arch/um/os-Linux/sys-x86/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
   7 #ifdef __i386__
   8
   9 static jmp_buf buf;
  10
  11 static void segfault(int sig)
  12 {
  13         longjmp(buf, 1);
  14 }
  15
  16 static int page_ok(unsigned long page)
  17 {
  18         unsigned long *address = (unsigned long *) (page << UM_KERN_PAGE_SHIFT);
  19         unsigned long n = ~0UL;
  20         void *mapped = NULL;
  21         int ok = 0;
  22
  23         /*
  24          * First see if the page is readable.  If it is, it may still
  25          * be a VDSO, so we go on to see if it's writable.  If not
  26          * then try mapping memory there.  If that fails, then we're
  27          * still in the kernel area.  As a sanity check, we'll fail if
  28          * the mmap succeeds, but gives us an address different from
  29          * what we wanted.
  30          */
  31         if (setjmp(buf) == 0)
  32                 n = *address;
  33         else {
  34                 mapped = mmap(address, UM_KERN_PAGE_SIZE,
  35                               PROT_READ | PROT_WRITE,
  36                               MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  37                 if (mapped == MAP_FAILED)
  38                         return 0;
  39                 if (mapped != address)
  40                         goto out;
  41         }
  42
  43         /*
  44          * Now, is it writeable?  If so, then we're in user address
  45          * space.  If not, then try mprotecting it and try the write
  46          * again.
  47          */
  48         if (setjmp(buf) == 0) {
  49                 *address = n;
  50                 ok = 1;
  51                 goto out;
  52         } else if (mprotect(address, UM_KERN_PAGE_SIZE,
  53                             PROT_READ | PROT_WRITE) != 0)
  54                 goto out;
  55
  56         if (setjmp(buf) == 0) {
  57                 *address = n;
  58                 ok = 1;
  59         }
  60
  61  out:
  62         if (mapped != NULL)
  63                 munmap(mapped, UM_KERN_PAGE_SIZE);
  64         return ok;
  65 }
  66
  67 unsigned long os_get_top_address(void)
  68 {
  69         struct sigaction sa, old;
  70         unsigned long bottom = 0;
  71         /*
  72          * A 32-bit UML on a 64-bit host gets confused about the VDSO at
  73          * 0xffffe000.  It is mapped, is readable, can be reprotected writeable
  74          * and written.  However, exec discovers later that it can't be
  75          * unmapped.  So, just set the highest address to be checked to just
  76          * below it.  This might waste some address space on 4G/4G 32-bit
  77          * hosts, but shouldn't hurt otherwise.
  78          */
  79         unsigned long top = 0xffffd000 >> UM_KERN_PAGE_SHIFT;
  80         unsigned long test, original;
  81
  82         printf("Locating the bottom of the address space ... ");
  83         fflush(stdout);
  84
  85         /*
  86          * We're going to be longjmping out of the signal handler, so
  87          * SA_DEFER needs to be set.
  88          */
  89         sa.sa_handler = segfault;
  90         sigemptyset(&sa.sa_mask);
  91         sa.sa_flags = SA_NODEFER;
  92         if (sigaction(SIGSEGV, &sa, &old)) {
  93                 perror("os_get_top_address");
  94                 exit(1);
  95         }
  96
  97         /* Manually scan the address space, bottom-up, until we find
  98          * the first valid page (or run out of them).
  99          */
 100         for (bottom = 0; bottom < top; bottom++) {
 101                 if (page_ok(bottom))
 102                         break;
 103         }
 104
 105         /* If we've got this far, we ran out of pages. */
 106         if (bottom == top) {
 107                 fprintf(stderr, "Unable to determine bottom of address "
 108                         "space.\n");
 109                 exit(1);
 110         }
 111
 112         printf("0x%x\n", bottom << UM_KERN_PAGE_SHIFT);
 113         printf("Locating the top of the address space ... ");
 114         fflush(stdout);
 115
 116         original = bottom;
 117
 118         /* This could happen with a 4G/4G split */
 119         if (page_ok(top))
 120                 goto out;
 121
 122         do {
 123                 test = bottom + (top - bottom) / 2;
 124                 if (page_ok(test))
 125                         bottom = test;
 126                 else
 127                         top = test;
 128         } while (top - bottom > 1);
 129
 130 out:
 131         /* Restore the old SIGSEGV handling */
 132         if (sigaction(SIGSEGV, &old, NULL)) {
 133                 perror("os_get_top_address");
 134                 exit(1);
 135         }
 136         top <<= UM_KERN_PAGE_SHIFT;
 137         printf("0x%x\n", top);
 138
 139         return top;
 140 }
 141
 142 #else
 143
 144 unsigned long os_get_top_address(void)
 145 {
 146         /* The old value of CONFIG_TOP_ADDR */
 147         return 0x7fc0000000;
 148 }
 149
 150 #endif