Object Size Checking (Using the GNU Compiler Collection (GCC))

### 7.10 Object Size Checking ¶

#### 7.10.1 Object Size Checking Built-in Functions ¶

GCC implements a limited buffer overflow protection mechanism that can prevent some buffer overflow attacks by determining the sizes of objects into which data is about to be written and preventing the writes when the size isn’t sufficient. The built-in functions described below yield the best results when used together and when optimization is enabled. For example, to detect object sizes across function boundaries or to follow pointer assignments through non-trivial control flow they rely on various optimization passes enabled with `-O2`. However, to a limited extent, they can be used without optimization as well.

Built-in Function: `size_t` __builtin_object_size `(const void * ptr, int type)` ¶

  This built-in construct returns a constant number of bytes from ptr to the end of the object ptr pointer points to (if known at compile time). To determine the sizes of dynamically allocated objects the function relies on the allocation functions called to obtain the storage to be declared with the `alloc_size` attribute (see Common Attributes). `__builtin_object_size` never evaluates its arguments for side effects. If there are any side effects in them, it returns `(size_t) -1` for type 0 or 1 and `(size_t) 0` for type 2 or 3. If there are multiple objects ptr can point to and all of them are known at compile time, the returned number is the maximum of remaining byte counts in those objects if type & 2 is 0 and minimum if nonzero. If it is not possible to determine which objects ptr points to at compile time, `__builtin_object_size` should return `(size_t) -1` for type 0 or 1 and `(size_t) 0` for type 2 or 3.

  type is an integer constant from 0 to 3. If the least significant bit is clear, objects are whole variables, if it is set, a closest surrounding subobject is considered the object a pointer points to. The second bit determines if maximum or minimum of remaining bytes is computed.

      struct V { char buf1[10]; int b; char buf2[10]; } var;
      char *p = &var.buf1[1];
      int *q = &var.b;

      /* Here the object p points to is var.  */
      assert (__builtin_object_size (p, 0) == sizeof (var) - 1);
      /* The subobject p points to is var.buf1.  */
      assert (__builtin_object_size (p, 1) == sizeof (var.buf1) - 1);
      /* The object q points to is var.  */
      assert (__builtin_object_size (q, 0)
              == (char *) (&var + 1) - (char *) &var.b);
      /* The subobject q points to is var.b.  */
      assert (__builtin_object_size (q, 1) == sizeof (var.b));

Built-in Function: `size_t` __builtin_dynamic_object_size `(const void * ptr, int type)` ¶

  This builtin is similar to `__builtin_object_size` in that it returns a number of bytes from ptr to the end of the object ptr pointer points to, except that the size returned may not be a constant. This results in successful evaluation of object size estimates in a wider range of use cases and can be more precise than `__builtin_object_size`, but it incurs a performance penalty since it may add a runtime overhead on size computation. Semantics of type as well as return values in case it is not possible to determine which objects ptr points to at compile time are the same as in the case of `__builtin_object_size`.

#### 7.10.2 Object Size Checking and Source Fortification ¶

Hardening of function calls using the `_FORTIFY_SOURCE` macro is one of the key uses of the object size checking built-in functions. To make implementation of these features more convenient and improve optimization and diagnostics, there are built-in functions added for many common string operation functions, e.g., for `memcpy` `__builtin___memcpy_chk` built-in is provided. This built-in has an additional last argument, which is the number of bytes remaining in the object the dest argument points to or `(size_t) -1` if the size is not known.

The built-in functions are optimized into the normal string functions like `memcpy` if the last argument is `(size_t) -1` or if it is known at compile time that the destination object will not be overflowed. If the compiler can determine at compile time that the object will always be overflowed, it issues a warning.

The intended use can be e.g.

    #undef memcpy
    #define bos0(dest) __builtin_object_size (dest, 0)
    #define memcpy(dest, src, n) \
      __builtin___memcpy_chk (dest, src, n, bos0 (dest))

    char *volatile p;
    char buf[10];
    /* It is unknown what object p points to, so this is optimized
       into plain memcpy - no checking is possible.  */
    memcpy (p, "abcde", n);
    /* Destination is known and length too.  It is known at compile
       time there will be no overflow.  */
    memcpy (&buf[5], "abcde", 5);
    /* Destination is known, but the length is not known at compile time.
       This will result in __memcpy_chk call that can check for overflow
       at run time.  */
    memcpy (&buf[5], "abcde", n);
    /* Destination is known and it is known at compile time there will
       be overflow.  There will be a warning and __memcpy_chk call that
       will abort the program at run time.  */
    memcpy (&buf[6], "abcde", 5);

Such built-in functions are provided for `memcpy`, `mempcpy`, `memmove`, `memset`, `strcpy`, `stpcpy`, `strncpy`, `strcat` and `strncat`.

#### 7.10.2.1 Formatted Output Function Checking ¶

Built-in Function: `int` __builtin___sprintf_chk `(char *s, int flag, size_t os, const char *fmt, ...)` ¶
Built-in Function: `int` __builtin___snprintf_chk `(char *s, size_t maxlen, int flag, size_t os, const char *fmt, ...)` ¶
Built-in Function: `int` __builtin___vsprintf_chk `(char *s, int flag, size_t os, const char *fmt, va_list ap)` ¶
Built-in Function: `int` __builtin___vsnprintf_chk `(char *s, size_t maxlen, int flag, size_t os, const char *fmt, va_list ap)` ¶

  The added flag argument is passed unchanged to `__sprintf_chk` etc. functions and can contain implementation specific flags on what additional security measures the checking function might take, such as handling `%n` differently.

  The os argument is the object size s points to, like in the other built-in functions. There is a small difference in the behavior though, if os is `(size_t) -1`, the built-in functions are optimized into the non-checking functions only if flag is 0, otherwise the checking function is called with os argument set to `(size_t) -1`.

  In addition to this, there are checking built-in functions `__builtin___printf_chk`, `__builtin___vprintf_chk`, `__builtin___fprintf_chk` and `__builtin___vfprintf_chk`. These have just one additional argument, flag, right before format string fmt. If the compiler is able to optimize them to `fputc` etc. functions, it does, otherwise the checking function is called and the flag argument passed to it.