linux-kernel - Re: [PATCH 3/4] kbuild: slim down package for building external modules

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Message-ID: <CAK7LNARZon-tqr+oKfr0r+nSfK9R6iz1AZrY9pLoaAhOqWfL6w@mail.gmail.com>
Date: Fri, 21 Feb 2025 03:12:25 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Jeffrey Hugo <quic_jhugo@...cinc.com>, Nicolas Schier <n.schier@....de>, 
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Kees Cook <kees@...nel.org>, Nathan Chancellor <nathan@...nel.org>, Ben Hutchings <ben@...adent.org.uk>, 
	regressions@...ts.linux.dev
Subject: Re: [PATCH 3/4] kbuild: slim down package for building external modules

On Fri, Feb 21, 2025 at 2:43 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Thu, Feb 20, 2025 at 10:24:32AM -0700, Jeffrey Hugo wrote:
> > On 2/20/2025 9:49 AM, Greg KH wrote:
> > > What do you need/want to parse the .config file in the first place?  Why
> > > not rely on the generated .h files instead as those can be parsed the
> > > same way as before, right?
> >
> > Two usecases -
> >
> > First when secure boot is enabled, DKMS looks for CONFIG_MODULE_SIG_HASH to
> > figure out signing the modules so that they can be loaded.  Removing .config
> > causes an error in DKMS and the module signing process will fail.  The
> > resulting modules (already compiled successfully) will fail to load.
> > Whatever the user needed those modules for will no longer work.
>
> Shouldn't the "normal" kbuild process properly sign the module if it
> needs to be signed?  Or are you trying to do this "by hand"?

You can ignore this.
This is not related to the upstream module signing.

He is just explaining how DKMS greps CONFIG_MODULE_SIG_HASH.

See this line:
https://github.com/dell/dkms/blob/v3.1.5/dkms.in#L1113

The upstream kernel documentation said "do not grep .config" many years ago.
The latest DKMS has been fixed.



>
> > If the user is on Ubuntu, and has built a kernel 6.12 or later, they need to
> > build upstream DKMS and use it as none of the Canonical provided DKMS builds
> > have the fix.  This feels like a situation that would make the user afraid
> > to upgrade their kernel (to your point above).
> >
> > This feels very much like an "at runtime" issue, assuming external modules
> > are supported.  I may be wrong here.
>
> external modules can be broken at any moment in time, you know that.
> There's never a guarantee for that at all.
>
> > Second, our usecase is that we look at the .config to tell if a particular
> > driver is included in the kernel build (config =y or =m). This driver
> > provides diagnostic information which is useful to our product, but not
> > required for operation.  It does not have headers that are exposed to the
> > rest of the kernel, so checking the generated .h files does not work.  If
> > the driver is not built, we provide a backported version that is then built
> > out of tree.
>
> You can check the same .h files for those config options, no need to
> manually parse a .config file.  What's wrong with including
> include/generated/autoconf.h properly?  That's what the build system
> uses, right?

Upstream uses include/config/auto.conf
External modules can do the same.




-- 
Best Regards
Masahiro Yamada