[PATCH] winbindd: control number of winbindd's client connections

Volker Lendecke Volker.Lendecke at SerNet.DE
Mon Jun 8 14:31:47 MDT 2015

On Mon, Jun 08, 2015 at 10:25:53PM +0300, Uri Simchoni wrote:
> I'll try what you suggest. I tried documenting my understanding in
> comments inside.

Thanks for reading through my mail :-)

> On Mon, Jun 8, 2015 at 12:13 PM, Volker Lendecke
> <Volker.Lendecke at sernet.de> wrote:
> >
> > Sorry to be a bit late in the game, I was too busy to really give the
> > issue some thought last week.
> >
> > How about the following approach to solve the problem:
> >
> > Every time we are about to ship some request to a winbind child we could
> > to check whether the corresponding winbind client is still around. I
> > know that this means that we need to carry state information down to
> > some deep levels. The information about which client is being served is
> > not readily available in wb_child_request_trigger, but I'm sure there's
> > ways to get it there.
> >
> > We can even go one step further and keep listening for readability on
> > the client socket while a request is in progress, waiting for its turn
> > down to the child process. While a child RPC request is in progress,
> > we need to stop listening for the client disconnect, as this would badly
> > destroy the parent->child protocol sequence.
> >
> > If that is done, isn't most of the problem already solved?  If we are
> > running into fd limits, just bump up the number of fds. Fds are relatively
> > cheap these days, in particular if with modern winbind you're using
> > epoll. 3.3.16 won't have that yet, but master should.  Also, there can
> > be at most one fd per system thread assuming normal use of libwbclient.
> It has taken me a while to figure out this one. What I understand is
> "don't bother with stopping and resuming accept, just set winbind max
> clients to something high enough, and bump the max FDs accordingly.
> Even if you don't know what high enough is, you can set it to
> something really really high because winbindd's ability to accept
> clients (with epoll) exceeds the system's ability to create clients
> (with fork - 1 fd per process, assuming client is not malicious and
> well behaved)" That, assuming we keep the number of accepted
> connections equal to the number of clients, and we do that by checking
> whether the client is still there.

Right. This assumes that we have some reasonable means to
limit the winbind connections to one per process, which is
not the case right now given the timeout and reconnect

> In that case I would also change (fix) the documentation of "winbind
> max clients" - it is not a limit but more of a hint.

Also right. And we should just remove that silly DEBUG(1)

> What if the client is not well-behaved? I suppose we can attack the
> problem of malicious clients by passing the client's pid via the unix
> domain socket and keeping a map that enforces the one connection per
> client rule - that's outside the scope of this patch set as this
> possibility existed before.

Also correct. Before implementing this scheme, we need to
watch out for the recent changes in libwbclient that make it
possible to open multiple connections from a single process
using the Ctx variants of the calls. If I read "man unix"
right, SCM_RIGHTS only gives us the PID. But as you said,
we're not safe against malicious clients right now, so I
would postpone that a bit.


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