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Commit 4a2c7a78 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
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[PATCH] make fork() atomic wrt pgrp/session signals 

Eric W. Biederman wrote:
>
> Ok. SUSV3/Posix is clear, fork is atomic with respect
> to signals.  Either a signal comes before or after a
> fork but not during. (See the rationale section).
> http://www.opengroup.org/onlinepubs/000095399/functions/fork.html


>
> The tasklist_lock does not stop forks from adding to a process
> group. The forks stall while the tasklist_lock is held, but a fork
> that began before we grabbed the tasklist_lock simply completes
> afterwards, and the child does not receive the signal.

This also means that SIGSTOP or sig_kernel_coredump() signal can't
be delivered to pgrp/session reliably.

With this patch copy_process() returns -ERESTARTNOINTR when it
detects a pending signal, fork() will be restarted transparently
after handling the signals.

This patch also deletes now unneeded "group_stop_count > 0" check,
copy_process() can no longer succeed while group stop in progress.

Signed-off-by: default avatarOleg Nesterov <oleg@tv-sign.ru>
Acked-By: default avatarEric Biederman <ebiederm@xmission.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 47e65328

kernel/fork.c

+17 −20
Original line number Diff line number Diff line
@@ -1136,16 +1136,6 @@ static task_t *copy_process(unsigned long clone_flags, 
			!cpu_online(task_cpu(p))))

		set_task_cpu(p, smp_processor_id());



	/*

	 * Check for pending SIGKILL! The new thread should not be allowed

	 * to slip out of an OOM kill. (or normal SIGKILL.)

	 */

	if (sigismember(&current->pending.signal, SIGKILL)) {

		write_unlock_irq(&tasklist_lock);

		retval = -EINTR;

		goto bad_fork_cleanup_namespace;

	}



	/* CLONE_PARENT re-uses the old parent */

	if (clone_flags & (CLONE_PARENT|CLONE_THREAD))

		p->real_parent = current->real_parent;
@@ -1154,6 +1144,23 @@ static task_t *copy_process(unsigned long clone_flags, 
	p->parent = p->real_parent;



	spin_lock(&current->sighand->siglock);



	/*

	 * Process group and session signals need to be delivered to just the

	 * parent before the fork or both the parent and the child after the

	 * fork. Restart if a signal comes in before we add the new process to

	 * it's process group.

	 * A fatal signal pending means that current will exit, so the new

	 * thread can't slip out of an OOM kill (or normal SIGKILL).

 	 */

 	recalc_sigpending();

	if (signal_pending(current)) {

		spin_unlock(&current->sighand->siglock);

		write_unlock_irq(&tasklist_lock);

		retval = -ERESTARTNOINTR;

		goto bad_fork_cleanup_namespace;

	}



	if (clone_flags & CLONE_THREAD) {

		/*

		 * Important: if an exit-all has been started then
@@ -1170,16 +1177,6 @@ static task_t *copy_process(unsigned long clone_flags, 
		p->group_leader = current->group_leader;

		list_add_tail_rcu(&p->thread_group, &p->group_leader->thread_group);



		if (current->signal->group_stop_count > 0) {

			/*

			 * There is an all-stop in progress for the group.

			 * We ourselves will stop as soon as we check signals.

			 * Make the new thread part of that group stop too.

			 */

			current->signal->group_stop_count++;

			set_tsk_thread_flag(p, TIF_SIGPENDING);

		}



		if (!cputime_eq(current->signal->it_virt_expires,

				cputime_zero) ||

		    !cputime_eq(current->signal->it_prof_expires,