Test whether memory can be accessed

test_access.c

#define _GNU_SOURCE
#include <stdio.h>
#include <udis86.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <ucontext.h>
#include <execinfo.h>
#include <setjmp.h>

sighandler_t * _ptr = (sighandler_t*) NULL;
jmp_buf _buf;

void _fuckit_seghandle_userspace() {
  longjmp(&_buf, 1);    
}
void _fuckit_seghandle(int sig, siginfo_t* si, void* unused) {
  ucontext_t* uc = (ucontext_t*) unused;
  greg_t * gregs = uc->uc_mcontext.gregs;
  gregs[REG_RIP] = &_fuckit_seghandle_userspace;
}
void _fuckcpp_setup_segfault_handler(int reset) {
  struct sigaction sa, osa;
  sa.sa_flags = SA_SIGINFO;
  sigemptyset (&sa.sa_mask);
  if (!reset)
    sa.sa_sigaction = &_fuckit_seghandle;
  else
    sa.sa_sigaction = _ptr;
  sigaction(SIGSEGV, &sa, &osa);
  _ptr = osa.sa_sigaction;
}
int _real_check(char* addr, size_t size, int wr, void* buf) {
    memcpy(buf, addr, size);
    if (wr) 
	memcpy(addr, buf, size);
}
int can_access_mem(void* addr, size_t size, int wr) {
    #define LIM (64)
    char buf[size<=LIM ? size : LIM];
    if (!setjmp(&_buf)) {
	_fuckcpp_setup_segfault_handler(0);
	volatile char * p = (char*)addr;
	ssize_t remain = size;
	while ( remain > 0 && ((int)addr & ~LIM) ) { // align first
	    _real_check(addr, 1, wr, buf);
	    remain --;
	    addr ++;
	}
	for (; remain > 0; remain -= LIM) {
	    _real_check(addr, rename<=LIM ? rename : LIM, wr, buf);
	    addr += LIM;
	}
	_fuckcpp_setup_segfault_handler(1);
	return 1;
    } else {
	_fuckcpp_setup_segfault_handler(1);
	return 0;	
    }
}
#define CAN_ACCESS_T(TYPE, x) (can_access_mem((x), sizeof(TYPE), 0))
#define CAN_WRITE_T(TYPE, x) (can_access_mem((x), sizeof(TYPE), 1))
#define CAN_ACCESS(x) (can_access_mem((x), sizeof(x), 0))
#define CAN_WRITE(x) (can_access_mem((x), sizeof(x),1))

int main()
{
    int a;
    printf("can read %p: %d\n", &main, can_access_mem(&main, 10, 0));
    printf("can write %p: %d\n", &main, can_access_mem(&main, 10, 1));
    printf("can write %p: %d\n", &a, CAN_WRITE(&a));
    printf("can write %p: %d\n", a, CAN_WRITE(a));
    printf("can write %p: %d\n", 543124, CAN_WRITE_T(void*, 543124));
    printf("can write %p: %d\n", 0, can_access_mem(0, 4, 1));
    printf("can read %p: %d\n", 18446744073699065856, can_access_mem(18446744073699065856, 4096, 0));
    printf("can write %p: %d\n", 18446744073699065856, can_access_mem(18446744073699065856, 4096, 1));
    return 0;
}