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    SubjectRe: ptrace fixes for 3.2
    Hello, Oleg.

    On Tue, Jan 03, 2012 at 04:44:04PM +0100, Oleg Nesterov wrote:
    > It fails because ->real_parent sees its child in EXIT_DEAD state
    > while the tracer is going to change the state back to EXIT_ZOMBIE
    > in wait_task_zombie().

    Argh.... EXIT_ZOMBIE -> DEAD -> ZOMBIE dancing in wait_task_zombie()
    is just nasty. Didn't realize it was doing that. :(

    > The offending commit is 823b018e which moved the EXIT_DEAD check,
    > but in fact we should not blame it. The original code was not
    > correct as well because it didn't take ptrace_reparented() into
    > account and because we can't really trust ->ptrace.
    >
    > This patch adds the additional check to close this particular
    > race but it doesn't solve the whole problem. We simply can't
    > rely on ->ptrace in this case, it can be cleared if the tracer
    > is multithreaded by the exiting ->parent.

    I'm not following this part. Can you please explain it in a bit more
    detail?

    > I think we should kill EXIT_DEAD altogether, we should always
    > remove the soon-to-be-reaped child from ->children or at least
    > we should never do the DEAD->ZOMBIE transition. But this is too
    > complex for 3.2.

    Agreed. Removing the reverse transition shouldn't be too difficult
    and can be done without affecting fast non-ptrace path. ie. if the
    child is ptraced, drop readlock, grab writelock, recheck, buffer
    states to copy out to userland, detach and transit to DEAD if
    necessary.

    > Also, I think wait_consider_task() needs more fixes. I do not
    > think we should clear ->notask_error without WEXITED in this
    > case, but this is what we do in the EXIT_ZOMBIE case.

    Hmmm... I'm not sure about that. Why do you think so?

    > Reported-by: Denys Vlasenko <vda.linux@googlemail.com>
    > Cc: v3.0.. <stable@kernel.org>
    > Signed-off-by: Oleg Nesterov <oleg@redhat.com>

    Anyways, the fix looks good to me.

    Acked-by: Tejun Heo <tj@kernel.org>

    Thank you.

    --
    tejun


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