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Improvements to C/C++ analysis

The following changes in version 1.24 affect C/C++ analysis in all applications.

General improvements

You can now suppress alerts using either single-line block comments (/* ... */) or line comments (// ...).

New queries

Query Tags Purpose
Implicit function declarations (cpp/Likely Bugs/Underspecified Functions/ImplicitFunctionDeclaration.ql) correctness, maintainability This query finds calls to undeclared functions that are compiled by a C compiler. Results are shown on LGTM by default.

Changes to existing queries

A new taint-tracking library is used by all the security queries that track tainted values (cpp/path-injection, cpp/cgi-xss, cpp/sql-injection, cpp/uncontrolled-process-operation, cpp/unbounded-write, cpp/tainted-format-string, cpp/tainted-format-string-through-global, cpp/uncontrolled-arithmetic, cpp/uncontrolled-allocation-size, cpp/user-controlled-bypass, cpp/cleartext-storage-buffer, cpp/tainted-permissions-check). These queries now have more precise results and also offer path explanations so you can explore the results easily. There is a performance cost to this, and the LGTM query suite will overall run slower than before.

Query Expected impact Change
Boost_asio TLS Settings Misconfiguration (cpp/boost/tls-settings-misconfiguration) Query id change The identifier was updated to use dashes in place of underscores (previous identifier cpp/boost/tls_settings_misconfiguration).
Buffer not sufficient for string (cpp/overflow-calculated) More true positive results This query now identifies a wider variety of buffer allocations using the semmle.code.cpp.models.interfaces.Allocation library.
Hard-coded Japanese era start date (cpp/japanese-era/exact-era-date) This query is no longer run on LGTM.
Memory is never freed (cpp/memory-never-freed) More true positive results This query now identifies a wider variety of buffer allocations using the semmle.code.cpp.models.interfaces.Allocation library.
Memory may not be freed (cpp/memory-may-not-be-freed) More true positive results This query now identifies a wider variety of buffer allocations using the semmle.code.cpp.models.interfaces.Allocation library.
Mismatching new/free or malloc/delete (cpp/new-free-mismatch) Fewer false positive results Improved handling of template code gives greater precision.
Missing return statement (cpp/missing-return) Fewer false positive results and more accurate locations Functions containing asm statements are no longer highlighted by this query. The locations reported by this query are now more accurate in some cases.
No space for zero terminator (cpp/no-space-for-terminator) More results with greater precision The query gives more precise results for a wider variety of buffer allocations. String arguments to formatting functions are now (usually) expected to be null terminated strings. Use of the semmle.code.cpp.models.interfaces.Allocation library identifies problems with a wider variety of buffer allocations. This query is also more conservative when identifying which pointers point to null-terminated strings.
Overflow in uncontrolled allocation size (cpp/uncontrolled-allocation-size) Fewer false positive results The query now produces fewer, more accurate results. Cases where the tainted allocation size is range checked are more reliably excluded.
Overloaded assignment does not return 'this' (cpp/assignment-does-not-return-this) Fewer false positive results This query no longer reports incorrect results in template classes.
Pointer overflow check (cpp/pointer-overflow-check),
Possibly wrong buffer size in string copy (cpp/bad-strncpy-size),
Signed overflow check (cpp/signed-overflow-check)		 More correct results A new library is used for determining which expressions have identical value, giving more precise results. There is a performance cost to this, and the LGTM suite will overall run slower than before.
Unsafe array for days of the year (cpp/leap-year/unsafe-array-for-days-of-the-year) This query is no longer run on LGTM.
Unsigned comparison to zero (cpp/unsigned-comparison-zero) More correct results This query now also looks for comparisons of the form 0 <= x.

Changes to libraries

  • The built-in C++20 "spaceship operator" (<=>) is now supported via the QL class SpaceshipExpr. Overloaded forms are modeled as calls to functions named operator<=>.
  • The data-flow library (semmle.code.cpp.dataflow.DataFlow and semmle.code.cpp.dataflow.TaintTracking) has been improved, which affects and improves some security queries. The improvements are:
    • Track flow through functions that combine taint tracking with flow through fields.
    • Track flow through clone-like functions, that is, functions that read contents of a field from a parameter and stores the value in the field of a returned object.
  • The security pack taint tracking library (semmle.code.cpp.security.TaintTracking) uses a new intermediate representation. This provides a more precise analysis of flow through parameters and pointers. For new queries, however, we continue to recommend using semmle.code.cpp.dataflow.TaintTracking.
  • The global value numbering library (semmle.code.cpp.valuenumbering.GlobalValueNumbering) uses a new intermediate representation to provide a more precise analysis of heap-allocated memory and pointers to stack variables.
  • New libraries have been created to provide a more consistent and useful interface for modeling allocation and deallocation. These replace the old semmle.code.cpp.commons.Alloc library.
    • The new semmle.code.cpp.models.interfaces.Allocation library models allocations, such as new expressions and calls to malloc.
    • The new semmle.code.cpp.models.interfaces.Deallocation library models deallocations, such as delete expressions and calls to free.
    • The predicate freeCall in semmle.code.cpp.commons.Alloc has been deprecated. The Allocation and Deallocation models in semmle.code.cpp.models.interfaces should be used instead.
  • The new class StackVariable should be used in place of LocalScopeVariable in most cases. The difference is that StackVariable does not include variables declared with static or thread_local.
    • As a rule of thumb, custom queries about the values of variables should be changed from LocalScopeVariable to StackVariable, while queries about the name or scope of variables should remain unchanged.
    • The LocalScopeVariableReachability library is deprecated in favor of StackVariableReachability. The functionality is the same.
  • Taint tracking and data flow now features better modeling of commonly-used library functions:
    • gets and similar functions,
    • the most common operations on std::string,
    • strdup and similar functions, and
    • formatting functions such as sprintf.
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