goto, jump to entry point of mmaped ELF binary - c

Question

i can check entry point of my binary with "$readelf cbinary -a" and through the code. But how to check its entry point virtual adr when binary is mmaped and then jump there?
int fd;
int PageSize;
char *fileName = "/home/dssiam/workspace_eclipse/hello/src/cprog";
if ((PageSize = sysconf(_SC_PAGE_SIZE)) < 0) {
perror("sysconf() Error=");
}
if ((fd = open(fileName, O_RDWR, S_IXUSR | S_IXGRP | S_IXOTH)) == -1)
{
perror("err open file:");
exit(1);
}
else
{
fd = open(fileName, O_RDWR, S_IXUSR | S_IXGRP | S_IXOTH);
}
void *address;
int len;
off_t my_offset = 0;
len = PageSize*3; //Map one page
address = mmap(NULL, len, PROT_WRITE, MAP_SHARED, fd, my_offset);
if (address == MAP_FAILED)
{
perror("mmap error. ");
}
lseek(fd, 24, SEEK_SET);
unsigned long entry_point;
read(fd, &entry_point, sizeof(entry_point)); //IT RETURN entry point adr of my binary at "/home/dssiam/workspace_eclipse/hello/src/cprog" but not in VM
printf("entry: 0x%lx\n", entry_point);
close(fd);
void *ptr = (void *)0x80484b0; // 0x80484b0 - entry_point vaddress
goto *ptr; //no jump here
so i can jump to the start of my main program, but i cant jump to the binary "cprog" stored at my hdd and mmaped region too.
any help would be appreciated.

Answer 1

The code has lots of mistakes (wrong mmap protection, wrong mmap start address, arbitrary pagesize, C standard specifically prohibits this kind of computed goto) but the biggest problem is that this method simply will not work, except maybe for the most basic cases.
You cannot just mmap a single function from elf file into the memory and expect it to work -- you will need to perform relocations for relocatable code, and even for PIC (position independent code), you still need to create GOT.
I am going to guess that what you really want to dynamically load complied files, so use a standard way to do this: compile your file into .so dynamic library, then use dlopen/dlsym to access functions from the file.