[gnutls-devel] [TLS] multiple clients in one process (was: Re: Deployment ... Re: This working group has failed)

Andy Lutomirski luto at amacapital.net
Sun Dec 1 03:34:37 CET 2013

On Nov 30, 2013 6:03 PM, "Nico Williams" <nico at cryptonector.com> wrote:
> On Sat, Nov 30, 2013 at 2:58 AM, Nikos Mavrogiannopoulos
> <nmav at gnutls.org> wrote:
> > On Fri, 2013-11-29 at 18:03 -0600, Nico Williams wrote:
> >> It is not safe to use PKCS#11 on the child-side of fork() without
> >> [...]
> >
> > I was referring to his issue. As far as I understood he didn't use PKCS
> > #11 at all, so there should be no breakage. When ones uses PKCS #11 in
> > gnutls he is (currently) required to call gnutls_pkcs11_reinit() on a
> > fork.
> Remember, we're talking about layered software.  This might be a
> process using PAM and loading multiple PAMs that use GnuTLS, so: which
> one of them should reinit GnuTLS?  No, the library *must*
> self-[re-]initialize.  Always.  Period.  Full stop.  There must be no
> question of some caller being first to initialize, there must be no
> way for two callers to step on each others' toes by re-initializing
> the library and destroying the other caller's state.
> Yes, nothing should fork() and try to use a non-async-signal-safe
> interface on the child-side of the fork.

Huh?  It should be entirely safe for a single-threaded program to open a
TLS connection, close it, fork, and open another connection.

The async-signal-safe-only thing applies to multithreaded programs only.


 That does give a library an
> excuse to have undefined fork-safety.  But it's much, much better to
> have well-defined fork-safety.  It's OK to lose access to all
> objects/whatever on the child-side of fork(), and it's even OK to leak
> them (since the data structures in question may not be in a consistent
> state, the library may not be able to walk them, and free() is not
> async-signal-safe).  It'd even be better to abort() if called on the
> child-side of fork() than to have undefined behavior -- after all,
> callers of fork() are not supposed to call async-signal-unsafe code.
> The key is to have deterministic and *noticeable* behavior.
> The recipe is simple:
>  - use a InitOnceInitialize/pthread_once interface to initialize once as
>    (don't require that the caller initialize the library)
>  - use a pthread_atfork() child-side handler to reinitialize all
> global state (all locks, ...), leaking all of it OR to set a flag,
> then abort() if called with this flag set
> Nico
> --
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