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

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Message-ID: <874jh3t7e9.fsf@oldenburg.str.redhat.com>
Date:   Thu, 30 Nov 2023 19:37:02 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Tycho Andersen <tycho@...ho.pizza>
Cc:     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>,
        mathieu.desnoyers@...icios.com
Subject: Re: [RFC 1/3] pidfd: allow pidfd_open() on non-thread-group leaders

* Tycho Andersen:

> From: Tycho Andersen <tandersen@...flix.com>
>
> We are using the pidfd family of syscalls with the seccomp userspace
> notifier. When some thread triggers a seccomp notification, we want to do
> some things to its context (munge fd tables via pidfd_getfd(), maybe write
> to its memory, etc.). However, threads created with ~CLONE_FILES or
> ~CLONE_VM mean that we can't use the pidfd family of syscalls for this
> purpose, since their fd table or mm are distinct from the thread group
> leader's. In this patch, we relax this restriction for pidfd_open().

Does this mean that pidfd_getfd cannot currently be used to get
descriptors for a TID if that TID doesn't happen to share its descriptor
set with the thread group leader?

I'd like to offer a userspace API which allows safe stashing of
unreachable file descriptors on a service thread.

Cc:ing Mathieu because of our previous discussions?

Thanks,
Florian