src/basic/sigbus.c

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