Re: [RFC 1/3] seccomp: add a return code to trap to userspace

[Andy Lutomirski]

> On Feb 26, 2018, at 4:49 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> 
>> On Wed, Feb 14, 2018 at 9:19 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>> On Wed, Feb 14, 2018 at 3:29 PM, Tycho Andersen <tycho@xxxxxxxx> wrote:
>>>> On Tue, Feb 13, 2018 at 01:09:20PM -0800, Kees Cook wrote:
>>>> On Sun, Feb 4, 2018 at 2:49 AM, Tycho Andersen <tycho@xxxxxxxx> wrote:
>>>> I wonder if this communication should be netlink, which gives a more
>>>> well-structured way to describe what's on the wire? The reason I ask
>>>> is because if we ever change the seccomp_data structure, we'll now
>>>> have two places where we need to deal with it (the first being within
>>>> the BPF itself). My initial idea was to prefix the communication with
>>>> a size field, then send the structure, and then I had nightmares, and
>>>> realized this was basically netlink reinvented.
>>> 
>>> I suggested netlink in LA, and everyone (especially Andy) groaned very
>>> loudly :). I'm happy to switch it to netlink if you like, although i
>>> think memcpy() of structs should be safe here, since the return value
>>> from read or write can indicate the size of things.
>> 
>> I could easily get on board with "netlink" (i.e. NLA) messages sent
>> over an fd.  I will object strongly to the use of netlink *sockets*.
> 
> Yeah, I was thinking NLA over the fd; not a netlink socket.
> 
>>>> An ERRNO filter would block a USER_NOTIF because it's unconditional.
>>>> TRACE could be either, USER_NOTIF could be either.
>>>> 
>>>> This means TRACE rules would be bumped by a USER_NOTIF... hmm.
>>> 
>>> Yes, I didn't exactly know what to do here. ERRNO, TRAP, and KILL all
>>> seemed more important than USER_NOTIF, but TRACE didn't. I don't have
>>> a strong opinion about what to do here, because users can adjust their
>>> filters accordingly. Let me know what you prefer.
>> 
>> If we switched to eBPF functions, this whole issue goes away.
> 
> Yeah, though we'd still need some kind of "wait for answer" eBPF
> function. It feels wrong to re-use maps for that...
> 

BPF_CALL.

Alexei, can we make it so that each bpf program type can easily limit which BPF_CALL helpers can be use and allow bpf program types to add their own helpers?c