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

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Message-ID: <ZXCS1SMQLhSPtjdp@tycho.pizza>
Date:   Wed, 6 Dec 2023 08:27:17 -0700
From:   Tycho Andersen <tycho@...ho.pizza>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Christian Brauner <brauner@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Tycho Andersen <tandersen@...flix.com>
Subject: Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

On Thu, Nov 30, 2023 at 08:43:18PM +0100, Florian Weimer wrote:
> * Mathieu Desnoyers:
> 
> >>> I'd like to offer a userspace API which allows safe stashing of
> >>> unreachable file descriptors on a service thread.
> 
> >> By "safe" here do you mean not accessible via pidfd_getfd()?
> 
> No, unreachable by close/close_range/dup2/dup3.  I expect we can do an
> intra-process transfer using /proc, but I'm hoping for something nicer.

It occurred to me that we could get the seccomp() protected-memory
functionality almost all the way via some combination of
memfd_create(MFD_ALLOW_SEALING), fcntl(F_SEAL_WRITE|F_SEAL_SEAL), and
mmap(PROT_NONE). Some other thread could come along and unmap/remap,
but perhaps with some kind of F_SEAL_NOUNMAP married to one of these
special files we could both get what we want?

I submitted a talk to FOSDEM just for grins, if anyone is planning to
attend that.

Tycho