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    SubjectRe: [RFC, PATCH 1/3] introduce SYS_CLONE_MASK
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    Oleg Nesterov <oleg@tv-sign.ru> writes:

    > On 04/08, Eric W. Biederman wrote:
    >
    >> If we are going to have kernel only flags please use an additional
    >> argument to do_fork and copy_process.
    >
    > Yes, we can do this. But we have a number of architectures which use
    > sys_clone() to implement kernel_thread(). It would be nice to have an
    > architecture neutral kernel_thread() implementation as you proposed.
    > We should change all of them if we want to add a new parameter to
    > do_fork().
    >
    > Perhaps it is better to add reparent_kthread() (next patch) to kthread()
    > and forget about CLONE_KERNEL_THREAD.

    Please.

    > Anyway, re-parenting to swapper breaks pstree, it doesn't show kernel
    > threads. And if ->parent == /sbin/init, we can't remove us from ->children
    > (unless we forbid sub-thread-of-init exec). So the only safe change is
    > set ->exit_state = -1.

    Yes. We certainly need ->exit_state = -1.
    Earlier I had forgotten about second the use of ->children to update
    the parent pointer of processes when their parent exits.

    There is a practical question how much we care about pstree being
    confused (I assume it doesn't crash). If this is just a confusion
    issue then I say go for it. PPID == 0 is a very legitimate way to say
    the kernel is the parent process.

    There are a few more cases where we are likely to get PPID == 0 in the
    future and /sbin/init already has that now. Plus there is a lot of
    historic precedent. The odd part is PPID = 0 having multiple
    children.

    If we decide maintaining a tree is important I would much rather put
    init_task on the task_list so we can see it in /proc then go the other
    way around.

    I would like a confirmation that it PPID == 0 is what is confusing
    pstree just to make certain we haven't half filled in some field
    in init_task and are thus giving in correct /proc output. But that is
    all the double checking I would do.

    >> Your current scheme also has the bad side that if user space supplied
    >> a kernel flag it is hard to detect it and return -EINVAL. Which
    >> limits future expansion. Silently dropping clone flags is a real
    >> pain, if you are trying to detect if a new flag has been implemented.
    >
    > Yes. But that is what we are doing now. copy_process() just ignores
    > unknown flags.

    Agreed. I fixed that in sys_unshare but I should really submit a
    patch to do the same for sys_clone at some point.

    When know flags aren't implemented we certainly return -EINVAL.

    Given that this line of work looks to fix the race that messes allows
    a threaded init to generate unkillable zombies I can probably find
    some time in the next while to work on it.

    Eric
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