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

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Message-ID: <CAHk-=wh+3PWi2NuoQ0hbSyLpOHjaBWKcgX6N7+PfPkXzNAfMwA@mail.gmail.com>
Date:   Thu, 28 Jan 2021 13:23:11 -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 1:03 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> I really think the whole compiler version check is purely voodoo programming.

.. but there are obviously potentially things we - in the kernel - do
that may make certain compiler versions incompatible. We long long ago
used to have things like "you can't have an empty struct because gcc
version x.y.z doesn't support it", so even a UP spinlock would be

       typedef struct { int gcc_is_buggy; } raw_spinlock_t;

but only if you compiled it with a version of gcc older than 3.0. So
compiling one file with one compiler, and another with a newer one,
would result in the data structures simply not having the same layout.

That's not because of compiler versions per se, it's because of our
version checks.

THAT workaround is long gone, but I didn't check what other ones we
might have now. But the gcc version checks we _do_ have are not
necessarily about major versions at all (ie I trivially found checks
for 4.9, 4.9.2, 5.1, 7.2 and 9.1).

               Linus