src/basic/sigbus.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2 /***
   3   This file is part of systemd.
   4
   5   Copyright 2014 Lennart Poettering
   6 ***/
   7
   8 #include <errno.h>
   9 #include <signal.h>
  10 #include <stddef.h>
  11 #include <sys/mman.h>
  12
  13 #include "macro.h"
  14 #include "sigbus.h"
  15 #include "util.h"
  16
  17 #define SIGBUS_QUEUE_MAX 64
  18
  19 static struct sigaction old_sigaction;
  20 static unsigned n_installed = 0;
  21
  22 /* We maintain a fixed size list of page addresses that triggered a
  23    SIGBUS. We access with list with atomic operations, so that we
  24    don't have to deal with locks between signal handler and main
  25    programs in possibly multiple threads. */
  26
  27 static void* volatile sigbus_queue[SIGBUS_QUEUE_MAX];
  28 static volatile sig_atomic_t n_sigbus_queue = 0;
  29
  30 static void sigbus_push(void *addr) {
  31         unsigned u;
  32
  33         assert(addr);
  34
  35         /* Find a free place, increase the number of entries and leave, if we can */
  36         for (u = 0; u < SIGBUS_QUEUE_MAX; u++)
  37                 if (__sync_bool_compare_and_swap(&sigbus_queue[u], NULL, addr)) {
  38                         __sync_fetch_and_add(&n_sigbus_queue, 1);
  39                         return;
  40                 }
  41
  42         /* If we can't, make sure the queue size is out of bounds, to
  43          * mark it as overflow */
  44         for (;;) {
  45                 unsigned c;
  46
  47                 __sync_synchronize();
  48                 c = n_sigbus_queue;
  49
  50                 if (c > SIGBUS_QUEUE_MAX) /* already overflow */
  51                         return;
  52
  53                 if (__sync_bool_compare_and_swap(&n_sigbus_queue, c, c + SIGBUS_QUEUE_MAX))
  54                         return;
  55         }
  56 }
  57
  58 int sigbus_pop(void **ret) {
  59         assert(ret);
  60
  61         for (;;) {
  62                 unsigned u, c;
  63
  64                 __sync_synchronize();
  65                 c = n_sigbus_queue;
  66
  67                 if (_likely_(c == 0))
  68                         return 0;
  69
  70                 if (_unlikely_(c >= SIGBUS_QUEUE_MAX))
  71                         return -EOVERFLOW;
  72
  73                 for (u = 0; u < SIGBUS_QUEUE_MAX; u++) {
  74                         void *addr;
  75
  76                         addr = sigbus_queue[u];
  77                         if (!addr)
  78                                 continue;
  79
  80                         if (__sync_bool_compare_and_swap(&sigbus_queue[u], addr, NULL)) {
  81                                 __sync_fetch_and_sub(&n_sigbus_queue, 1);
  82                                 *ret = addr;
  83                                 return 1;
  84                         }
  85                 }
  86         }
  87 }
  88
  89 static void sigbus_handler(int sn, siginfo_t *si, void *data) {
  90         unsigned long ul;
  91         void *aligned;
  92
  93         assert(sn == SIGBUS);
  94         assert(si);
  95
  96         if (si->si_code != BUS_ADRERR || !si->si_addr) {
  97                 assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
  98                 raise(SIGBUS);
  99                 return;
 100         }
 101
 102         ul = (unsigned long) si->si_addr;
 103         ul = ul / page_size();
 104         ul = ul * page_size();
 105         aligned = (void*) ul;
 106
 107         /* Let's remember which address failed */
 108         sigbus_push(aligned);
 109
 110         /* Replace mapping with an anonymous page, so that the
 111          * execution can continue, however with a zeroed out page */
 112         assert_se(mmap(aligned, page_size(), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED, -1, 0) == aligned);
 113 }
 114
 115 void sigbus_install(void) {
 116         struct sigaction sa = {
 117                 .sa_sigaction = sigbus_handler,
 118                 .sa_flags = SA_SIGINFO,
 119         };
 120
 121         n_installed++;
 122
 123         if (n_installed == 1)
 124                 assert_se(sigaction(SIGBUS, &sa, &old_sigaction) == 0);
 125
 126         return;
 127 }
 128
 129 void sigbus_reset(void) {
 130
 131         if (n_installed <= 0)
 132                 return;
 133
 134         n_installed--;
 135
 136         if (n_installed == 0)
 137                 assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
 138
 139         return;
 140 }