Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT modules

From: Masahiro Yamada <masahiroy@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Michal Marek <michal.lkml@markovi.net>,
	linux-hardening@vger.kernel.org,
	Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Justin Forbes <jforbes@redhat.com>,
	Ondrej Mosnacek <omosnace@redhat.com>,
	Frank Eigler <fche@redhat.com>, Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT modules
Date: Sat, 6 Mar 2021 00:31:36 +0900	[thread overview]
Message-ID: <CAK7LNARDEwyjDj3u=8Y176TehU9FD548LFTrqi3AmBR6jrnA_Q@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wiT3FGuKuqLniBN2T_PZwD0GH4kf3XNCzq2tfChqn0+SQ@mail.gmail.com>

On Fri, Mar 5, 2021 at 4:13 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Thu, Mar 4, 2021 at 7:36 AM Masahiro Yamada <masahiroy@kernel.org> wrote:
> >
> > All the kernel-space objects are rebuilt
> > when the compiler is upgraded.
>
> I very much NAK'ed that one. Why did that go in?

When the compiler is upgraded, all objects
should be rebuilt by the new compiler,
- this keeps Kbuild deterministic,
irrespective of whether it is a fresh build,
or incremental build.

If we do not force the full rebuild,
the banner at boot time is no point.

[    0.000000] Linux version 5.8.0-44-generic (buildd@lgw01-amd64-039)
(gcc (Ubuntu 10.2.0-13ubuntu1) 10.2.0, GNU ld (GNU Binutils for
Ubuntu) 2.35.1) #50-Ubuntu SMP Tue Feb 9 06:29:41 UTC 2021 (Ubuntu
5.8.0-44.50-generic 5.8.18)

It claims it was built by
gcc (Ubuntu 10.2.0-13ubuntu1) 10.2.0
but we would never know if it is true
for whole objects.
Some of them might have been compiled by
an older compiler.

> Or maybe I NAK'ed another version of it (I think the one I NAK'ed was
> from Josh), and didn't realize that there were multiple ones.
>
> > Linus complaint about GCC plugins not being rebuilt.
>
> Yes, and that was a separate complaint and not at all tied to the other objects.
>
> Gcc plugins aren't kernel object files at all. They are very much like
> dynamically loadable libraries to gcc itself.
>
>                Linus

-- 
Best Regards
Masahiro Yamada