src/basic/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 <errno.h>
  23 #include <signal.h>
  24 #include <stddef.h>
  25 #include <sys/mman.h>
  26
  27 #include "macro.h"
  28 #include "sigbus.h"
  29 #include "util.h"
  30
  31 #define SIGBUS_QUEUE_MAX 64
  32
  33 static struct sigaction old_sigaction;
  34 static unsigned n_installed = 0;
  35
  36 /* We maintain a fixed size list of page addresses that triggered a
  37    SIGBUS. We access with list with atomic operations, so that we
  38    don't have to deal with locks between signal handler and main
  39    programs in possibly multiple threads. */
  40
  41 static void* volatile sigbus_queue[SIGBUS_QUEUE_MAX];
  42 static volatile sig_atomic_t n_sigbus_queue = 0;
  43
  44 static void sigbus_push(void *addr) {
  45         unsigned u;
  46
  47         assert(addr);
  48
  49         /* Find a free place, increase the number of entries and leave, if we can */
  50         for (u = 0; u < SIGBUS_QUEUE_MAX; u++)
  51                 if (__sync_bool_compare_and_swap(&sigbus_queue[u], NULL, addr)) {
  52                         __sync_fetch_and_add(&n_sigbus_queue, 1);
  53                         return;
  54                 }
  55
  56         /* If we can't, make sure the queue size is out of bounds, to
  57          * mark it as overflow */
  58         for (;;) {
  59                 unsigned c;
  60
  61                 __sync_synchronize();
  62                 c = n_sigbus_queue;
  63
  64                 if (c > SIGBUS_QUEUE_MAX) /* already overflow */
  65                         return;
  66
  67                 if (__sync_bool_compare_and_swap(&n_sigbus_queue, c, c + SIGBUS_QUEUE_MAX))
  68                         return;
  69         }
  70 }
  71
  72 int sigbus_pop(void **ret) {
  73         assert(ret);
  74
  75         for (;;) {
  76                 unsigned u, c;
  77
  78                 __sync_synchronize();
  79                 c = n_sigbus_queue;
  80
  81                 if (_likely_(c == 0))
  82                         return 0;
  83
  84                 if (_unlikely_(c >= SIGBUS_QUEUE_MAX))
  85                         return -EOVERFLOW;
  86
  87                 for (u = 0; u < SIGBUS_QUEUE_MAX; u++) {
  88                         void *addr;
  89
  90                         addr = sigbus_queue[u];
  91                         if (!addr)
  92                                 continue;
  93
  94                         if (__sync_bool_compare_and_swap(&sigbus_queue[u], addr, NULL)) {
  95                                 __sync_fetch_and_sub(&n_sigbus_queue, 1);
  96                                 *ret = addr;
  97                                 return 1;
  98                         }
  99                 }
 100         }
 101 }
 102
 103 static void sigbus_handler(int sn, siginfo_t *si, void *data) {
 104         unsigned long ul;
 105         void *aligned;
 106
 107         assert(sn == SIGBUS);
 108         assert(si);
 109
 110         if (si->si_code != BUS_ADRERR || !si->si_addr) {
 111                 assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
 112                 raise(SIGBUS);
 113                 return;
 114         }
 115
 116         ul = (unsigned long) si->si_addr;
 117         ul = ul / page_size();
 118         ul = ul * page_size();
 119         aligned = (void*) ul;
 120
 121         /* Let's remember which address failed */
 122         sigbus_push(aligned);
 123
 124         /* Replace mapping with an anonymous page, so that the
 125          * execution can continue, however with a zeroed out page */
 126         assert_se(mmap(aligned, page_size(), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED, -1, 0) == aligned);
 127 }
 128
 129 void sigbus_install(void) {
 130         struct sigaction sa = {
 131                 .sa_sigaction = sigbus_handler,
 132                 .sa_flags = SA_SIGINFO,
 133         };
 134
 135         n_installed++;
 136
 137         if (n_installed == 1)
 138                 assert_se(sigaction(SIGBUS, &sa, &old_sigaction) == 0);
 139
 140         return;
 141 }
 142
 143 void sigbus_reset(void) {
 144
 145         if (n_installed <= 0)
 146                 return;
 147
 148         n_installed--;
 149
 150         if (n_installed == 0)
 151                 assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
 152
 153         return;
 154 }