src/basic/sigbus.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <errno.h>
   4 #include <signal.h>
   5 #include <stddef.h>
   6 #include <sys/mman.h>
   7
   8 #include "macro.h"
   9 #include "memory-util.h"
  10 #include "sigbus.h"
  11
  12 #define SIGBUS_QUEUE_MAX 64
  13
  14 static struct sigaction old_sigaction;
  15 static unsigned n_installed = 0;
  16
  17 /* We maintain a fixed size list of page addresses that triggered a
  18    SIGBUS. We access with list with atomic operations, so that we
  19    don't have to deal with locks between signal handler and main
  20    programs in possibly multiple threads. */
  21
  22 static void* volatile sigbus_queue[SIGBUS_QUEUE_MAX];
  23 static volatile sig_atomic_t n_sigbus_queue = 0;
  24
  25 static void sigbus_push(void *addr) {
  26         unsigned u;
  27
  28         assert(addr);
  29
  30         /* Find a free place, increase the number of entries and leave, if we can */
  31         for (u = 0; u < SIGBUS_QUEUE_MAX; u++)
  32                 if (__sync_bool_compare_and_swap(&sigbus_queue[u], NULL, addr)) {
  33                         __sync_fetch_and_add(&n_sigbus_queue, 1);
  34                         return;
  35                 }
  36
  37         /* If we can't, make sure the queue size is out of bounds, to
  38          * mark it as overflow */
  39         for (;;) {
  40                 unsigned c;
  41
  42                 __sync_synchronize();
  43                 c = n_sigbus_queue;
  44
  45                 if (c > SIGBUS_QUEUE_MAX) /* already overflow */
  46                         return;
  47
  48                 if (__sync_bool_compare_and_swap(&n_sigbus_queue, c, c + SIGBUS_QUEUE_MAX))
  49                         return;
  50         }
  51 }
  52
  53 int sigbus_pop(void **ret) {
  54         assert(ret);
  55
  56         for (;;) {
  57                 unsigned u, c;
  58
  59                 __sync_synchronize();
  60                 c = n_sigbus_queue;
  61
  62                 if (_likely_(c == 0))
  63                         return 0;
  64
  65                 if (_unlikely_(c >= SIGBUS_QUEUE_MAX))
  66                         return -EOVERFLOW;
  67
  68                 for (u = 0; u < SIGBUS_QUEUE_MAX; u++) {
  69                         void *addr;
  70
  71                         addr = sigbus_queue[u];
  72                         if (!addr)
  73                                 continue;
  74
  75                         if (__sync_bool_compare_and_swap(&sigbus_queue[u], addr, NULL)) {
  76                                 __sync_fetch_and_sub(&n_sigbus_queue, 1);
  77                                 *ret = addr;
  78                                 return 1;
  79                         }
  80                 }
  81         }
  82 }
  83
  84 static void sigbus_handler(int sn, siginfo_t *si, void *data) {
  85         unsigned long ul;
  86         void *aligned;
  87
  88         assert(sn == SIGBUS);
  89         assert(si);
  90
  91         if (si->si_code != BUS_ADRERR || !si->si_addr) {
  92                 assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
  93                 raise(SIGBUS);
  94                 return;
  95         }
  96
  97         ul = (unsigned long) si->si_addr;
  98         ul = ul / page_size();
  99         ul = ul * page_size();
 100         aligned = (void*) ul;
 101
 102         /* Let's remember which address failed */
 103         sigbus_push(aligned);
 104
 105         /* Replace mapping with an anonymous page, so that the
 106          * execution can continue, however with a zeroed out page */
 107         assert_se(mmap(aligned, page_size(), PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_FIXED, -1, 0) == aligned);
 108 }
 109
 110 void sigbus_install(void) {
 111         struct sigaction sa = {
 112                 .sa_sigaction = sigbus_handler,
 113                 .sa_flags = SA_SIGINFO,
 114         };
 115
 116         /* make sure that sysconf() is not called from a signal handler because
 117         * it is not guaranteed to be async-signal-safe since POSIX.1-2008 */
 118         (void) page_size();
 119
 120         n_installed++;
 121
 122         if (n_installed == 1)
 123                 assert_se(sigaction(SIGBUS, &sa, &old_sigaction) == 0);
 124
 125         return;
 126 }
 127
 128 void sigbus_reset(void) {
 129
 130         if (n_installed <= 0)
 131                 return;
 132
 133         n_installed--;
 134
 135         if (n_installed == 0)
 136                 assert_se(sigaction(SIGBUS, &old_sigaction, NULL) == 0);
 137
 138         return;
 139 }