mmap-jump.c

#ifndef __powerpc64__
#error "unsupported architecture"
#endif

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

#include <assert.h>
#include <unistd.h>
#include <sys/mman.h>

int main(void)
{
	unsigned long pgsize, numinsns;
	unsigned int *insns;
	int i;

	/* allocate a page for the executable image */
	pgsize = sysconf(_SC_PAGESIZE);
	numinsns = pgsize / sizeof(unsigned int);
	insns = (unsigned int *) mmap(NULL, pgsize,
				      PROT_READ | PROT_WRITE | PROT_EXEC,
				      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
	if (insns == MAP_FAILED) {
		perror("mmap");
		return EXIT_FAILURE;
	}

	/* fill image with no-ops, not using any stack operations
	   for now so a callee stack frame is not created here */
	for (i = 0; i < numinsns - 1; i++)
		insns[i] = 0x60000000;          /* nop */

	/* last instruction in the image to return back
	   to the caller */
	insns[numinsns - 1] = 0x4e800020;       /* blr */

	puts("jumping to mmap-ed region");

	/* use a linked branch as that update the return address
	   automatically in LR and use an unconditional branch to
	   LR in the mmap-ed image to return back */
	asm volatile("mtctr     %0" : : "r"((unsigned long) insns));
	asm volatile("bctrl");

	puts("returned from mmap-ed region");

	/* cleanup */
	munmap((void *) insns, pgsize);
	return EXIT_SUCCESS;
}