linux-kernel - Re: [PATCH v4 06/15] fs,fork,exit: export symbols necessary for KUnit UAPI support

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Message-ID: <20250716132337-ee01c8f1-0942-4d45-a906-67d4884a765e@linutronix.de>
Date: Wed, 16 Jul 2025 13:33:05 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Christoph Hellwig <hch@...radead.org>
Cc: Al Viro <viro@...iv.linux.org.uk>, 
	Luis Chamberlain <mcgrof@...nel.org>, Kees Cook <kees@...nel.org>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, linux-mm@...ck.org, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 06/15] fs,fork,exit: export symbols necessary for
 KUnit UAPI support

On Wed, Jul 16, 2025 at 04:11:04AM -0700, Christoph Hellwig wrote:
> On Wed, Jul 16, 2025 at 10:39:57AM +0200, Thomas Weißschuh wrote:
> > Let's take kernel_execve() as example, there is no way around using this
> > function in one way or another. It only has two existing callers.
> > init/main.c: It is completely unsuitable for this usecase.
> > kernel/umh.c: It is also what Al suggested and I am all for it.
> > Unfortunately it is missing features. Citation from my response to Al:
> 
> But why does the code that calls it need to be modular?  I get why
> the actual test cases should be modular, but the core test runner is
> small and needs a lot of kernel internals.  Just require it to be
> built-in and all this mess goes away.

KUnit UAPI calls into KUnit proper which itself is modular.
As such it needs to be modular, too.
This also makes a small always-builtin component annoying as it will need
to call into KUnit through indirect calls.

But again:
I see that it makes sense to restrict random out-of-tree modules
from accessing kernel internals. But here the symbols are only exported for
one single, in-tree user. What is the advantage of forcing this user to be
non-modular to get access?

> That being said some of this stuff, like get_fs_type / put_filesystem
> or replace_fd seem like the wrong level of abstractions for something
> running tests anyway.

This was modelled after usermode helper and usermode driver.
To me it makes sense, and I don't see an obvious way to get rid of these.

Or do you mean to introduce a new in-core helper to abstract this away?
Then everybody would need to pay the cost for this helper even if it is only
used from some modular code.

Thomas