Warn when sizeof is used in strncmp ticket #2095

This checks for the case where the user thought sizeof(buf) gave the
size in bytes of 'buf' in code like the following:

  const char *buf = "Hello World";
  strncmp(buf, other, sizeof(buf));

lib/checkother.cpp

@@ -421,6 +421,37 @@ void CheckOther::sizeofForArrayParameterError(const Token *tok)
                );
 }
 
+void CheckOther::checkSizeofForStrncmpSize()
+{
+    const SymbolDatabase *symbolDatabase = _tokenizer->getSymbolDatabase();
+    const char pattern1[] = "strncmp ( %any , %any% , sizeof ( %var% ) )";
+    const char pattern2[] = "strncmp ( %any , %any% , sizeof %var% )";
+    for (const Token *tok = _tokenizer->tokens(); tok; tok = tok->next()) {
+        if (Token::Match(tok, pattern1) || Token::Match(tok, pattern2)) {
+            int tokIdx = 7;
+            if (tok->tokAt(tokIdx)->str() == "(")
+                ++tokIdx;
+            const Token *tokVar = tok->tokAt(tokIdx);
+            if (tokVar->varId() > 0) {
+                const Variable *var = symbolDatabase->getVariableFromVarId(tokVar->varId());
+                if (var && var->nameToken()->strAt(-1) == "*") {
+                    sizeofForStrncmpError(tokVar);
+                }
+            }
+        }
+    }
+}
+
+void CheckOther::sizeofForStrncmpError(const Token *tok)
+{
+    reportError(tok, Severity::warning, "strncmpLen",
+                "Passing sizeof(pointer) as the last argument to strncmp.\n"
+                "Passing a pointer to sizeof returns the size of the pointer, not "
+                "the number of characters pointed to by that pointer. This "
+                "means that only 4 or 8 (on 64-bit systems) characters are "
+                "compared, which is probably not what was expected.");
+}
+
 //---------------------------------------------------------------------------
 //    switch (x)
 //    {

lib/checkother.h

@@ -58,6 +58,7 @@ public:
         checkOther.checkRedundantAssignmentInSwitch();
         checkOther.checkAssignmentInAssert();
         checkOther.checkSizeofForArrayParameter();
+        checkOther.checkSizeofForStrncmpSize();
         checkOther.checkSizeofForNumericParameter();
         checkOther.checkSelfAssignment();
         checkOther.checkDuplicateIf();
@@ -196,6 +197,9 @@ public:
     /** @brief %Check for using sizeof with array given as function argument */
     void checkSizeofForArrayParameter();
 
+    /** @brief %Check for using sizeof with a char pointer */
+    void checkSizeofForStrncmpSize();
+
     /** @brief %Check for using sizeof with numeric given as function argument */
     void checkSizeofForNumericParameter();
 
@@ -264,6 +268,7 @@ public:
     void catchExceptionByValueError(const Token *tok);
     void memsetZeroBytesError(const Token *tok, const std::string &varname);
     void sizeofForArrayParameterError(const Token *tok);
+    void sizeofForStrncmpError(const Token *tok);
     void sizeofForNumericParameterError(const Token *tok);
     void incorrectStringCompareError(const Token *tok, const std::string& func, const std::string &string, const std::string &len);
     void incorrectStringBooleanError(const Token *tok, const std::string& string);
@@ -294,6 +299,7 @@ public:
         c.fflushOnInputStreamError(0, "stdin");
         c.misusedScopeObjectError(NULL, "varname");
         c.sizeofForArrayParameterError(0);
+        c.sizeofForStrncmpError(0);
         c.sizeofForNumericParameterError(0);
         c.coutCerrMisusageError(0, "cout");
 

test/testother.cpp

@@ -139,6 +139,7 @@ private:
         TEST_CASE(duplicateExpression2); // ticket #2730
 
         TEST_CASE(alwaysTrueFalseStringCompare);
+        TEST_CASE(checkStrncmpSizeof);
         TEST_CASE(checkSignOfUnsignedVariable);
 
         TEST_CASE(checkForSuspiciousSemicolon1);
@@ -3440,6 +3441,16 @@ private:
         ASSERT_EQUALS("[test.cpp:3]: (warning) Comparison of identical string variables.\n", errout.str());
     }
 
+    void checkStrncmpSizeof() {
+        check(
+            "int fun(const char *buf1)\n"
+            "{\n"
+            "  const char *buf1_ex = \"foobarbaz\";\n"
+            "  return strncmp(buf1, buf1_ex, sizeof(buf1_ex)) == 0;\n"
+            "}");
+        ASSERT_EQUALS("[test.cpp:4]: (warning) Passing sizeof(pointer) as the last argument to strncmp.\n", errout.str());
+    }
+
     void check_signOfUnsignedVariable(const char code[]) {
         // Clear the error buffer..
         errout.str("");