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

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Date:   Thu, 28 Jan 2021 15:34:09 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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 01:23:11PM -0800, Linus Torvalds wrote:
> 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.

Right, this is what I'm trying to say.  We have features based on
compiler version checks.  Peterz pointed out asm goto as a previous
example.

> 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).

Then maybe the check should be same major.minor?

And convert it to a strongly worded warning/disclaimer?

-- 
Josh