The patch titled
reparent/untrace: do nothing if no childs/tracees
has been added to the -mm tree. Its filename is
reparent-untrace-do-nothing-if-no-childs-tracees.patch
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------------------------------------------------------
Subject: reparent/untrace: do nothing if no childs/tracees
From: Oleg Nesterov <oleg@xxxxxxxxxx>
forget_original_parent() and exit_ptrace() can avoid taking the global
tasklist_lock if there are no childs/tracees. But I failed to invent the
comment to explain why/when this is safe to do, that is why the separate
patch/changelog.
The problem is, we can race with the concurrent release_task() which can
remove the last child form our ->children/ptraced list. This means that
list_empty() can return the "false" positive, it is possible that
release_task() is still in progress, it can use the caller's task_struct
somehow, and it is even possible that list_del(sibling/ptrace_entry) has
not yet completed.
But this is fine, before our task_struct will be released we will take
tasklist_lock at least once in release_task(), this will synchronize us
with the possible release_task/ptrace_unlink in flight.
However, forget_original_parent() has another problem. We can race with
another thread which has already picked us for reparenting before we set
PF_EXITING, so this patch also checks thread_group_empty().
It is possible to be more clever, we can take tasklist for reading, or
ensure that ->thread_group.prev is not PF_EXITING, but this is nasty.
Perhaps even this optimization is too ugly.
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: "Metzger, Markus T" <markus.t.metzger@xxxxxxxxx>
Cc: Roland McGrath <roland@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/exit.c | 10 ++++++++++
kernel/ptrace.c | 3 +++
2 files changed, 13 insertions(+)
diff -puN kernel/exit.c~reparent-untrace-do-nothing-if-no-childs-tracees kernel/exit.c
--- a/kernel/exit.c~reparent-untrace-do-nothing-if-no-childs-tracees
+++ a/kernel/exit.c
@@ -806,6 +806,16 @@ static void forget_original_parent(struc
struct task_struct *p, *n, *reaper;
LIST_HEAD(dead_children);
+ if (thread_group_empty(father)) {
+ /*
+ * Make sure no other thread can reparent to
+ * us after the list_empty(->children) check.
+ */
+ smp_rmb();
+ if (list_empty(&father->children))
+ return;
+ }
+
write_lock_irq(&tasklist_lock);
reaper = find_new_reaper(father);
diff -puN kernel/ptrace.c~reparent-untrace-do-nothing-if-no-childs-tracees kernel/ptrace.c
--- a/kernel/ptrace.c~reparent-untrace-do-nothing-if-no-childs-tracees
+++ a/kernel/ptrace.c
@@ -323,6 +323,9 @@ void exit_ptrace(struct task_struct *tra
struct task_struct *p, *n;
LIST_HEAD(ptrace_dead);
+ if (list_empty(&tracer->ptraced))
+ return;
+
write_lock_irq(&tasklist_lock);
list_for_each_entry_safe(p, n, &tracer->ptraced, ptrace_entry) {
if (__ptrace_detach(tracer, p))
_
Patches currently in -mm which might be from oleg@xxxxxxxxxx are
origin.patch
linux-next.patch
pipe_rdwr_fasync-fix-the-error-handling-to-prevent-the-leak-crash.patch
get_mm_hiwater_xxx-trivial-s-define-inline.patch
getrusage-fill-ru_maxrss-value.patch
ptrace-kill-__ptrace_detach-fix-exit_state-check.patch
ptrace-simplify-ptrace_exit-ignoring_children-path.patch
ptrace-simplify-ptrace_exit-ignoring_children-pathpatch-fix.patch
ptrace-reintroduce-__ptrace_detach-as-a-callee-of-ptrace_exit.patch
ptrace-reintroduce-__ptrace_detach-as-a-callee-of-ptrace_exit-fix.patch
ptrace-fix-possible-zombie-leak-on-ptrace_detach.patch
reparent_thread-dont-call-kill_orphaned_pgrp-if-task_detached.patch
reparent_thread-fix-the-is-it-traced-check.patch
reparent_thread-fix-a-zombie-leak-if-sbin-init-ignores-sigchld.patch
forget_original_parent-split-out-the-un-ptrace-part.patch
forget_original_parent-do-not-abuse-child-ptrace_entry.patch
forget_original_parent-do-not-abuse-child-ptrace_entry-fix.patch
move-exit_ptrace-from-forget_original_parent-to-do_exit.patch
reparent-untrace-do-nothing-if-no-childs-tracees.patch
tracehook_notify_death-use-task_detached-helper.patch
workqueue-avoid-recursion-in-run_workqueue.patch
kthreads-move-sched-realeted-initialization-from-kthreadd-context.patch
kthreads-simplify-the-startup-synchronization.patch
kthreads-rework-kthread_stop.patch
kthreads-simplify-migration_thread-exit-path.patch
pids-document-task_pgrp-task_session-is-not-safe-without-tasklist-rcu.patch
pids-document-task_pgrp-task_session-is-not-safe-without-tasklist-rcu-fix.patch
pids-improve-get_task_pid-to-fix-the-unsafe-sys_wait4-task_pgrp.patch
pids-refactor-vnr-nr_ns-helpers-to-make-them-safe.patch
pids-kill-now-unused-signal_struct-__pgrp-__session-and-friends.patch
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