Re: [GIT PULL] kdbus for 4.1-rc1

Eric Dumazet
2015-04-15T22:56:10Z

On Wed, 2015-04-15 at 23:48 +0100, Al Viro wrote:
> On Wed, Apr 15, 2015 at 03:28:58PM -0700, Andy Lutomirski wrote:
> > On Wed, Apr 15, 2015 at 3:18 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > > On Wed, Apr 15, 2015 at 03:11:17PM -0700, Andy Lutomirski wrote:
> > >
> > >> This is functionally identical to passing AF_UNIX socket fds over
> > >> SCM_RIGHTS, but I want something much lighter weight.
> > >
> > > Most of the weight in SCM_RIGHTS comes from the fact that you can
> > > pass AF_UNIX sockets over it, which requires a garbage collector.
> > > Exclude that and suddenly it becomes very cheap...
> >
> > I should have been more specific. I don't mean the performance of
> > SCM_RIGHTS itself; I mean the memory overhead of keeping tons of fds
> > around, each with their socket data structures and buffers.
> >
> > I think that dbus could be quite efficiently implemented with a
> > userspace daemon that just introduces peers to each other, but the fd
> > explosion could be rather bad for some use cases.
> >
> > I'll be the first to admit that I don't have a clean API in mind.
> > There was a lightweight fd proposal way back when, but it never went
> > anywhere, and it might not be suitable anyway.
>
> Wait, are you talking about the overhead of descriptors used for capability
> tokens (essentially zero - one system-wide struct file per capability +
> one pointer in descriptor table of anyone who holds it + two bits in
> bitmaps in the sam descriptor tables) or about the overhead of descriptors
> used to send/receive those over? The latter don't have to be sockets
> at all - they could bloody well be files on some ipcfs, or character device,
> or FIFOs, etc.

This kind of remind me futex : From an apparent simple idea we got to
the point of having more than 3000 lines of code in kernel/futex.c

It is sad that af_unix was chosen to support fd passing in the first
place. This is serious DOS vector.