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6.56 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)

is a built-in construct that returns a constant number of bytes from ptr to the end of the object ptr pointer points to (if known at compile time). __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], *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));

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.

There are also checking built-in functions for formatted output functions. 

     int __builtin___sprintf_chk (char *s, int flag, size_t os, const char *fmt, ...);
     int __builtin___snprintf_chk (char *s, size_t maxlen, int flag, size_t os,
                                   const char *fmt, ...);
     int __builtin___vsprintf_chk (char *s, int flag, size_t os, const char *fmt,
                                   va_list ap);
     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.