Re: [patch] futex API cleanups, futex-api-cleanup-2.5.69-A2

[Rusty Russell <rusty@rustcorp.com.au>]

In message <20030519233353.GD13706@mail.jlokier.co.uk> you write:
> Ingo Molnar wrote:
> > FUTEX_FD is an instant DoS, it allows the pinning of one page per file
> > descriptor, per thread. With a default limit of 1024 open files per
> > thread, and 256 threads (on a sane/conservative setup), this means 1 GB of
> > RAM can be pinned down by a normal unprivileged user.
> 
> The correct solution [;)] is EP_FUTEX - allow a futex to be specified
> as the source of an epoll event.

Hi Jamie!

No, you don't understand (or maybe you were being facetious 8).

A normal futex blocks: ie. you can only have one futex being slept on
per process, so pinning a page is probably fine.

The NGPT guys wanted to allow waiting on more than one futex per
process.  If you want to allow this through *any* mechanism, you have to:

1) Allocate something for each futex.  The normal case uses the stack,
   so never fails.

2) Control the number which can be used at once.

The current implementation needs a tighter #2.

Cheers,
Rusty.
--
  Anyone who quotes me in their sig is an idiot. -- Rusty Russell.