An example on how to overload calloc():
1. Create calloclib.c:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
void* calloc(size_t n, size_t size) {
// To confirm you're using the right calloc() function
// uncomment the next line.
//fprintf(stderr, "Using calloc from libcalloc\n");
size_t new_size;
void * ptr;
new_size = n*size;
if ((n | size) & ~(((size_t)-1 >> (sizeof(size_t) << 2)))) {
if ((size != 0) && ((new_size / size) != n)) {
// this has caused a wrap around, fail request
errno = ENOMEM;
return(NULL);
}
}
ptr = malloc(new_size);
if (ptr) {
memset(ptr, 0, new_size);
}
return ptr;
}
2. Calloc test program:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
int main(int argc, char *argv[]) {
void *ptr = calloc(65537, 65537);
// void *ptr = calloc(1, 10);
if (ptr == NULL) {
fprintf(stderr, "calloc error: errno=%d\n", errno);
} else {
fprintf(stderr, "calloc ptr @ 0x%p\n", ptr);
free(ptr);
}
return EXIT_SUCCESS;
}
3. Build the calloclib shared library
nto[ARCH]-gcc -shared -o
libcalloc.so calloclib.c
e.g. ntox86-gcc -shared -o
libcalloc.so calloclib.c
4. Build your application or calloctest
ntox86-gcc -o calloctest calloctest.c
5. Place both calloctest and
libcalloc.so on your target. To use libcalloc start the test program (or any of your applications) by preloading it like this:
// void *ptr = calloc(65537, 65537);
# DL_DEBUG=libs LD_PRELOAD=/root/calloc/
libcalloc.so /root/calloc/calloctest
load_object: attempt load of /root/calloc/
libcalloc.so
load_elf32: found DT_TEXTREL, mapping a private copy of text sections!
load_elf32: loaded lib at addr b8200000(text) b8201518(data)
calloc error: errno=12
// overflow detected and error generated.
