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Date:   Tue, 24 Nov 2020 19:12:40 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     William Mcvicker <willmcvicker@google.com>
Cc:     Jessica Yu <jeyu@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org,
        kernel-team@android.com, Saravana Kannan <saravanak@google.com>
Subject: Re: [PATCH v1 0/2] Add support to capture external module's SCM
 version
Message-ID: <X71NGClOP5Lqg6M5@kroah.com>
References: <20201121011652.2006613-1-willmcvicker@google.com>
 <20201123090257.GB6334@infradead.org>
 <20201123221338.GA2726675@google.com>
 <20201124093117.GA21089@linux-8ccs>
 <20201124180516.GA737971@google.com>
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On Tue, Nov 24, 2020 at 10:05:16AM -0800, William Mcvicker wrote:
> On Tue, Nov 24, 2020 at 10:31:18AM +0100, Jessica Yu wrote:
> > +++ William Mcvicker [23/11/20 14:13 -0800]:
> > > On Mon, Nov 23, 2020 at 09:02:57AM +0000, Christoph Hellwig wrote:
> > > > On Sat, Nov 21, 2020 at 01:16:49AM +0000, Will McVicker wrote:
> > > > > These two patches add module support to capture an external module's SCM
> > > > > version as a MODULE_INFO() attribute. This allows users to identity the SCM
> > > > > version of a given kernel module by using the modinfo tool or on the device
> > > > > via sysfs:
> > > > 
> > > > As this obviously is of no use for in-tree modules it falls under the we
> > > > don't add code to support things that are not in tree rule and has no
> > > > business in the kernel.
> > > 
> > > Hi Christoph,
> > > 
> > > Ah sorry, I didn't intend this to come across as only for external modules.
> > > That just seemed like the easiest way to explain how the scmversion attribute
> > > can be different from the vermagic. We mainly need this for in-tree kernel
> > > modules since that's where most our drivers are. Let me re-phrase this with
> > > that in mind. Basically, I like to look at this as an improved version of the
> > > existing srcversion module attribute since it allows you to easily identify the
> > > module version with a quick SCM version string check instead of doing a full
> > > checksum on the module source.
> > > 
> > > For example, we have a setup to test kernel changes on the hikey and db845c
> > > devices without updating the kernel modules. Without this scmversion module
> > > attribute, you can't identify the original module version using `uname
> > > -r`. And for kernel modules in the initramfs, you can't even use modinfo to get
> > > the module vermagic.  With this patch, you are able to get the SCM version for
> > > *all* kernel modules (on disk and in the initramfs) via the sysfs node:
> > > /sys/module/<mod>/scmversion. This also works the other way around when
> > > developers update their kernel modules to fix some bug (like a security
> > > vulnerability) but don't need to update the full kernel.
> > 
> > Hi Will,
> > 
> > If this were also intended for in-tree kernel modules, then why do
> > intree modules only get the UTS_RELEASE string in their scmversion
> > field, which basically already exists in the vermagic? Or do you plan
> > to change that?
> > 
> > Jessica
> 
> Hi Jessica,
> 
> Thanks for asking! The reason in-tree kernel modules get the UTS_RELEASE string
> is for a few reasons:
> 
> (1) It contains the SCM version (since UTS_RELEASE has that).
> (2) It allows you to get the SCM version via the sysfs node (useful for modules
> in the initramfs).
> (3) It helps identify that that particular kernel module was in-tree when
> originally compiled.
> (4) Using UTS_RELEASE also allows us to respect the privacy of kernels with
> "# CONFIG_LOCALVERSION_AUTO is not set" by not including the SCM version in the
> module scmversion attribute.
> 
> Now, if we don't care about knowing if a module was in-tree or not (since
> we only care about in-tree modules here anyway), I can update the patch to have
> a consistent format regardless of in-tree or external. Personally, I like the
> UTS_RELEASE version better because it gives me more information from the sysfs
> node which is useful when debugging issues related to modules loaded in
> initramfs.

We already know if a module was built in-or-out of tree, the "O" taint
flag is set, so that information is already in the module today, right?
Can't that be used somehow here?

thanks,

greg k-h