than the more generic misc-swapped-arguments check.

clang-tidy/misc/StringConstructorCheck.cpp

117

S and L are neither descriptive nor seem to be a commonly used name for this purpose. Can you expand variable names a bit?

[etienneb]

rebased

clang-tidy/misc/CMakeLists.txt

add_clang_library(clangTidyMiscModule
   MoveConstantArgumentCheck.cpp
   MoveConstructorInitCheck.cpp
   MultipleStatementMacroCheck.cpp
   NewDeleteOverloadsCheck.cpp
   NoexceptMoveConstructorCheck.cpp
   NonCopyableObjects.cpp
   SizeofContainerCheck.cpp
   StaticAssertCheck.cpp
+  StringConstructorCheck.cpp
   StringIntegerAssignmentCheck.cpp
   StringLiteralWithEmbeddedNulCheck.cpp
   SuspiciousMissingCommaCheck.cpp
   SuspiciousSemicolonCheck.cpp
   SwappedArgumentsCheck.cpp
   ThrowByValueCatchByReferenceCheck.cpp
   UndelegatedConstructor.cpp
   UnusedAliasDeclsCheck.cpp

clang-tidy/misc/MiscTidyModule.cpp

 #include "MoveConstantArgumentCheck.h"
 #include "MoveConstructorInitCheck.h"
 #include "MultipleStatementMacroCheck.h"
 #include "NewDeleteOverloadsCheck.h"
 #include "NoexceptMoveConstructorCheck.h"
 #include "NonCopyableObjects.h"
 #include "SizeofContainerCheck.h"
 #include "StaticAssertCheck.h"
+#include "StringConstructorCheck.h"
 #include "StringIntegerAssignmentCheck.h"
 #include "StringLiteralWithEmbeddedNulCheck.h"
 #include "SuspiciousMissingCommaCheck.h"
 #include "SuspiciousSemicolonCheck.h"
 #include "SwappedArgumentsCheck.h"
 #include "ThrowByValueCatchByReferenceCheck.h"
 #include "UndelegatedConstructor.h"
 #include "UniqueptrResetReleaseCheck.h"
CheckFactories.registerCheck<NewDeleteOverloadsCheck>(
         "misc-new-delete-overloads");
     CheckFactories.registerCheck<NoexceptMoveConstructorCheck>(
         "misc-noexcept-move-constructor");
     CheckFactories.registerCheck<NonCopyableObjectsCheck>(
         "misc-non-copyable-objects");
     CheckFactories.registerCheck<SizeofContainerCheck>("misc-sizeof-container");
     CheckFactories.registerCheck<StaticAssertCheck>(
         "misc-static-assert");
+    CheckFactories.registerCheck<StringConstructorCheck>(
+        "misc-string-constructor");
     CheckFactories.registerCheck<StringIntegerAssignmentCheck>(
         "misc-string-integer-assignment");
     CheckFactories.registerCheck<StringLiteralWithEmbeddedNulCheck>(
         "misc-string-literal-with-embedded-nul");
     CheckFactories.registerCheck<SuspiciousMissingCommaCheck>(
         "misc-suspicious-missing-comma");
     CheckFactories.registerCheck<SuspiciousSemicolonCheck>(
         "misc-suspicious-semicolon");

clang-tidy/misc/StringConstructorCheck.h

+//===--- StringConstructorCheck.h - clang-tidy-------------------*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_STRING_CONSTRUCTOR_H
+#define LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_STRING_CONSTRUCTOR_H
+
+#include "../ClangTidy.h"
+
+namespace clang {
+namespace tidy {
+namespace misc {
+
+/// Finds suspicious string constructor and check their parameters.
+///
+/// For the user-facing documentation see:
+/// http://clang.llvm.org/extra/clang-tidy/checks/misc-string-constructor.html
+class StringConstructorCheck : public ClangTidyCheck {
+public:
+  StringConstructorCheck(StringRef Name, ClangTidyContext *Context);
+  void storeOptions(ClangTidyOptions::OptionMap &Opts);
+  void registerMatchers(ast_matchers::MatchFinder *Finder) override;
+  void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
+
+private:
+  const bool WarnOnLargeLength;
+  const unsigned int LargeLengthThreshold;
+};
+
+} // namespace misc
+} // namespace tidy
+} // namespace clang
+
+#endif // LLVM_CLANG_TOOLS_EXTRA_CLANG_TIDY_MISC_STRING_CONSTRUCTOR_H

