> On Mar 16, 2018, at 7:47 AM, Christian Brauner <christian.brauner@xxxxxxxxxxx> 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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