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.
-
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 objectptr
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 thealloc_size
attribute (see Common Function Attributes).__builtin_object_size
never evaluates its arguments for side effects. If there are any side effects in them, it returns(size_t) -1
fortype
0 or 1 and(size_t) 0
fortype
2 or 3. If there are multiple objectsptr
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 iftype
& 2 is 0 and minimum if nonzero. If it is not possible to determine which objectsptr
points to at compile time,__builtin_object_size
should return(size_t) -1
fortype
0 or 1 and(size_t) 0
fortype
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));
-
size_t __builtin_dynamic_object_size(const void *ptr, int type)#
is similar to
__builtin_object_size
in that it returns a number of bytes fromptr
to the end of the objectptr
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 oftype
as well as return values in case it is not possible to determine which objectsptr
points to at compile time are the same as in the case of__builtin_object_size
.
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.