#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/mman.h>
#include "longjmp.h"
#include "kern_constants.h"
static jmp_buf buf;
static void segfault(int sig)
{
longjmp(buf, 1);
}
static int page_ok(unsigned long page)
{
unsigned long *address = (unsigned long *) (page << UM_KERN_PAGE_SHIFT);
unsigned long n = ~0UL;
void *mapped = NULL;
int ok = 0;
/*
* First see if the page is readable. If it is, it may still
* be a VDSO, so we go on to see if it's writable. If not
* then try mapping memory there. If that fails, then we're
* still in the kernel area. As a sanity check, we'll fail if
* the mmap succeeds, but gives us an address different from
* what we wanted.
*/
if (setjmp(buf) == 0)
n = *address;
else {
mapped = mmap(address, UM_KERN_PAGE_SIZE,
PROT_READ | PROT_WRITE,
MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
if (mapped == MAP_FAILED)
return 0;
if (mapped != address)
goto out;
}
/*
* Now, is it writeable? If so, then we're in user address
* space. If not, then try mprotecting it and try the write
* again.
*/
if (setjmp(buf) == 0) {
*address = n;
ok = 1;
goto out;
} else if (mprotect(address, UM_KERN_PAGE_SIZE,
PROT_READ | PROT_WRITE) != 0)
goto out;
if (setjmp(buf) == 0) {
*address = n;
ok = 1;
}
out:
if (mapped != NULL)
munmap(mapped, UM_KERN_PAGE_SIZE);
return ok;
}
unsigned long os_get_top_address(void)
{
struct sigaction sa, old;
unsigned long bottom = 0;
/*
* A 32-bit UML on a 64-bit host gets confused about the VDSO at
* 0xffffe000. It is mapped, is readable, can be reprotected writeable
* and written. However, exec discovers later that it can't be
* unmapped. So, just set the highest address to be checked to just
* below it. This might waste some address space on 4G/4G 32-bit
* hosts, but shouldn't hurt otherwise.
*/
unsigned long top = 0xffffd000 >> UM_KERN_PAGE_SHIFT;
unsigned long test, original;
printf("Locating the bottom of the address space ... ");
fflush(stdout);
/*
* We're going to be longjmping out of the signal handler, so
* SA_DEFER needs to be set.
*/
sa.sa_handler = segfault;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_NODEFER;
if (sigaction(SIGSEGV, &sa, &old)) {
perror("os_get_top_address");
exit(1);
}
/* Manually scan the address space, bottom-up, until we find
* the first valid page (or run out of them).
*/
for (bottom = 0; bottom < top; bottom++) {
if (page_ok(bottom))
break;
}
/* If we've got this far, we ran out of pages. */
if (bottom == top) {
fprintf(stderr, "Unable to determine bottom of address "
"space.\n");
exit(1);
}
printf("0x%x\n", bottom << UM_KERN_PAGE_SHIFT);
printf("Locating the top of the address space ... ");
fflush(stdout);
original = bottom;
/* This could happen with a 4G/4G split */
if (page_ok(top))
goto out;
do {
test = bottom + (top - bottom) / 2;
if (page_ok(test))
bottom = test;
else
top = test;
} while (top - bottom > 1);
out:
/* Restore the old SIGSEGV handling */
if (sigaction(SIGSEGV, &old, NULL)) {
perror("os_get_top_address");
exit(1);
}
top <<= UM_KERN_PAGE_SHIFT;
printf("0x%x\n", top);
return top;
}
