posix/register-atfork.c

   1 /* Copyright (C) 2002-2023 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <libc-lock.h>
  19 #include <stdbool.h>
  20 #include <register-atfork.h>
  21 #include <intprops.h>
  22 #include <stdio.h>
  23
  24 #define DYNARRAY_ELEMENT           struct fork_handler
  25 #define DYNARRAY_STRUCT            fork_handler_list
  26 #define DYNARRAY_PREFIX            fork_handler_list_
  27 #define DYNARRAY_INITIAL_SIZE      48
  28 #include <malloc/dynarray-skeleton.c>
  29
  30 static struct fork_handler_list fork_handlers;
  31 static uint64_t fork_handler_counter;
  32
  33 static int atfork_lock = LLL_LOCK_INITIALIZER;
  34
  35 int
  36 __register_atfork (void (*prepare) (void), void (*parent) (void),
  37                    void (*child) (void), void *dso_handle)
  38 {
  39   lll_lock (atfork_lock, LLL_PRIVATE);
  40
  41   if (fork_handler_counter == 0)
  42     fork_handler_list_init (&fork_handlers);
  43
  44   struct fork_handler *newp = fork_handler_list_emplace (&fork_handlers);
  45   if (newp != NULL)
  46     {
  47       newp->prepare_handler = prepare;
  48       newp->parent_handler = parent;
  49       newp->child_handler = child;
  50       newp->dso_handle = dso_handle;
  51
  52       /* IDs assigned to handlers start at 1 and increment with handler
  53          registration.  Un-registering a handlers discards the corresponding
  54          ID.  It is not reused in future registrations.  */
  55       if (INT_ADD_OVERFLOW (fork_handler_counter, 1))
  56         __libc_fatal ("fork handler counter overflow");
  57       newp->id = ++fork_handler_counter;
  58     }
  59
  60   /* Release the lock.  */
  61   lll_unlock (atfork_lock, LLL_PRIVATE);
  62
  63   return newp == NULL ? ENOMEM : 0;
  64 }
  65 libc_hidden_def (__register_atfork)
  66
  67 static struct fork_handler *
  68 fork_handler_list_find (struct fork_handler_list *fork_handlers,
  69                         void *dso_handle)
  70 {
  71   for (size_t i = 0; i < fork_handler_list_size (fork_handlers); i++)
  72     {
  73       struct fork_handler *elem = fork_handler_list_at (fork_handlers, i);
  74       if (elem->dso_handle == dso_handle)
  75         return elem;
  76     }
  77   return NULL;
  78 }
  79
  80 void
  81 __unregister_atfork (void *dso_handle)
  82 {
  83   lll_lock (atfork_lock, LLL_PRIVATE);
  84
  85   struct fork_handler *first = fork_handler_list_find (&fork_handlers,
  86                                                        dso_handle);
  87   /* Removing is done by shifting the elements in the way the elements
  88      that are not to be removed appear in the beginning in dynarray.
  89      This avoid the quadradic run-time if a naive strategy to remove and
  90      shift one element at time.  */
  91   if (first != NULL)
  92     {
  93       struct fork_handler *new_end = first;
  94       first++;
  95       for (; first != fork_handler_list_end (&fork_handlers); ++first)
  96         {
  97           if (first->dso_handle != dso_handle)
  98             {
  99               *new_end = *first;
 100               ++new_end;
 101             }
 102         }
 103
 104       ptrdiff_t removed = first - new_end;
 105       for (size_t i = 0; i < removed; i++)
 106         fork_handler_list_remove_last (&fork_handlers);
 107     }
 108
 109   lll_unlock (atfork_lock, LLL_PRIVATE);
 110 }
 111
 112 uint64_t
 113 __run_prefork_handlers (_Bool do_locking)
 114 {
 115   uint64_t lastrun;
 116
 117   if (do_locking)
 118     lll_lock (atfork_lock, LLL_PRIVATE);
 119
 120   /* We run prepare handlers from last to first.  After fork, only
 121      handlers up to the last handler found here (pre-fork) will be run.
