Re: Signal delivery to kernel threads/processes?

From: Don Lewis (truckman_at_FreeBSD.org)
Date: 01/17/04

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    Date: Sat, 17 Jan 2004 01:57:36 -0800 (PST)
To: rwatson@FreeBSD.org

    On 16 Jan, Robert Watson wrote:
    >
    > Bill Paul raised an interesting question with me recently -- he observed
    > that a userspace process running with root privileges could deliver a
    > signal to a kthread, and that this might produce undesired behavior. I
    > was sure that, at some point, we had a check disallowing this, but I don't
    > see it in either RELENG_4 or HEAD. Do we rely on the ability to
    > send/receive signals to interrupt kthreads, that we know of? While the
    > signal delivery itself doesn't make sense, waking up a tsleep() with
    > PCATCH could well be useful behavior. Should a kthread have to declare if
    > it wants to receive interruptions? Given plans, at some point, to make
    > kthreads be real threads, and be part of a kernel process, that would
    > raise some challenges for code relying on the ability to be interrupted
    > with a signal in kernel space, as signals generated by kill() are
    > targetted at processes, not threads.
    >
    > Any thoughts? It's tempting simply to add the following to cr_cansignal()
    > to at least disallow sourcing the signals in userspace:
    >
    > if (p->p_flag & P_SYSTEM)
    > return (EPERM);
    >
    > But I don't have a broad enough view of what goes on in the kernel to
    > reason about what disasters this might cause if signalling is relied on.

    The only thing I worried about is what happens to kthreads on
    shutdown. It looks like this is handled by kthread_suspend() which
    tells the thread that it has received a SIGSTOP, but this isn't done
    with the normal signal delivery mechanism.
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