Example A-7 : Program That Explores the Address Space
#include <stddef.h> /* for standard malloc(3C) */
#include <unistd.h> /* for sbrk(2) */
#include <stdio.h> /* for printf */
#include <sys/types.h> /* for __psint_t */
/* include <sys/stat.h> */
#include <sys/fcntl.h> /* for O_RDWR */
#include <sys/mman.h> /* for mmap(2) */
#define DISPLAY(v,t) {printf("%s:\t%0lx\n",t,(__psint_t)v);}
int main()
{
/*
|| Get a mask that truncates an address to a page boundary.
*/
__psint_t psize = getpagesize();
__psint_t pmask = ~(psize-1);
/*
|| Get a file descriptor for the nothing device.
|| Use that FD to map two segments of memory containing 00.
*/
int zero = open("/dev/zero",O_RDWR);
void * zmap1 = mmap(0,16384,PROT_WRITE,MAP_SHARED,zero,0);
void * zmap2 = mmap(0,16384,PROT_WRITE,MAP_SHARED,zero,0);
/*
|| Map one segment at a designated address reserved for
|| user maps by the MIPS ABI.
*/
void * abi_map = (char *)mmap((void *)0x30040000L,16384,
PROT_WRITE,MAP_SHARED+MAP_FIXED,zero, 0);
/*
|| Get the address of this program.
*/
char * poke = (char *)((__psint_t)main);
/*
|| Get some program addresses supplied by ld(1), but note
|| the warnings in end(3C) -- these addresses "have no standard
|| definition" when multiple text/data segments exist.
*/
extern int _ftext[];
void * ld_ftext = (void *)_ftext;
extern int _etext[];
void * ld_etext = (void *)_etext;
extern int _fdata[];
void * ld_fdata = (void *)_fdata;
extern int _edata[];
void * ld_edata = (void *)_edata;
extern int _fbss[];
void * ld_fbss = (void *)_fbss;
extern int _end[];
void * ld_end = (void *)_end;
/*
|| Get the address of some code in the libc DSO.
*/
void * libc_adr = (void *)fprintf;
/*
|| Get the current start and end of the heap.
*/
void * malloc_adr = (void *)malloc((size_t)256);
void * brk_adr = sbrk(0);
/*
|| Get the address of an item in our stack space.
*/
void * stack_adr = (void *)&psize;
/*
|| Display all the above.
*/
DISPLAY(psize,"Page size")
DISPLAY(zmap1,"Mapped segment 1")
DISPLAY(zmap2,"Mapped segment 2")
DISPLAY(abi_map,"ABI mapped segment")
DISPLAY(ld_ftext,"Text starts")
DISPLAY(ld_etext,"Text ends")
DISPLAY(ld_fdata,"Initialized data starts")
DISPLAY(ld_edata,"Initialized data ends")
DISPLAY(ld_fbss,"Uninitialized starts")
DISPLAY(ld_end,"Uninitialized ends")
DISPLAY(malloc_adr,"Heap data starts")
DISPLAY(brk_adr,"Heap data ends")
DISPLAY(stack_adr,"Stack data")
DISPLAY(libc_adr,"Spot in one DSO")
/*
|| See if we can get away with patching our own text.
*/
if (!mprotect((void *)(pmask&(__psint_t)poke),psize,PROT_WRITE+PROT_EXEC))
{
poke[0] = poke[0];
printf("I wrote into program text\n");
}
else
{
perror("mprotect(text)");
}
}