src/shared/sigbus.c

   1 /*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
   2
   3 /***
   4   This file is part of systemd.
   5
   6   Copyright 2014 Lennart Poettering
   7
   8   systemd is free software; you can redistribute it and/or modify it
   9   under the terms of the GNU Lesser General Public License as published by
  10   the Free Software Foundation; either version 2.1 of the License, or
  11   (at your option) any later version.
  12
  13   systemd is distributed in the hope that it will be useful, but
  14   WITHOUT ANY WARRANTY; without even the implied warranty of
  15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  16   Lesser General Public License for more details.
  17
  18   You should have received a copy of the GNU Lesser General Public License
  19   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  20 ***/
  21
  22 #include <signal.h>
  23 #include <sys/mman.h>
  24
  25 #include "macro.h"
  26 #include "util.h"
  27 #include "sigbus.h"
  28
  29 #define SIGBUS_QUEUE_MAX 64
  30
  31 static struct sigaction old_sigaction;
  32 static unsigned n_installed = 0;
  33
  34 /* We maintain a fixed size list of page addresses that triggered a
  35    SIGBUS. We access with list with atomic operations, so that we
  36    don't have to deal with locks between signal handler and main
  37    programs in possibly multiple threads. */
  38
  39 static void* volatile sigbus_queue[SIGBUS_QUEUE_MAX];
  40 static volatile sig_atomic_t n_sigbus_queue = 0;
  41
  42 static void sigbus_push(void *addr) {
  43         unsigned u;
  44
  45         assert(addr);
  46
  47         /* Find a free place, increase the number of entries and leave, if we can */
  48         for (u = 0; u < SIGBUS_QUEUE_MAX; u++)
  49                 if (__sync_bool_compare_and_swap(&sigbus_queue[u], NULL, addr)) {
  50                         __sync_fetch_and_add(&n_sigbus_queue, 1);
  51                         return;
  52                 }
  53
  54         /* If we can't, make sure the queue size is out of bounds, to
  55          * mark it as overflow */
  56         for (;;) {
  57                 unsigned c;
  58
  59                 __sync_synchronize();
  60                 c = n_sigbus_queue;
  61
  62                 if (c > SIGBUS_QUEUE_MAX) /* already overflow */
  63                         return;
  64
  65                 if (__sync_bool_compare_and_swap(&n_sigbus_queue, c, c + SIGBUS_QUEUE_MAX))
  66                         return;
  67         }
  68 }
  69
  70 int sigbus_pop(void **ret) {
  71         assert(ret);
  72
  73         for (;;) {
  74                 unsigned u, c;
  75
  76                 __sync_synchronize();
  77                 c = n_sigbus_queue;
  78
  79                 if (_likely_(c == 0))
  80                         return 0;
  81
  82                 if (_unlikely_(c >= SIGBUS_QUEUE_MAX))
  83                         return -EOVERFLOW;
  84
  85                 for (u = 0; u < SIGBUS_QUEUE_MAX; u++) {
  86                         void *addr;
  87
  88                         addr = sigbus_queue[u];
  89                         if (!addr)
  90                                 continue;
  91
  92                         if (__sync_bool_compare_and_swap(&sigbus_queue[u], addr, NULL)) {
  93                                 __sync_fetch_and_sub(&n_sigbus_queue, 1);
  94                                 *ret = addr;
  95                                 return 1;
  96                         }
  97                 }
  98         }
  99 }
 100
 101 static void sigbus_handler(int sn, siginfo_t *si, void *data) {
 102         unsigned long ul;
 103         void *aligned;
 104
 105         assert(sn == SIGBUS);
 106         assert(si);
 107
 108         if (si->si_code != BUS_ADRERR || !si->si_addr) {
 109                 assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
 110                 raise(SIGBUS);
 111                 return;
 112         }
 113
 114         ul = (unsigned long) si->si_addr;
 115         ul = ul / page_size();
 116         ul = ul * page_size();
 117         aligned = (void*) ul;
 118
 119         /* Let's remember which address failed */
 120         sigbus_push(aligned);
 121
 122         /* Replace mapping with an anonymous page, so that the
 123          * execution can continue, however with a zeroed out page */
 124         assert_se(mmap(aligned, page_size(), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED, -1, 0) == aligned);
 125 }
 126
 127 void sigbus_install(void) {
 128         struct sigaction sa = {
 129                 .sa_sigaction = sigbus_handler,
 130                 .sa_flags = SA_SIGINFO,
 131         };
 132
 133         n_installed++;
 134
 135         if (n_installed == 1)
 136                 assert_se(sigaction(SIGBUS, &sa, &old_sigaction) == 0);
 137
 138         return;
 139 }
 140
 141 void sigbus_reset(void) {
 142
 143         if (n_installed <= 0)
 144                 return;
 145
 146         n_installed--;
 147
 148         if (n_installed == 0)
 149                 assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
 150
 151         return;
 152 }