src/basic/sigbus.c

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