clang-tidy/misc/StringConstructorCheck.cpp

+//===--- StringConstructorCheck.cpp - clang-tidy---------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "StringConstructorCheck.h"
+#include "clang/AST/ASTContext.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang {
+namespace tidy {
+namespace misc {
+
+AST_MATCHER_P(IntegerLiteral, isBiggerThan, unsigned, N) {
+  return Node.getValue().getZExtValue() > N;
+}
+
+StringConstructorCheck::StringConstructorCheck(StringRef Name,
+                                               ClangTidyContext *Context)
+    : ClangTidyCheck(Name, Context),
+      WarnOnLargeLength(Options.get("WarnOnLargeLength", 1) != 0),
+      LargeLengthThreshold(Options.get("LargeLengthThreshold", 0x800000)) {}
+
+void StringConstructorCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
+  Options.store(Opts, "WarnOnLargeLength", WarnOnLargeLength);
+  Options.store(Opts, "LargeLengthThreshold", LargeLengthThreshold);
+}
+
+void StringConstructorCheck::registerMatchers(MatchFinder *Finder) {
+  if (!getLangOpts().CPlusPlus)
+    return;
+
+  const auto ZeroExpr = expr(ignoringParenImpCasts(integerLiteral(equals(0))));
+  const auto CharExpr = expr(ignoringParenImpCasts(characterLiteral()));
+  const auto NegativeExpr = expr(ignoringParenImpCasts(
+      unaryOperator(hasOperatorName("-"),
+                    hasUnaryOperand(integerLiteral(unless(equals(0)))))));
+  const auto LargeLengthExpr = expr(ignoringParenImpCasts(
+      integerLiteral(isBiggerThan(LargeLengthThreshold))));
+  const auto CharPtrType = type(anyOf(pointerType(), arrayType()));
+
+  // Match a string-literal; even through a declaration with initializer.
+  const auto BoundStringLiteral = stringLiteral().bind("str");
+  const auto ConstStrLiteralDecl = varDecl(
+      isDefinition(), hasType(constantArrayType()), hasType(isConstQualified()),
+      hasInitializer(ignoringParenImpCasts(BoundStringLiteral)));
+  const auto ConstPtrStrLiteralDecl = varDecl(
+      isDefinition(),
+      hasType(pointerType(pointee(isAnyCharacter(), isConstQualified()))),
+      hasInitializer(ignoringParenImpCasts(BoundStringLiteral)));
+  auto ConstStrLiteral = expr(ignoringParenImpCasts(anyOf(
+      BoundStringLiteral, declRefExpr(hasDeclaration(anyOf(
+                              ConstPtrStrLiteralDecl, ConstStrLiteralDecl))))));
+
+  // Check the fill constructor. Fills the string with n consecutive copies of
+  // character c. [i.e string(size_t n, char c);].
+  Finder->addMatcher(
+      cxxConstructExpr(
+          hasDeclaration(cxxMethodDecl(hasName("basic_string"))),
+          hasArgument(0, hasType(qualType(isInteger()))),
+          hasArgument(1, hasType(qualType(isInteger()))),
+          anyOf(
+              // Detect the expression: string('x', 40);
+              hasArgument(0, CharExpr.bind("swapped-parameter")),
+              // Detect the expression: string(0, ...);
+              hasArgument(0, ZeroExpr.bind("empty-string")),
+              // Detect the expression: string(-4, ...);
+              hasArgument(0, NegativeExpr.bind("negative-length")),
+              // Detect the expression: string(0x1234567, ...);
+              hasArgument(0, LargeLengthExpr.bind("large-length"))))
+          .bind("constructor"),
+      this);
+
+  // Check the literal string constructor with char pointer and length
+  // parameters. [i.e. string (const char* s, size_t n);]
+  Finder->addMatcher(
+      cxxConstructExpr(
+          hasDeclaration(cxxMethodDecl(hasName("basic_string"))),
+          hasArgument(0, hasType(CharPtrType)),
+          hasArgument(1, hasType(isInteger())),
+          anyOf(
+              // Detect the expression: string("...", 0);
+              hasArgument(1, ZeroExpr.bind("empty-string")),
+              // Detect the expression: string("...", -4);
+              hasArgument(1, NegativeExpr.bind("negative-length")),
+              // Detect the expression: string("lit", 5)
+              allOf(hasArgument(0, ConstStrLiteral.bind("literal-with-length")),
+                    hasArgument(1, ignoringParenImpCasts(
+                                       integerLiteral().bind("int")))),
+              // Detect the expression: string("lit", 0x1234567);
+              hasArgument(1, LargeLengthExpr.bind("large-length")))),
+      this);
+}
+
+void StringConstructorCheck::check(const MatchFinder::MatchResult &Result) {
+  if (const auto *E = Result.Nodes.getNodeAs<Expr>("swapped-parameter")) {
+    diag(E->getLocStart(), "constructor parameters are probably swapped");
+  } else if (const auto *E = Result.Nodes.getNodeAs<Expr>("empty-string")) {
+    diag(E->getLocStart(), "constructor creating an empty string");
+  } else if (const auto *E = Result.Nodes.getNodeAs<Expr>("negative-length")) {
+    diag(E->getLocStart(), "negative value used as length parameter");
+  } else if (const auto *E = Result.Nodes.getNodeAs<Expr>("large-length")) {
+    if (WarnOnLargeLength)
+      diag(E->getLocStart(), "suspicious large length parameter");
+  } else if (const auto *E =
+                 Result.Nodes.getNodeAs<Expr>("literal-with-length")) {
+    const auto *S = Result.Nodes.getNodeAs<StringLiteral>("str");
+    const auto *L = Result.Nodes.getNodeAs<IntegerLiteral>("int");
+    if (L->getValue().ugt(S->getLength())) {
+      diag(E->getLocStart(), "length is bigger then string literal size");
+    }
+  }
+}
+
+} // namespace misc
+} // namespace tidy
+} // namespace clang

