musl - Re: pthread_mutex_t shared between processes with different pid namespaces

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Message-ID: <87cyg6iyo8.fsf@oldenburg.str.redhat.com>
Date: Tue, 28 Jan 2025 19:24:55 +0100
From: Florian Weimer <fweimer@...hat.com>
To: Rich Felker <dalias@...c.org>
Cc: Daniele Personal <d.dario76@...il.com>,  musl@...ts.openwall.com
Subject: Re: pthread_mutex_t shared between processes with different
 pid namespaces

* Rich Felker:

> Yes, the pid namespace boundary is your problem. Process-shared
> mutexes only work on the same logical system with a unique set of
> thread identifiers. If you're trying to share them across different
> pid namespaces, the same pid/tid may refer to different
> processes/threads in different ones, and it's not usable as a mutex
> ownership identity.

Is this required for implementing the unlock-if-not-owner error code on
mutex unlock?

By the way, there is a proposal to teach the kernel to rewrite the
ownership list of task exit:

  [PATCH v2 0/4] futex: Drop ROBUST_LIST_LIMIT
  <https://lore.kernel.org/linux-kernel/20250127202608.223864-1-andrealmeid@igalia.com/>

I'm worried about the compatibility impact.

Thanks,
Florian

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