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    SubjectRe: [RFC 0/3] seccomp trap to userspace
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    > On Mar 16, 2018, at 7:47 AM, Christian Brauner <christian.brauner@mailbox.org> wrote:
    >
    >> On Fri, Mar 16, 2018 at 12:46:55AM +0000, Andy Lutomirski wrote:


    I bet I confused everyone with a blatant typo:

    >>
    >> Hmm, I think we have to be very careful to avoid nasty races. I think
    >> the correct approach is to notice the signal and send a message to the
    >> listener that a signal is pending but to take no additional action.
    >> If the handler ends up completing the syscall with a successful
    >> return, we don't want to replace it with -EINTR. IOW the code looks
    >> kind of like:
    >>
    >> send_to_listener("hey I got a signal");

    That should be “hey I got a syscall”. D’oh!

    >> wait_ret = wait_interruptible for the listener to reply;
    >> if (wait_ret == -EINTR) {
    >
    > Hm, so from the pseudo-code it looks like: The handler would inform the
    > listener that it received a signal (either from the syscall requester or
    > from somewhere else) and then wait for the listener to reply to that
    > message. This would allow the listener to decide what action it wants
    > the handler to take based on the signal, i.e. either cancel the request
    > or retry? The comment makes it sound like that the handler doesn't
    > really wait on the listener when it receives a signal it simply moves
    > on.

    It keeps waiting killably but not interruptibly.

    > So no "taking no additional action" here means not have the handler
    > decide to abort but the listener?

    If by “handler” you mean kernel, then yes.

    There’s no userspace syscall handler involved. From the kernel’s perspective, a syscall is never still in progress when a signal handler is invoked — we only actually invoke syscall handlers in prepare_exit_to_usermode() or the non-x86 equivalent and the functions it calls. While a syscall is running, the kernel might notice that a signal is pending and do one of a few things:

    1. Just keep going. Not all syscalls can be interrupted.

    2. Try to finish early. If a send() call has already sent some but not all data, it can stop waiting and return the number of bytes sent.

    3. Abort with -EINTR.

    4. Abort with -ERESTARTSYS or one of its relatives. These fiddle with user registers in a somewhat unpleasant way to pretend that the syscall never actually happened. This works for syscalls that wait with an absolute timeout, for example.

    5. Set up restart_syscall() magic, rewrite regs so it looks like the user was about to call restart_syscall() when the signal happened, and abort.

    In all cases, the signal is dealt with afterwards. This could result in changing regs to call the handler or in simply returning.

    1-3 should work fully in seccomp. The only issue is that the kernel doesn’t know *which* to do, nor can the kernel force the listener to abort cleanly, so I think we have no real choice but to let the listener decide.

    4 could be supported just like 1-3. 5 is awful, and I don’t think we should support it for user listeners.

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