Improvements to C/C++ analysis

New queries

Query	Tags	Purpose
Cast between HRESULT and a Boolean type (cpp/hresult-boolean-conversion)	security, external/cwe/cwe-253	Finds logic errors caused by mistakenly treating the Windows HRESULT type as a Boolean instead of testing it with the appropriate macros. Results are shown on LGTM by default.
Cast from char* to wchar_t* (cpp/incorrect-string-type-conversion)	security, external/cwe/cwe-704	Detects potentially dangerous casts from char* to wchar_t*. Results are shown on LGTM by default.
Dead code due to goto or break statement (cpp/dead-code-goto)	maintainability, external/cwe/cwe-561	Detects dead code following a goto or break statement. Results are shown on LGTM by default.
Inconsistent direction of for loop (cpp/inconsistent-loop-direction)	correctness, external/cwe/cwe-835	Detects for loops where the increment and guard condition don't appear to correspond. Results are shown on LGTM by default.
Incorrect 'not' operator usage (cpp/incorrect-not-operator-usage)	security, external/cwe/cwe-480	Finds uses of the logical not (!) operator that look like they should be bit-wise not (~). Results are hidden on LGTM by default.
Non-virtual destructor in base class (cpp/virtual-destructor)	reliability, readability, language-features	This query, NonVirtualDestructorInBaseClass.ql, is a replacement in LGTM for the query: No virtual destructor (AV Rule 78.ql). The new query ignores base classes with non-public destructors since we consider those to be adequately protected. The new version retains the query identifier, cpp/virtual-destructor, and results are displayed by default on LGTM. The old query is no longer run on LGTM.
NULL application name with an unquoted path in call to CreateProcess (cpp/unsafe-create-process-call)	security, external/cwe/cwe-428	Finds unsafe uses of the CreateProcess function. Results are hidden on LGTM by default.
Setting a DACL to NULL in a SECURITY_DESCRIPTOR (cpp/unsafe-dacl-security-descriptor)	security, external/cwe/cwe-732	Finds code that creates world-writable objects on Windows by setting their DACL to NULL. Results are shown on LGTM by default.

Changes to existing LGTM queries

Query	Expected impact	Change
Comparison result is always the same (cpp/constant-comparison)	Fewer false positive results	Comparisons in template instantiations are now excluded from results.
Empty branch of conditional (cpp/empty-block)	Fewer false positive results	Now recognizes commented blocks more reliably.
Expression has no effect (cpp/useless-expression)	Fewer false positive results	Expressions in template instantiations are now excluded from results.
Missing return statement (cpp/missing-return)	Fewer false positive results, visible by default	Improved results when a function returns a template-dependent type, or makes a non-returning call to another function. Precision increased from 'medium' to 'high' so that alerts are shown by default in LGTM.
Multiplication result converted to larger type (cpp/integer-multiplication-cast-to-long)	Fewer false positive results	Char-typed numbers are no longer considered to be potentially large.
No virtual destructor (cpp/jsf/av-rule-78)	No results in LGTM	This query is part of the Joint Strike Fighter suite which defines strict coding rules for air vehicles. Its query identifier has been revised to reflect this. On LGTM this query has been replaced by the similar query "Non-virtual destructor in base class", see New queries above. The new query highlights only code that is likely to be a problem in the majority of projects.
Overloaded assignment does not return 'this' (cpp/assignment-does-not-return-this)	Fewer false positive results	Any return statements that are unreachable are now ignored.
Resource not released in destructor (cpp/resource-not-released-in-destructor)	Fewer false positive results	No longer highlights uses of C++ placement new and results are no longer reported for resources where the destructor body is not in the snapshot database.
Self comparison (cpp/comparison-of-identical-expressions)	Fewer false positive results	Code inside macro invocations is now excluded from the query.
Static array access may cause overflow (cpp/static-buffer-overflow)	More correct results	Data flow to the size argument of a buffer operation is now checked in this query.
Suspicious add with sizeof (cpp/suspicious-add-sizeof)	Fewer false positive results	Arithmetic with void pointers (where allowed) is now excluded from results.
Unsigned comparison to zero (cpp/unsigned-comparison-zero)	Fewer false positive results	Comparisons in template instantiations are now excluded from results.
Wrong type of arguments to formatting function (cpp/wrong-type-format-argument)	Fewer false positive results	False positive results involving typedefs have been removed. Expected argument types are determined more accurately, especially for wide string and pointer types. Custom (non-standard) formatting functions are also identified more accurately.

Changes to other queries

Query	Expected impact	Change
Array offset used before range check (cpp/offset-use-before-range-check)	More results and fewer false positive results	Now recognizes array accesses in different positions within the expression. Code where the range is checked before and after the array access is no longer highlighted.
AV Rule 164 (cpp/jsf/av-rule-164)	Fewer false positive results	Now accounts for explicit casts.
Call to memory access function may overflow buffer (cpp/overflow-buffer)	More correct results	Array indexing with a negative index is now detected by this query.
Global could be static (cpp/jpl-c/limited-scope-file and cpp/power-of-10/global-could-be-static)	Fewer false positive results	Variables with declarations in header files are now excluded from results.
Memory is never freed (cpp/memory-never-freed)	Fewer false positive results	No longer highlights uses of C++ placement new, which returns a pointer that does not need to be freed.
Negation of unsigned value (cpp/jsf/av-rule-165)	Fewer false positive results	Now accounts for explicit casts.
Suspicious call to memset (cpp/suspicious-call-to-memset)	Fewer false positive results	Types involving decltype are now correctly compared.
Variable scope too large (cpp/jpl-c/limited-scope-function and cpp/power-of-10/variable-scope-too-large)	Fewer false positive results	Variables with declarations in header files, or that are used at file scope, are now excluded from results.

Changes to QL libraries

• New hash consing library (semmle.code.cpp.valuenumbering.HashCons) for structural comparison of expressions. Unlike the existing library for global value numbering, this library implements a pure syntactic comparison of expressions and will equate expressions even if they may not compute the same value.
• The Buffer.qll library has more conservative treatment of arrays embedded in structs. This reduces false positive results in a number of security queries, especially cpp/overflow-buffer.
  • Pre-C99 encodings of flexible array members are recognized more reliably.
  • Arrays of zero size are now treated as a special case.
• The library semmle.code.cpp.dataflow.RecursionPrevention is now deprecated. It was an aid for transitioning data-flow queries from 1.16 to 1.17, and it no longer has any function. Imports of this library should simply be deleted.