Re: DoS with POSIX file locks?

Miklos Szeredi
2006-03-21T17:58:53Z

> > Apps using LinuxThreads seem to be candidates:
> >
> > According to POSIX 1003.1c, a successful `exec*' in one of the
> > threads should automatically terminate all other threads in the
> > program. This behavior is not yet implemented in LinuxThreads.
> > Calling `pthread_kill_other_threads_np' before `exec*' achieves
> > much of the same behavior, except that if `exec*' ultimately
> > fails, then all other threads are already killed.
> >
> > steal_locks() was probably added as a workaround for this case, no?
>
> Possibly, but LinuxThreads were never really POSIX thread compliant
> anyway. Anyhow, the problem isn't really LinuxThreads, it is rather that
> the existence of the standalone CLONE_FILES flag allows you to do a lot
> of weird inheritance crap with 'posix locks' that the POSIX standards
> committees never even had to consider.

Yes. The execve-with-multiple-threads/posix-locks interaction is not
documented for LinuxThreads but removing steal_locks() makes that
implementation slighly differently incompatible to POSIX. Some
application _might_ be relying on the current behavior.

It's just a question of how much confidence do we have, that no app
will break if steal_locks() is removed. This function was added by
Chris Wright on 2003-12-29 (Cset 1.1371.111.3):

Add steal_locks helper for use in conjunction with unshare_files to
make sure POSIX file lock semantics aren't broken due to
unshare_files.

Chris, do you remember if this was due to some concrete breakage or
just a preemtive measure?

Miklos