Re: [musl] pthread_mutex_t shared between processes with different pid namespaces

[Rich Felker <dalias@libc.org>]

On Mon, Feb 03, 2025 at 06:25:41PM +0100, Florian Weimer wrote:
> * Daniele Personal:
> 
> > On Sat, 2025-02-01 at 17:03 +0100, Florian Weimer wrote:
> >> * Daniele Personal:
> >> 
> >> > > Is this required for implementing the unlock-if-not-owner error
> >> > > code
> >> > > on mutex unlock?
> >> > 
> >> > No, I don't see problems related to EOWNERDEAD.
> >> 
> >> Sorry, what I meant is that the TID is needed for efficient reporting
> >> of
> >> usage errors.  It's not imposed by the robust list protocol as such..
> >> There could be a PID-namespace-compatible robust mutex type that does
> >> not have this problem (but with less error checking).
> >> 
> >> Thanks,
> >> Florian
> >> 
> >
> > Are you saying that there are pthread_mutexes which can be shared
> > across processes run on different pid namespaces? If yes I'm definitely
> > interested on this. Can you tell me something more?
> 
> You would have to add a new mutex type that is a mix of
> PTHREAD_MUTEX_NORMAL amd PTHREAD_MUTEX_ROBUST.  Closer to the latter,
> but without the ownership checks.

This is inaccurate. Robust mutexes fundamentally depend on having the
owner's tid in the owner field, and on this value not matching the tid
of any other task that might hold the mutex. If these properties don't
hold, the mutex may fail to unlock when the owner dies, or incorrectly
unlock when another task mimicking the owner dies.

The Linux robust mutex protocol fundamentally does not work across pid
namespaces.

Rich