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    SubjectRe: [PATCH] do_signal_stop: use signal_group_exit()
    On 02/16, Oleg Nesterov wrote:
    >
    > On 02/15, Andrew Morton wrote:
    > >
    > > ug. On about the fourth boot with the current -mm lineup I hit:
    > >
    > > : BUG: unable to handle kernel paging request at 0000000000200200
    > ^^^^^^^^^^^^^^^^
    > == LIST_POISON2
    >
    > > : IP: [<ffffffff802444f5>] free_pid+0x35/0x8e
    >
    > most probably == hlist_del_rcu(pid_chain)
    >
    > > : Call Trace:
    > > : [<ffffffff80237727>] ? release_task+0x152/0x2e5
    > > : [<ffffffff80237f81>] ? do_wait+0x6c7/0xa1c
    > > : [<ffffffff8022f4cc>] ? default_wake_function+0x0/0xe
    > > : [<ffffffff8023e670>] ? sys_rt_sigaction+0x7a/0x98
    > > : [<ffffffff80238360>] ? sys_wait4+0x8a/0xa1
    > > : [<ffffffff8020be4b>] ? system_call_after_swapgs+0x7b/0x80
    >
    > (Can't understand why there is no detach_pid() in this stack trace,
    > but it is the only possible caller of free_pid()).
    >
    > So, detach_pid()->free_pid() hit an already unhashed pid. But this
    > is not possible?
    >
    > This means we already did detach_pid(), but in that case the previous
    > detach_pid() has set task->pids[].pid = NULL, and we should OOPS earlier,
    > somewhere at "if (!hlist_empty(&pid->tasks[tmp]))".

    Yes, this is not possible.

    But what possible is: we have the unbalanced put_pid(pid) which frees the
    live pid. The next alloc_pid() gets the same memory, and initializes all
    pid->tasks[] lists.

    Now, if that pid was used as PIDTYPE_PGID/PIDTYPE_SID, the next detach_pid()
    from this pgrp/sid sees that the pid is not used (all lists are hlist_empty),
    frees this pid again, and the bug manifests itself this way.

    tiocspgrp:

    put_pid(real_tty->pgrp);
    // ------ WINDOW ------
    real_tty->pgrp = get_pid(pgrp);

    When bash spawns the command, both parent and child do ioctl(TIOCSPGRP,child),
    and it is possible that both do put_pid() on the same parent's pid.

    Damn, when you know what the bug is, the test case is trivial:

    $ while true; do perl -e0; done

    The kernel crashes.

    From 2.6.25-rc2-mm1.bz2 patch:
    >
    > - .ioctl = tty_ioctl,
    > + .unlocked_ioctl = tty_ioctl,

    and this is why this didn't happen before, I guess.

    > I'll try to think more about this, but I doubt very much I'll find the
    > reason :(

    Ohhh... 7 (!!!) hours of hacking + some vodka did the trick.

    (Kamalesh, I think you hit the same bug).

    Oleg.



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