docs/clang-tidy/checks/list.rst

.. toctree::
    misc-misplaced-widening-cast
    misc-move-constructor-init
    misc-multiple-statement-macro
    misc-new-delete-overloads
    misc-noexcept-move-constructor
    misc-non-copyable-objects
    misc-sizeof-container
    misc-static-assert
+   misc-string-constructor
    misc-string-integer-assignment
    misc-string-literal-with-embedded-nul
    misc-suspicious-missing-comma
    misc-suspicious-semicolon
    misc-swapped-arguments
    misc-throw-by-value-catch-by-reference
    misc-undelegated-constructor
    misc-uniqueptr-reset-release

docs/clang-tidy/checks/misc-string-constructor.rst

+.. title:: clang-tidy - misc-string-constructor
+
+misc-string-constructor
+=======================
+
+Finds string constructors that are suspicious and probably errors.
+
+
+A common mistake is to swap parameters to the 'fill' string-constructor.
+
+Examples:
+
+.. code:: c++
+
+  std::string('x', 50) str; // should be std::string(50, 'x') 
+
+
+Calling the string-literal constructor with a length bigger than the literal is
+suspicious and adds extra random characters to the string.
+
+Examples:
+
+.. code:: c++
+
+  std::string("test", 200);   // Will include random characters after "test".
+
+
+Creating an empty string from constructors with parameters is considered
+suspicious. The programmer should use the empty constructor instead.
+
+Examples:
+
+.. code:: c++
+
+  std::string("test", 0);   // Creation of an empty string.
+

test/clang-tidy/misc-string-constructor.cpp

+// RUN: %check_clang_tidy %s misc-string-constructor %t
+
+namespace std {
+template <typename T>
+class allocator {};
+template <typename T>
+class char_traits {};
+template <typename C, typename T = std::char_traits<C>, typename A = std::allocator<C> >
+struct basic_string {
+  basic_string();
+  basic_string(const C*, unsigned int size);
+  basic_string(unsigned int size, C c);
+};
+typedef basic_string<char> string;
+typedef basic_string<wchar_t> wstring;
+}
+
+const char* kText = "";
+const char kText2[] = "";
+extern const char kText3[];
+
+void Test() {
+  std::string str('x', 4);
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: constructor parameters are probably swapped [misc-string-constructor]
+  std::wstring wstr(L'x', 4);
+  // CHECK-MESSAGES: [[@LINE-1]]:16: warning: constructor parameters are probably swapped
+  std::string s0(0, 'x');
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: constructor creating an empty string
+  std::string s1(-4, 'x');
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: negative value used as length parameter
+  
+  std::string q0("test", 0);
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: constructor creating an empty string
+  std::string q1(kText, -4);
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: negative value used as length parameter
+  std::string q2("test", 200);
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: length is bigger then string literal size
+  std::string q3(kText, 200);
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: length is bigger then string literal size
+  std::string q4(kText2, 200);
+  // CHECK-MESSAGES: [[@LINE-1]]:15: warning: length is bigger then string literal size
+}
+
+void Valid() {
+  std::string empty();
+  std::string str(4, 'x');
+  std::wstring wstr(4, L'x');
+  std::string s1("test", 4);
+  std::string s2("test", 3);
+}