wait_task_zombie() first does EXIT_ZOMBIE->EXIT_DEAD transition and drops tasklist_lock. If this task is not the natural child and it is traced, we change its state back to EXIT_ZOMBIE for ->real_parent.
The last transition is racy, this is even documented in 50b8d257486a "ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE race". wait_consider_task() tries to detect this transition and clear ->notask_error but we can't rely on ptrace_reparented(), debugger can exit and do ptrace_unlink() before its sub-thread sets EXIT_ZOMBIE. And there is another problem which were missed before: this transition can also race with reparent_leader() which doesn't reset >exit_signal if EXIT_DEAD, assuming that this task must be reaped by someone else. So the tracee can be re-parented with ->exit_signal != SIGCHLD, and if /sbin/init doesn't use __WALL it becomes unreapable. Change reparent_leader() to update ->exit_signal even if EXIT_DEAD. Note: this is the simple temporary hack for -stable, it doesn't try to solve all problems, it will be reverted by the next changes. Cc: [email protected] Reported-by: Jan Kratochvil <[email protected]> Reported-and-tested-by: Michal Schmidt <[email protected]> Signed-off-by: Oleg Nesterov <[email protected]> --- kernel/exit.c | 15 +++++++++++---- 1 files changed, 11 insertions(+), 4 deletions(-) diff --git a/kernel/exit.c b/kernel/exit.c index 1e77fc6..5281522 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -560,9 +560,6 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p, struct list_head *dead) { list_move_tail(&p->sibling, &p->real_parent->children); - - if (p->exit_state == EXIT_DEAD) - return; /* * If this is a threaded reparent there is no need to * notify anyone anything has happened. @@ -570,9 +567,19 @@ static void reparent_leader(struct task_struct *father, struct task_struct *p, if (same_thread_group(p->real_parent, father)) return; - /* We don't want people slaying init. */ + /* + * We don't want people slaying init. + * + * Note: we do this even if it is EXIT_DEAD, wait_task_zombie() + * can change ->exit_state to EXIT_ZOMBIE. If this is the final + * state, do_notify_parent() was already called and ->exit_signal + * doesn't matter. + */ p->exit_signal = SIGCHLD; + if (p->exit_state == EXIT_DEAD) + return; + /* If it has exited notify the new parent about this child's death. */ if (!p->ptrace && p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) { -- 1.5.5.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
