linux-kernel - Re: [PATCH] pidfd: prevent creation of pidfds for kthreads

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Message-ID: <20240819-staudamm-rederei-cb7092f54e76@brauner>
Date: Mon, 19 Aug 2024 10:41:15 +0200
From: Christian Brauner <brauner@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Oleg Nesterov <oleg@...hat.com>, Aleksa Sarai <cyphar@...har.com>, 
	Tycho Andersen <tandersen@...flix.com>, Daan De Meyer <daan.j.demeyer@...il.com>, 
	Tejun Heo <tj@...nel.org>, stable@...r.kernel.org
Subject: Re: [PATCH] pidfd: prevent creation of pidfds for kthreads

On Sat, Aug 17, 2024 at 08:58:18PM GMT, Eric Biggers wrote:
> Hi Christian,
> 
> On Wed, Jul 31, 2024 at 12:01:12PM +0200, Christian Brauner wrote:
> > It's currently possible to create pidfds for kthreads but it is unclear
> > what that is supposed to mean. Until we have use-cases for it and we
> > figured out what behavior we want block the creation of pidfds for
> > kthreads.
> > 
> > Fixes: 32fcb426ec00 ("pid: add pidfd_open()")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Christian Brauner <brauner@...nel.org>
> > ---
> >  kernel/fork.c | 25 ++++++++++++++++++++++---
> >  1 file changed, 22 insertions(+), 3 deletions(-)
> 
> Unfortunately this commit broke systemd-shutdown's ability to kill processes,
> which makes some filesystems no longer get unmounted at shutdown.
> 
> It looks like systemd-shutdown relies on being able to create a pidfd for any
> process listed in /proc (even a kthread), and if it gets EINVAL it treats it a
> fatal error and stops looking for more processes...

Thanks for the report!
I talked to Daan De Meyer who made that change and he said that this
must a systemd version that hasn't gotten his fixes yet. In any case, if
this causes regression then I'll revert it right now. See the appended
revert.

View attachment "0001-Revert-pidfd-prevent-creation-of-pidfds-for-kthreads.patch" of type "text/x-diff" (2040 bytes)