Re: [GIT PULL] kdbus for 4.1-rc1

[Andy Lutomirski]

On Wed, Apr 15, 2015 at 4:27 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Apr 15, 2015 at 03:54:10PM -0700, Andy Lutomirski wrote:
>> Huh, interesting.
>>
>> I was imagining that each of a server's peers (capability holders)
>> would have a fresh struct file, but maybe this wouldn't be needed at
>> all.  You'd still need a way to get replies to your request, but the
>> API could just as easily be:
>>
>> int send_to_capability(int dest, int source, const void *data, size_t len, ...);
>>
>> where dest would be the destination's fd and source would be whatever
>> receive queue I expect the response on.
>>
>> So maybe this is feasible.  It doesn't solve broadcasts, but dbus
>> unicast could easily layer over a facility like this and the context
>> switch problem would go away for unicast.
>>
>> Heck, I'd use it for my own proprietary stuff, too.  It would be way
>> easier than the absurd tangle of socketpairs I currently use.
>
> BTW, the main issue with AF_UNIX passing is that recepient isn't asleep
> awaiting for descriptors - they are thrown by sender at whoever's receiving
> and sit there until somebody gets around to picking them.
>
> _IF_ we had
> client: I want a desciptor <goes to sleep, interruptibly>
> kernel: assign it a sequence number
> server: sees request (including sequence number)
> server: give this fd to originator of request #N
> kernel: check if originator is still there, insert the damn thing into their
> descriptor table if they still are and return the obtained number
> or
> server: tell the originator of request #N to fuck off
> kernel: check if originator is still there and gleefully pass the "fuck off" if
> they still are
>
> we wouldn't have the in-flight state at all, and there goes the garbage
> collection shite.  With some elaboration, it could even carry the
> authentication traffic - "fuck off" might be "answer this challenge", with
> the next "I want a descriptor" carrying reply...

I wonder if we could get away with having the receiver pre-allocate
some placeholder fds and then have the kernel replace a placeholder
with a passed fd immediately when the fd is sent and enqueue *that* in
the cmsg data.  If you send an fd to someone who hasn't assigned any
placeholders to the receiving socket, then you get an error.

To keep the accounting sane, a placeholder would be a bona fide fd,
presumably a reference to a global placeholder anon_inode.

--Andy
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