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

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Date:   Thu, 4 Mar 2021 09:08:12 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        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>,
        Frank Eigler <fche@...hat.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH RFC] gcc-plugins: Handle GCC version mismatch for OOT
 modules

On Thu, Mar 04, 2021 at 09:27:28PM +0900, Masahiro Yamada wrote:
> I agree with rebuilding GCC plugins when the compiler is upgraded
> for *in-tree* building.
> Linus had reported it a couple of months before,
> and I just submitted a very easy fix.

Hm?  So does that mean that a GCC version change won't trigger a
tree-wide rebuild?  So you're asserting that a GCC mismatch is ok for
in-tree code, but not for external modules???  That seems backwards.

For in-tree, why not just rebuild the entire tree?  Some kernel features
are dependent on compiler version or capability, so not rebuilding the
tree could introduce silent breakage.

For external modules, a tree-wide rebuild isn't an option so the risk is
assumed by the user.  I posted a patch earlier [1] which prints a
warning if the compiler major/minor version changes with an external
module build.

[1] https://lkml.kernel.org/r/20210201211322.t2rxmvnrystc2ky7@treble

> Rebuilding plugins for external modules is not easy;
> plugins are placed in the read-only directory,
> /usr/src/linux-headers-$(uname -r)/scripts/gcc-plugins/.
> 
> The external modules must not (cannot) update in-tree
> build artifacts.  "Rebuild" means creating copies in a different
> writable directory.
> Doing that requires a lot of design changes.

Ok.  So it sounds like the best/easiest option is the original patch in
this thread:  when building an external module with a GCC mismatch, just
disable the GCC plugin, with a warning (or an error for randstruct).

-- 
Josh