arch/um/os-Linux/sys-i386/task_size.c

   1 #include <stdio.h>
   2 #include <stdlib.h>
   3 #include <signal.h>
   4 #include <sys/mman.h>
   5 #include "longjmp.h"
   6 #include "kern_constants.h"
   7
   8 static jmp_buf buf;
   9
  10 static void segfault(int sig)
  11 {
  12         longjmp(buf, 1);
  13 }
  14
  15 static int page_ok(unsigned long page)
  16 {
  17         unsigned long *address = (unsigned long *) (page << UM_KERN_PAGE_SHIFT);
  18         unsigned long n = ~0UL;
  19         void *mapped = NULL;
  20         int ok = 0;
  21
  22         /*
  23          * First see if the page is readable.  If it is, it may still
  24          * be a VDSO, so we go on to see if it's writable.  If not
  25          * then try mapping memory there.  If that fails, then we're
  26          * still in the kernel area.  As a sanity check, we'll fail if
  27          * the mmap succeeds, but gives us an address different from
  28          * what we wanted.
  29          */
  30         if (setjmp(buf) == 0)
  31                 n = *address;
  32         else {
  33                 mapped = mmap(address, UM_KERN_PAGE_SIZE,
  34                               PROT_READ | PROT_WRITE,
  35                               MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  36                 if (mapped == MAP_FAILED)
  37                         return 0;
  38                 if (mapped != address)
  39                         goto out;
  40         }
  41
  42         /*
  43          * Now, is it writeable?  If so, then we're in user address
  44          * space.  If not, then try mprotecting it and try the write
  45          * again.
  46          */
  47         if (setjmp(buf) == 0) {
  48                 *address = n;
  49                 ok = 1;
  50                 goto out;
  51         } else if (mprotect(address, UM_KERN_PAGE_SIZE,
  52                             PROT_READ | PROT_WRITE) != 0)
  53                 goto out;
  54
  55         if (setjmp(buf) == 0) {
  56                 *address = n;
  57                 ok = 1;
  58         }
  59
  60  out:
  61         if (mapped != NULL)
  62                 munmap(mapped, UM_KERN_PAGE_SIZE);
  63         return ok;
  64 }
  65
  66 unsigned long os_get_top_address(void)
  67 {
  68         struct sigaction sa, old;
  69         unsigned long bottom = 0;
  70         /*
  71          * A 32-bit UML on a 64-bit host gets confused about the VDSO at
  72          * 0xffffe000.  It is mapped, is readable, can be reprotected writeable
  73          * and written.  However, exec discovers later that it can't be
  74          * unmapped.  So, just set the highest address to be checked to just
  75          * below it.  This might waste some address space on 4G/4G 32-bit
  76          * hosts, but shouldn't hurt otherwise.
  77          */
  78         unsigned long top = 0xffffd000 >> UM_KERN_PAGE_SHIFT;
  79         unsigned long test, original;
  80
  81         printf("Locating the bottom of the address space ... ");
  82         fflush(stdout);
  83
  84         /*
  85          * We're going to be longjmping out of the signal handler, so
  86          * SA_DEFER needs to be set.
  87          */
  88         sa.sa_handler = segfault;
  89         sigemptyset(&sa.sa_mask);
  90         sa.sa_flags = SA_NODEFER;
  91         if (sigaction(SIGSEGV, &sa, &old)) {
  92                 perror("os_get_top_address");
  93                 exit(1);
  94         }
  95
  96         /* Manually scan the address space, bottom-up, until we find
  97          * the first valid page (or run out of them).
  98          */
  99         for (bottom = 0; bottom < top; bottom++) {
 100                 if (page_ok(bottom))
 101                         break;
 102         }
 103
 104         /* If we've got this far, we ran out of pages. */
 105         if (bottom == top) {
 106                 fprintf(stderr, "Unable to determine bottom of address "
 107                         "space.\n");
 108                 exit(1);
 109         }
 110
 111         printf("0x%x\n", bottom << UM_KERN_PAGE_SHIFT);
 112         printf("Locating the top of the address space ... ");
 113         fflush(stdout);
 114
 115         original = bottom;
 116
 117         /* This could happen with a 4G/4G split */
 118         if (page_ok(top))
 119                 goto out;
 120
 121         do {
 122                 test = bottom + (top - bottom) / 2;
 123                 if (page_ok(test))
 124                         bottom = test;
 125                 else
 126                         top = test;
 127         } while (top - bottom > 1);
 128
 129 out:
 130         /* Restore the old SIGSEGV handling */
 131         if (sigaction(SIGSEGV, &old, NULL)) {
 132                 perror("os_get_top_address");
 133                 exit(1);
 134         }
 135         top <<= UM_KERN_PAGE_SHIFT;
 136         printf("0x%x\n", top);
 137
 138         return top;
 139 }