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56290d6e76
This provides a band-aid and addresses the scenario where fork is called from a signal handler while the process is in the malloc subsystem (or has acquired the libio list lock). It does not address the general issue of async-signal-safety of fork; multi-threaded processes are not covered, and some glibc subsystems have fork handlers which are not async-signal-safe.
268 lines
8.1 KiB
C
268 lines
8.1 KiB
C
/* Copyright (C) 2002-2016 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <assert.h>
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#include <stdlib.h>
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#include <unistd.h>
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#include <sys/types.h>
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#include <sysdep.h>
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#include <libio/libioP.h>
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#include <tls.h>
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#include <hp-timing.h>
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#include <ldsodefs.h>
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#include <stdio-lock.h>
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#include <atomic.h>
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#include <nptl/pthreadP.h>
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#include <fork.h>
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#include <arch-fork.h>
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#include <futex-internal.h>
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#include <malloc/malloc-internal.h>
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static void
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fresetlockfiles (void)
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{
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_IO_ITER i;
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for (i = _IO_iter_begin(); i != _IO_iter_end(); i = _IO_iter_next(i))
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if ((_IO_iter_file (i)->_flags & _IO_USER_LOCK) == 0)
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_IO_lock_init (*((_IO_lock_t *) _IO_iter_file(i)->_lock));
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}
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pid_t
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__libc_fork (void)
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{
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pid_t pid;
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struct used_handler
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{
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struct fork_handler *handler;
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struct used_handler *next;
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} *allp = NULL;
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/* Determine if we are running multiple threads. We skip some fork
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handlers in the single-thread case, to make fork safer to use in
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signal handlers. POSIX requires that fork is async-signal-safe,
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but our current fork implementation is not. */
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bool multiple_threads = THREAD_GETMEM (THREAD_SELF, header.multiple_threads);
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/* Run all the registered preparation handlers. In reverse order.
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While doing this we build up a list of all the entries. */
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struct fork_handler *runp;
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while ((runp = __fork_handlers) != NULL)
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{
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/* Make sure we read from the current RUNP pointer. */
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atomic_full_barrier ();
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unsigned int oldval = runp->refcntr;
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if (oldval == 0)
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/* This means some other thread removed the list just after
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the pointer has been loaded. Try again. Either the list
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is empty or we can retry it. */
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continue;
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/* Bump the reference counter. */
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if (atomic_compare_and_exchange_bool_acq (&__fork_handlers->refcntr,
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oldval + 1, oldval))
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/* The value changed, try again. */
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continue;
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/* We bumped the reference counter for the first entry in the
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list. That means that none of the following entries will
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just go away. The unloading code works in the order of the
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list.
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While executing the registered handlers we are building a
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list of all the entries so that we can go backward later on. */
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while (1)
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{
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/* Execute the handler if there is one. */
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if (runp->prepare_handler != NULL)
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runp->prepare_handler ();
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/* Create a new element for the list. */
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struct used_handler *newp
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= (struct used_handler *) alloca (sizeof (*newp));
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newp->handler = runp;
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newp->next = allp;
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allp = newp;
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/* Advance to the next handler. */
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runp = runp->next;
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if (runp == NULL)
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break;
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/* Bump the reference counter for the next entry. */
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atomic_increment (&runp->refcntr);
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}
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/* We are done. */
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break;
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}
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/* If we are not running multiple threads, we do not have to
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preserve lock state. If fork runs from a signal handler, only
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async-signal-safe functions can be used in the child. These data
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structures are only used by unsafe functions, so their state does
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not matter if fork was called from a signal handler. */
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if (multiple_threads)
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{
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_IO_list_lock ();
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/* Acquire malloc locks. This needs to come last because fork
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handlers may use malloc, and the libio list lock has an
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indirect malloc dependency as well (via the getdelim
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function). */
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__malloc_fork_lock_parent ();
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}
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#ifndef NDEBUG
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pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);
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#endif
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/* We need to prevent the getpid() code to update the PID field so
