linux-hardening - Re: [PATCH RFC] kbuild: Prevent compiler mismatch with external modules

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Message-ID: <CAHk-=wih0rLHsPXodpXJw_0F3bJqu=Pb_YNmPCSsYU_huoMwZA@mail.gmail.com>
Date:   Thu, 28 Jan 2021 12:24:45 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-hardening@...r.kernel.org,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Justin Forbes <jforbes@...hat.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Miroslav Benes <mbenes@...e.cz>,
        David Laight <David.Laight@...lab.com>,
        Jessica Yu <jeyu@...nel.org>
Subject: Re: [PATCH RFC] kbuild: Prevent compiler mismatch with external modules

On Thu, Jan 28, 2021 at 12:08 PM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> Add a check for compiler mismatch, but only check the major version.

I think this is wrong for multiple reasons.

The most fundamental reason is that it's pointless and doesn't
actually do what you claim it does.

Just doing a "make oldconfig" will reset the CONFIG_xyz_VERSION to
whatever is installed, and now your check doesn't actually do
anything, since you're not actually checking what the kernel was
compiled with!

So I think that check is pointless and entirely misleading. It doesn't
do what you want it to do, and what you claim it does.

I'm not convinced about the whole magic vs minor argument either. The
whole "new compiler features" thing is a red herring - even if you do
have new compiler features, that in itself has very little to do with
whether the resulting object files are compatible or not.

So I say NAK, on the basis that the patch is nonsensical, tests the
wrong thing, and doesn't really have a technical reason for it.

             Linus