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Subject[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: Oleg Nesterov <oleg@tv-sign.ru>

--- MM/kernel/fork.c~ 2006-03-13 15:04:52.000000000 +0300
+++ MM/kernel/fork.c 2006-03-16 21:51:12.000000000 +0300
@@ -1129,16 +1129,6 @@ static task_t *copy_process(unsigned lon
!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;
@@ -1147,6 +1137,23 @@ static task_t *copy_process(unsigned lon
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
@@ -1163,16 +1170,6 @@ static task_t *copy_process(unsigned lon
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,
-
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