sysdeps/nptl/fork.c

   1 /* Copyright (C) 2002-2018 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3    Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <http://www.gnu.org/licenses/>.  */
  18
  19 #include <assert.h>
  20 #include <stdlib.h>
  21 #include <unistd.h>
  22 #include <sys/types.h>
  23 #include <sysdep.h>
  24 #include <libio/libioP.h>
  25 #include <tls.h>
  26 #include <hp-timing.h>
  27 #include <ldsodefs.h>
  28 #include <stdio-lock.h>
  29 #include <atomic.h>
  30 #include <nptl/pthreadP.h>
  31 #include <fork.h>
  32 #include <arch-fork.h>
  33 #include <futex-internal.h>
  34 #include <malloc/malloc-internal.h>
  35
  36 static void
  37 fresetlockfiles (void)
  38 {
  39   _IO_ITER i;
  40
  41   for (i = _IO_iter_begin(); i != _IO_iter_end(); i = _IO_iter_next(i))
  42     if ((_IO_iter_file (i)->_flags & _IO_USER_LOCK) == 0)
  43       _IO_lock_init (*((_IO_lock_t *) _IO_iter_file(i)->_lock));
  44 }
  45
  46
  47 pid_t
  48 __libc_fork (void)
  49 {
  50   pid_t pid;
  51   struct used_handler
  52   {
  53     struct fork_handler *handler;
  54     struct used_handler *next;
  55   } *allp = NULL;
  56
  57   /* Determine if we are running multiple threads.  We skip some fork
  58      handlers in the single-thread case, to make fork safer to use in
  59      signal handlers.  POSIX requires that fork is async-signal-safe,
  60      but our current fork implementation is not.  */
  61   bool multiple_threads = THREAD_GETMEM (THREAD_SELF, header.multiple_threads);
  62
  63   /* Run all the registered preparation handlers.  In reverse order.
  64      While doing this we build up a list of all the entries.  */
  65   struct fork_handler *runp;
  66   while ((runp = __fork_handlers) != NULL)
  67     {
  68       /* Make sure we read from the current RUNP pointer.  */
  69       atomic_full_barrier ();
  70
  71       unsigned int oldval = runp->refcntr;
  72
  73       if (oldval == 0)
  74         /* This means some other thread removed the list just after
  75            the pointer has been loaded.  Try again.  Either the list
  76            is empty or we can retry it.  */
  77         continue;
  78
  79       /* Bump the reference counter.  */
  80       if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,
  81                                                 oldval + 1, oldval))
  82         /* The value changed, try again.  */
  83         continue;
  84
  85       /* We bumped the reference counter for the first entry in the
  86          list.  That means that none of the following entries will
  87          just go away.  The unloading code works in the order of the
  88          list.
  89
  90          While executing the registered handlers we are building a
  91          list of all the entries so that we can go backward later on.  */
  92       while (1)
  93         {
  94           /* Execute the handler if there is one.  */
  95           if (runp->prepare_handler != NULL)
  96             runp->prepare_handler ();
  97
  98           /* Create a new element for the list.  */
  99           struct used_handler *newp
 100             = (struct used_handler *) alloca (sizeof (*newp));
 101           newp->handler = runp;
 102           newp->next = allp;
 103           allp = newp;
 104
 105           /* Advance to the next handler.  */
 106           runp = runp->next;
 107           if (runp == NULL)
 108             break;
 109
 110           /* Bump the reference counter for the next entry.  */
 111           atomic_increment (&runp->refcntr);
 112         }
 113
 114       /* We are done.  */
 115       break;
 116     }
 117
 118   /* If we are not running multiple threads, we do not have to
 119      preserve lock state.  If fork runs from a signal handler, only
 120      async-signal-safe functions can be used in the child.  These data
 121      structures are only used by unsafe functions, so their state does
 122      not matter if fork was called from a signal handler.  */
 123   if (multiple_threads)
 124     {
 125       _IO_list_lock ();
 126
 127       /* Acquire malloc locks.  This needs to come last because fork
 128          handlers may use malloc, and the libio list lock has an
 129          indirect malloc dependency as well (via the getdelim
 130          function).  */
 131       call_function_static_weak (__malloc_fork_lock_parent);
 132     }
 133
 134 #ifdef ARCH_FORK
 135   pid = ARCH_FORK ();
 136 #else
 137 # error "ARCH_FORK must be defined so that the CLONE_SETTID flag is used"
 138   pid = INLINE_SYSCALL (fork, 0);
 139 #endif
 140
 141
 142   if (pid == 0)
 143     {
 144       struct pthread *self = THREAD_SELF;
 145
 146       /* See __pthread_once.  */
 147       if (__fork_generation_pointer != NULL)
 148         *__fork_generation_pointer += __PTHREAD_ONCE_FORK_GEN_INCR;
 149
 150 #if HP_TIMING_AVAIL
 151       /* The CPU clock of the thread and process have to be set to zero.  */
 152       hp_timing_t now;
 153       HP_TIMING_NOW (now);
 154       THREAD_SETMEM (self, cpuclock_offset, now);
 155       GL(dl_cpuclock_offset) = now;
 156 #endif
 157
 158 #ifdef __NR_set_robust_list
 159       /* Initialize the robust mutex list setting in the kernel which has
 160          been reset during the fork.  We do not check for errors because if
 161          it fails here, it must have failed at process startup as well and
 162          nobody could have used robust mutexes.
