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Message-ID: <20250208145215.GW10433@brightrain.aerifal.cx>
Date: Sat, 8 Feb 2025 09:52:15 -0500
From: Rich Felker <dalias@...c.org>
To: Daniele Dario <d.dario76@...il.com>
Cc: Florian Weimer <fweimer@...hat.com>, musl@...ts.openwall.com
Subject: Re: pthread_mutex_t shared between processes with different
pid namespaces
On Sat, Feb 08, 2025 at 03:40:18PM +0100, Daniele Dario wrote:
> Il sab 8 feb 2025, 13:39 Rich Felker <dalias@...c.org> ha scritto:
>
> > On Sat, Feb 08, 2025 at 10:20:45AM +0100, Daniele Dario wrote:
> > > But wouldn't this mean that robust mutexes functionality is totally
> > > incompatible with pid namespaces?
> >
> > No, only with trying to synchronize *across* different pid namespaces.
> >
> > > If the kernel relies on tid stored in memory by the process this always
> > > lacks the information about the pid namespace the tid belongs to.
> >
> > It's necessarily within the same pid namespace as the process itself.
> >
> > Functionally, you should consider different pid namespaces as
> > different systems that happen to be capable of sharing some resources.
> >
> > Rich
> >
>
> Yes, I'm just saying that sharing pthread_mutex_t instances across
> processes within the same pid namespace but on a system with more than a
> pid namespace could lead to issues anyway if the stored tid value is used
> by the kernel as who to contact without the knowledge of on which pid
> namespace.
>
> I not saying this is true, I'm trying to understand and if possible,
> improve things.
That's not a problem. The stored tid is used only in the context of a
process exiting, where the kernel code knows the relevant pid
namespace (the one the exiting process is in) and uses the tid
relative to that. If it didn't work this way, it would be a fatal bug
in the pid namespace implementation, which is supposed to allow
essentially transparent containerization (which includes processes in
the ns being able to use their tids as they could if they were outside
of any container/in global ns).
Rich
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