The C++ Programming Language
C++ includes a large subset of the C language. As far as the C subset is used, the recommendations in Defensive Coding in C apply.
For very large values of
n, an expression
new T[n] can return a pointer to a heap
region which is too small. In other words, not all array
elements are actually backed with heap memory reserved to the
array. Current GCC versions generate code that performs a
computation of the form
sizeof(T) * size_t(n) + cookie_size, where
currently at most 8. This computation can overflow, and GCC
versions prior to 4.8 generated code which did not detect this.
(Fedora 18 was the first release which fixed this in GCC.)
std::vector template can be used instead
an explicit array allocation. (The GCC implementation detects
If there is no alternative to
and the sources will be compiled with older GCC versions, code
which allocates arrays with a variable length must check for
overflow manually. For the
new T[n] example,
the size check could be
n || (n > 0 && n >
(size_t(-1) - 8) / sizeof(T)). (See Recommendations for Integer Arithmetic) If there are
additional dimensions (which must be constants according to the
C++ standard), these should be included as factors in the
These countermeasures prevent out-of-bounds writes and potential code execution. Very large memory allocations can still lead to a denial of service. Recommendations for Manually-written Decoders contains suggestions for mitigating this problem when processing untrusted data.
See Array Allocation for array allocation advice for C-style memory allocation.
Do not overload functions with versions that have different
security characteristics. For instance, do not implement a
strcat which works on
std::string arguments. Similarly, do not name
methods after such functions.
A stable binary interface (ABI) is vastly preferred for security updates. Without a stable ABI, all reverse dependencies need recompiling, which can be a lot of work and could even be impossible in some cases. Ideally, a security update only updates a single dynamic shared object, and is picked up automatically after restarting affected processes.
Outside of extremely performance-critical code, you should ensure that a wide range of changes is possible without breaking ABI. Some very basic guidelines are:
Avoid inline functions.
Use the pointer-to-implementation idiom.
Try to avoid templates. Use them if the increased type safety provides a benefit to the programmer.
Move security-critical code out of templated code, so that it can be patched in a central place if necessary.
The KDE project publishes a document with more extensive guidelines on ABI-preserving changes to C++ code, Policies/Binary Compatibility Issues With C++ (d-pointer refers to the pointer-to-implementation idiom).
GCC offers different language compatibility modes:
-std=c++98for the original 1998 C++ standard
-std=c++03for the 1998 standard with the changes from the TR1 technical report
-std=c++11for the 2011 C++ standard. This option should not be used.
-std=c++0xfor several different versions of C++11 support in development, depending on the GCC version. This option should not be used.
For each of these flags, there are variants which also enable GNU extensions (mostly language features also found in C99 or C11):
-std=gnu++11 should not be used.
If you enable C++11 support, the ABI of the standard C++ library
libstdc++ will change in subtle ways.
Currently, no C++ libraries are compiled in C++11 mode, so if
you compile your code in C++11 mode, it will be incompatible
with the rest of the system. Unfortunately, this is also the
case if you do not use any C++11 features. Currently, there is
no safe way to enable C++11 mode (except for freestanding
The meaning of C++0X mode changed from GCC release to GCC release. Earlier versions were still ABI-compatible with C++98 mode, but in the most recent versions, switching to C++0X mode activates C++11 support, with its compatibility problems.
Some C++11 features (or approximations thereof) are available
with TR1 support, that is, with
[option]`-std=gnu03 and in the
<tr1/*> header files. This includes
<tr1/functional>). For other C++11
features, the Boost C++ library contains replacements.
The C++ standard library includes most of its C counterpart by reference, see Defensive Coding in C.
This section collects functions and function templates which are part of the standard library and are difficult to use.
Functions which use output operators or iterators which do not come in pairs (denoting ranges) cannot perform iterator range checking. (See Iterators) Function templates which involve output iterators are particularly dangerous:
std::generate_n do not perform iterator
checking, either, but there is an explicit count which has to be
supplied by the caller, as opposed to an implicit length
indicator in the form of a pair of forward iterators.
These output-iterator-expecting functions should only be used
with unlimited-range output iterators, such as iterators
obtained with the
Other functions use single input or forward iterators, which can read beyond the end of the input range if the caller is not careful:
std::string class provides a convenient
way to handle strings. Unlike C strings,
std::string objects have an explicit length
(and can contain embedded NUL characters), and storage for its
characters is managed automatically. This section discusses
std::string, but these observations also
apply to other instances of the
The pointer returned by the
function does not necessarily point to a NUL-terminated string.
To obtain a C-compatible string pointer, use
c_str() instead, which adds the NUL
The pointers returned by the
c_str() functions and iterators are only
valid until certain events happen. It is required that the
std::string object still exists (even
if it was initially created as a copy of another string object).
Pointers and iterators are also invalidated when non-const
member functions are called, or functions with a non-const
reference parameter. The behavior of the GCC implementation
deviates from that required by the C++ standard if multiple
threads are present. In general, only the first call to a
non-const member function after a structural modification of the
string (such as appending a character) is invalidating, but this
also applies to member function such as the non-const version of
begin(), in violation of the C++ standard.
Particular care is necessary when invoking the
c_str() member function on a temporary
object. This is convenient for calling C functions, but the
pointer will turn invalid as soon as the temporary object is
destroyed, which generally happens when the outermost expression
enclosing the expression on which
is called completes evaluation. Passing the result of
c_str() to a function which does not store
or otherwise leak that pointer is safe, though.
std::array, subscribing with
operator does not perform bounds checks.
at(size_type) member function
instead. See Containers and
Furthermore, accessing the terminating NUL character using
operator is not possible. (In some
c_str() member function
writes the NUL character on demand.)
Never write to the pointers returned by
after casting away
const. If you need a
C-style writable string, use a
std::vector<char> object and its
data() member function. In this case, you
have to explicitly add the terminating NUL character.
GCC’s implementation of
currently based on reference counting. It is expected that a
future version will remove the reference counting, due to
performance and conformance issues. As a result, code that
implicitly assumes sharing by holding to pointers or iterators
for too long will break, resulting in run-time crashes or worse.
On the other hand, non-const iterator-returning functions will
no longer give other threads an opportunity for invalidating
existing iterators and pointers because iterator invalidation
does not depend on sharing of the internal character array
Many sequence containers similar to
operator(size_type) and a
at(size_type). This applies
and other instances of
operator(size_type) is not required by the
standard to perform bounds checking (and the implementation in
GCC does not). In contrast,
must perform such a check. Therefore, in code which is not
performance-critical, you should prefer
operator(size_type), even though it is
slightly more verbose.
member functions are undefined if a vector object is empty. You
vec.at(vec.size() - 1) as checked
replacements. For an empty vector,
defined; it returns an arbitrary pointer, but not necessarily
the NULL pointer.
Iterators do not perform any bounds checking. Therefore, all functions that work on iterators should accept them in pairs, denoting a range, and make sure that iterators are not moved outside that range. For forward iterators and bidirectional iterators, you need to check for equality before moving the first or last iterator in the range. For random-access iterators, you need to compute the difference before adding or subtracting an offset. It is not possible to perform the operation and check for an invalid operator afterwards.
Output iterators cannot be compared for equality. Therefore, it is impossible to write code that detects that it has been supplied an output area that is too small, and their use should be avoided.
These issues make some of the standard library functions difficult to use correctly, see Unpaired Iterators.
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