 163          Before we do that, we have to clear the list of robust mutexes
 164          because we do not inherit ownership of mutexes from the parent.
 165          We do not have to set self->robust_head.futex_offset since we do
 166          inherit the correct value from the parent.  We do not need to clear
 167          the pending operation because it must have been zero when fork was
 168          called.  */
 169 # if __PTHREAD_MUTEX_HAVE_PREV
 170       self->robust_prev = &self->robust_head;
 171 # endif
 172       self->robust_head.list = &self->robust_head;
 173 # ifdef SHARED
 174       if (__builtin_expect (__libc_pthread_functions_init, 0))
 175         PTHFCT_CALL (ptr_set_robust, (self));
 176 # else
 177       extern __typeof (__nptl_set_robust) __nptl_set_robust
 178         __attribute__((weak));
 179       if (__builtin_expect (__nptl_set_robust != NULL, 0))
 180         __nptl_set_robust (self);
 181 # endif
 182 #endif
 183
 184       /* Reset the lock state in the multi-threaded case.  */
 185       if (multiple_threads)
 186         {
 187           /* Release malloc locks.  */
 188           call_function_static_weak (__malloc_fork_unlock_child);
 189
 190           /* Reset the file list.  These are recursive mutexes.  */
 191           fresetlockfiles ();
 192
 193           /* Reset locks in the I/O code.  */
 194           _IO_list_resetlock ();
 195         }
 196
 197       /* Reset the lock the dynamic loader uses to protect its data.  */
 198       __rtld_lock_initialize (GL(dl_load_lock));
 199
 200       /* Run the handlers registered for the child.  */
 201       while (allp != NULL)
 202         {
 203           if (allp->handler->child_handler != NULL)
 204             allp->handler->child_handler ();
 205
 206           /* Note that we do not have to wake any possible waiter.
 207              This is the only thread in the new process.  The count
 208              may have been bumped up by other threads doing a fork.
 209              We reset it to 1, to avoid waiting for non-existing
 210              thread(s) to release the count.  */
 211           allp->handler->refcntr = 1;
 212
 213           /* XXX We could at this point look through the object pool
 214              and mark all objects not on the __fork_handlers list as
 215              unused.  This is necessary in case the fork() happened
 216              while another thread called dlclose() and that call had
 217              to create a new list.  */
 218
 219           allp = allp->next;
 220         }
 221
 222       /* Initialize the fork lock.  */
 223       __fork_lock = LLL_LOCK_INITIALIZER;
 224     }
 225   else
 226     {
 227       /* Release acquired locks in the multi-threaded case.  */
 228       if (multiple_threads)
 229         {
 230           /* Release malloc locks, parent process variant.  */
 231           call_function_static_weak (__malloc_fork_unlock_parent);
 232
 233           /* We execute this even if the 'fork' call failed.  */
 234           _IO_list_unlock ();
 235         }
 236
 237       /* Run the handlers registered for the parent.  */
 238       while (allp != NULL)
 239         {
 240           if (allp->handler->parent_handler != NULL)
 241             allp->handler->parent_handler ();
 242
 243           if (atomic_decrement_and_test (&allp->handler->refcntr)
 244               && allp->handler->need_signal)
 245             futex_wake (&allp->handler->refcntr, 1, FUTEX_PRIVATE);
 246
 247           allp = allp->next;
 248         }
 249     }
 250
 251   return pid;
 252 }
 253 weak_alias (__libc_fork, __fork)
 254 libc_hidden_def (__fork)
 255 weak_alias (__libc_fork, fork)