linux-kernel - Re: [PATCH v4 2/4] procfs: add "pidns" mount option

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Message-ID: <2025-08-06.1754489257-elated-baubles-defiant-growls-beloved-jewelry-9Ofm2b@cyphar.com>
Date: Thu, 7 Aug 2025 00:12:57 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Askar Safin <safinaskar@...omail.com>
Cc: amir73il@...il.com, brauner@...nel.org, corbet@....net, jack@...e.cz, 
	linux-api@...r.kernel.org, linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, luto@...capital.net, 
	shuah@...nel.org, viro@...iv.linux.org.uk
Subject: Re: [PATCH v4 2/4] procfs: add "pidns" mount option

On 2025-08-06, Askar Safin <safinaskar@...omail.com> wrote:
> > I just realised that we probably also want to support FSCONFIG_SET_PATH
> 
> I just checked kernel code. Indeed nobody uses FSCONFIG_SET_PATH.
> Moreover, fsparam_path macro is present since 5.1. And for all this
> time nobody used it. So, let's just remove FSCONFIG_SET_PATH. Nobody
> used it, so this will not break anything.
> 
> If you okay with that, I can submit patch, removing it.

I would prefer you didn't -- "*at()" semantics are very useful to a lot
of programs (*especially* AT_EMPTY_PATH). I would like the pidns= stuff
to support it, and probably also overlayfs...

I suspect the primary issue is that when migrating to the new mount API,
filesystem devs just went with the easiest thing to use
(FSCONFIG_SET_STRING) even though FSCONFIG_SET_PATH would be better. I
suspect the lack of documentation around fsconfig(2) played a part too.

My impression is that interest in the minutia about fsconfig(2) is quite
low on the list of priorities for most filesystem devs, and so the neat
aspects of fsconfig(2) haven't been fully utilised. (In LPC last year,
we struggled to come to an agreement on how filesystems should use the
read(2)-based error interface.)

We can very easily move fsparam_string() or fsparam_file_or_string()
parameters to fsparam_path() and a future fsparam_file_or_path(). I
would much prefer that as a user.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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