linux-kernel - Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

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Date:   Fri, 08 Dec 2023 14:58:03 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tycho Andersen <tycho@...ho.pizza>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

* Christian Brauner:

> On Fri, Dec 08, 2023 at 02:15:58PM +0100, Florian Weimer wrote:
>> * Christian Brauner:
>> 
>> > File descriptors are reachable for all processes/threads that share a
>> > file descriptor table. Changing that means breaking core userspace
>> > assumptions about how file descriptors work. That's not going to happen
>> > as far as I'm concerned.
>> 
>> It already has happened, though?  Threads are free to call
>> unshare(CLONE_FILES).  I'm sure that we have applications out there that
>
> If you unshare a file descriptor table it will affect all file
> descriptors of a given task. We don't allow hiding individual or ranges
> of file descriptors from close/dup. That's akin to a partially shared
> file descriptor table which is conceptually probably doable but just
> plain weird and nasty to get right imho.
>
> This really is either LSM territory to block such operations or use
> stuff like io_uring gives you.

Sorry, I misunderstood.  I'm imagining for something that doesn't share
partial tables and relies on explicit action to make available a
descriptor from a separate different table in another table, based on
some unique identifier (that is a bit more random than a file
descriptor).  So a bit similar to the the existing systemd service, but
not targeted at service restarts.

Thanks,
Florian