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that, if a signal arrives in the child very early and the signal
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handler uses getpid(), the value returned is correct. */
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pid_t parentpid = THREAD_GETMEM (THREAD_SELF, pid);
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THREAD_SETMEM (THREAD_SELF, pid, -parentpid);
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#ifdef ARCH_FORK
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pid = ARCH_FORK ();
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#else
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# error "ARCH_FORK must be defined so that the CLONE_SETTID flag is used"
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pid = INLINE_SYSCALL (fork, 0);
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#endif
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if (pid == 0)
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{
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struct pthread *self = THREAD_SELF;
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assert (THREAD_GETMEM (self, tid) != ppid);
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/* See __pthread_once. */
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if (__fork_generation_pointer != NULL)
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*__fork_generation_pointer += __PTHREAD_ONCE_FORK_GEN_INCR;
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/* Adjust the PID field for the new process. */
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THREAD_SETMEM (self, pid, THREAD_GETMEM (self, tid));
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#if HP_TIMING_AVAIL
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/* The CPU clock of the thread and process have to be set to zero. */
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hp_timing_t now;
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HP_TIMING_NOW (now);
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THREAD_SETMEM (self, cpuclock_offset, now);
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GL(dl_cpuclock_offset) = now;
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#endif
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#ifdef __NR_set_robust_list
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/* Initialize the robust mutex list which has been reset during
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the fork. We do not check for errors since if it fails here
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it failed at process start as well and noone could have used
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robust mutexes. We also do not have to set
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self->robust_head.futex_offset since we inherit the correct
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value from the parent. */
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# ifdef SHARED
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if (__builtin_expect (__libc_pthread_functions_init, 0))
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PTHFCT_CALL (ptr_set_robust, (self));
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# else
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extern __typeof (__nptl_set_robust) __nptl_set_robust
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__attribute__((weak));
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if (__builtin_expect (__nptl_set_robust != NULL, 0))
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__nptl_set_robust (self);
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# endif
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#endif
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/* Reset the lock state in the multi-threaded case. */
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if (multiple_threads)
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{
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/* Release malloc locks. */
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__malloc_fork_unlock_child ();
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/* Reset the file list. These are recursive mutexes. */
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fresetlockfiles ();
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/* Reset locks in the I/O code. */
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_IO_list_resetlock ();
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}
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/* Reset the lock the dynamic loader uses to protect its data. */
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__rtld_lock_initialize (GL(dl_load_lock));
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/* Run the handlers registered for the child. */
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while (allp != NULL)
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{
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if (allp->handler->child_handler != NULL)
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allp->handler->child_handler ();
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/* Note that we do not have to wake any possible waiter.
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This is the only thread in the new process. The count
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may have been bumped up by other threads doing a fork.
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We reset it to 1, to avoid waiting for non-existing
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thread(s) to release the count. */
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allp->handler->refcntr = 1;
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/* XXX We could at this point look through the object pool
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and mark all objects not on the __fork_handlers list as
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unused. This is necessary in case the fork() happened
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while another thread called dlclose() and that call had
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to create a new list. */
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allp = allp->next;
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}
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/* Initialize the fork lock. */
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__fork_lock = LLL_LOCK_INITIALIZER;
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}
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else
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{
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assert (THREAD_GETMEM (THREAD_SELF, tid) == ppid);
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/* Restore the PID value. */
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THREAD_SETMEM (THREAD_SELF, pid, parentpid);
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/* Release acquired locks in the multi-threaded case. */
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if (multiple_threads)
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{
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/* Release malloc locks, parent process variant. */
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__malloc_fork_unlock_parent ();
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/* We execute this even if the 'fork' call failed. */
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_IO_list_unlock ();
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}
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/* Run the handlers registered for the parent. */
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while (allp != NULL)
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{
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if (allp->handler->parent_handler != NULL)
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allp->handler->parent_handler ();
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if (atomic_decrement_and_test (&allp->handler->refcntr)
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&& allp->handler->need_signal)
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futex_wake (&allp->handler->refcntr, 1, FUTEX_PRIVATE);
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allp = allp->next;
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}
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}
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return pid;
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}
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weak_alias (__libc_fork, __fork)
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libc_hidden_def (__fork)
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weak_alias (__libc_fork, fork)
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