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    Subject[PATCH] fix the "remove task_ppid_nr_ns" commit
    The commit 84427eaef1fb91704c7112bdb598c810003b99f3 (remove task_ppid_nr_ns)
    moves the task_tgid_nr_ns(task->real_parent) outside of lock_task_sighand().
    This is wrong, ->real_parent could be freed/reused.

    Both ->parent/real_parent point to nothnig after __exit_signal() because we
    remove the child from ->children list, and thus the child can't be reparented
    when its parent exits.

    rcu_read_lock() protects ->parent/real_parent, but _only_ if we know it was
    valid before we take rcu lock.

    Revert this part of the patch.

    Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

    --- t/fs/proc/array.c~ppid_fix 2008-01-14 23:45:35.000000000 +0300
    +++ t/fs/proc/array.c 2008-01-14 23:59:27.000000000 +0300
    @@ -426,7 +426,6 @@ static int do_task_stat(struct task_stru
    cgtime = gtime = cputime_zero;

    rcu_read_lock();
    - ppid = task_tgid_nr_ns(task->real_parent, ns);
    if (lock_task_sighand(task, &flags)) {
    struct signal_struct *sig = task->signal;

    @@ -465,6 +464,7 @@ static int do_task_stat(struct task_stru
    }

    sid = task_session_nr_ns(task, ns);
    + ppid = task_tgid_nr_ns(task->real_parent, ns);
    pgid = task_pgrp_nr_ns(task, ns);

    unlock_task_sighand(task, &flags);


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