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_task_size(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;
  80
  81         printf("Locating the top 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         sigaction(SIGSEGV, &sa, &old);
  92
  93         if (!page_ok(bottom)) {
  94                 fprintf(stderr, "Address 0x%x no good?\n",
  95                         bottom << UM_KERN_PAGE_SHIFT);
  96                 exit(1);
  97         }
  98
  99         /* This could happen with a 4G/4G split */
 100         if (page_ok(top))
 101                 goto out;
 102
 103         do {
 104                 test = bottom + (top - bottom) / 2;
 105                 if (page_ok(test))
 106                         bottom = test;
 107                 else
 108                         top = test;
 109         } while (top - bottom > 1);
 110
 111 out:
 112         /* Restore the old SIGSEGV handling */
 113         sigaction(SIGSEGV, &old, NULL);
 114
 115         top <<= UM_KERN_PAGE_SHIFT;
 116         printf("0x%x\n", top);
 117         fflush(stdout);
 118
 119         return top;
 120 }