 122      Handlers registered during __run_prefork_handlers or
 123      __run_postfork_handlers will be positioned after this last handler, and
 124      since their prepare handlers won't be run now, their parent/child
 125      handlers should also be ignored.  */
 126   lastrun = fork_handler_counter;
 127
 128   size_t sl = fork_handler_list_size (&fork_handlers);
 129   for (size_t i = sl; i > 0;)
 130     {
 131       struct fork_handler *runp
 132         = fork_handler_list_at (&fork_handlers, i - 1);
 133
 134       uint64_t id = runp->id;
 135
 136       if (runp->prepare_handler != NULL)
 137         {
 138           if (do_locking)
 139             lll_unlock (atfork_lock, LLL_PRIVATE);
 140
 141           runp->prepare_handler ();
 142
 143           if (do_locking)
 144             lll_lock (atfork_lock, LLL_PRIVATE);
 145         }
 146
 147       /* We unlocked, ran the handler, and locked again.  In the
 148          meanwhile, one or more deregistrations could have occurred leading
 149          to the current (just run) handler being moved up the list or even
 150          removed from the list itself.  Since handler IDs are guaranteed to
 151          to be in increasing order, the next handler has to have:  */
 152
 153       /* A. An earlier position than the current one has.  */
 154       i--;
 155
 156       /* B. A lower ID than the current one does.  The code below skips
 157          any newly added handlers with higher IDs.  */
 158       while (i > 0
 159              && fork_handler_list_at (&fork_handlers, i - 1)->id >= id)
 160         i--;
 161     }
 162
 163   return lastrun;
 164 }
 165
 166 void
 167 __run_postfork_handlers (enum __run_fork_handler_type who, _Bool do_locking,
 168                          uint64_t lastrun)
 169 {
 170   size_t sl = fork_handler_list_size (&fork_handlers);
 171   for (size_t i = 0; i < sl;)
 172     {
 173       struct fork_handler *runp = fork_handler_list_at (&fork_handlers, i);
 174       uint64_t id = runp->id;
 175
 176       /* prepare handlers were not run for handlers with ID > LASTRUN.
 177          Thus, parent/child handlers will also not be run.  */
 178       if (id > lastrun)
 179         break;
 180
 181       if (do_locking)
 182         lll_unlock (atfork_lock, LLL_PRIVATE);
 183
 184       if (who == atfork_run_child && runp->child_handler)
 185         runp->child_handler ();
 186       else if (who == atfork_run_parent && runp->parent_handler)
 187         runp->parent_handler ();
 188
 189       if (do_locking)
 190         lll_lock (atfork_lock, LLL_PRIVATE);
 191
 192       /* We unlocked, ran the handler, and locked again.  In the meanwhile,
 193          one or more [de]registrations could have occurred.  Due to this,
 194          the list size must be updated.  */
 195       sl = fork_handler_list_size (&fork_handlers);
 196
 197       /* The just-run handler could also have moved up the list. */
 198
 199       if (sl > i && fork_handler_list_at (&fork_handlers, i)->id == id)
 200         /* The position of the recently run handler hasn't changed.  The
 201            next handler to be run is an easy increment away.  */
 202         i++;
 203       else
 204         {
 205           /* The next handler to be run is the first handler in the list
 206              to have an ID higher than the current one.  */
 207           for (i = 0; i < sl; i++)
 208             {
 209               if (fork_handler_list_at (&fork_handlers, i)->id > id)
 210                 break;
 211             }
 212         }
 213     }
 214
 215   if (do_locking)
 216     lll_unlock (atfork_lock, LLL_PRIVATE);
 217 }
 218
 219
 220 libc_freeres_fn (free_mem)
 221 {
 222   lll_lock (atfork_lock, LLL_PRIVATE);
 223
 224   fork_handler_list_free (&fork_handlers);
 225
 226   lll_unlock (atfork_lock, LLL_PRIVATE);
 227 }