gistfile1.txt

% make         
cc -D_FORTIFY_SOURCE=2 -Wall -g -O2    memcpy_test.c memcpy_chk.c   -o memcpy_test
In function ‘my_memcpy’,
    inlined from ‘main’ at memcpy_test.c:29:
memcpy_test.c:17: 警告: call to ‘my_memcpy_warn’ declared with attribute warning: my warning

% ./memcpy_test 
my buffer overflow detection
[1]    27873 abort (core dumped)  ./memcpy_test

memcpy_chk.c

#include <stddef.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

void* my_memcpy_chk (void *dest, const void *src, size_t len, size_t destlen)
{
  size_t i;
  const char *s = src;
  char *d = dest;

  if (len > destlen) {
    fprintf(stderr, "my buffer overflow detection\n");
	  abort();
  }

  for (i=0; i<len; i++)
	  d[i] = s[i];

  return dest;
}


char* my_memcpy_warn () __attribute__ ((alias("my_memcpy_chk")));

memcpy_test.c

#include <string.h>
#include <stdio.h>

char* my_memcpy_chk (char *dest, const char *src, size_t len, size_t destlen);

char* my_memcpy_warn (char *dest, const char *src, size_t len, size_t destlen)
  __attribute__((__warning__ ("my warning")));

static char* my_memcpy(char* dst, const char* src, size_t len) __attribute__((always_inline));

static char* my_memcpy(char* dst, const char* src, size_t len)
{
	size_t d = __builtin_object_size(dst, 0);

	if (d != (size_t) -1) {
		if (__builtin_constant_p(len) && len > d)
			return my_memcpy_warn(dst, src, len, d);
		else
			return my_memcpy_chk(dst, src, len, d);
	} else
		return memcpy (dst, src, len);
}

int main(void)
{
	const char *str = "12345";
	char buf[5];

	my_memcpy(buf, str, strlen(str)+1);

	return 0;
}