linux-kernel - Re: [PATCH] x86: Fix early boot crash on gcc-10, next try

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Date:   Sat, 25 Apr 2020 19:31:40 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Michael Matz <matz@...e.de>, Jakub Jelinek <jakub@...hat.com>,
        Sergei Trofimovich <slyfox@...too.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...omium.org>,
        Martin Liška <mliska@...e.cz>,
        Frédéric Pierret (fepitre) 
        <frederic.pierret@...es-os.org>, boris.ostrovsky@...cle.com,
        jgross@...e.com, linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH] x86: Fix early boot crash on gcc-10, next try

On Sat, Apr 25, 2020 at 11:04:40AM -0400, Arvind Sankar wrote:
> I'd put the clause about stack protector being disabled and the
> tail-call one together, to make clear that you still need the never
> return and always inline bits.

Done.

> Also, this function is implemented by multiple arch's and they all
> have similar comments -- might be better to consolidate the comment in
> the generic (dummy) one in include/linux/stackprotector.h laying out
> the restrictions that arch implementations should follow?

I'm not sure gcc-10 does the same thing on other arches - I'd let gcc
guys chime in here and other arch maintainers to decide what to do.

> There's also the one in init/main.c which is used by multiple
> architectures. On x86 at least, the call to arch_call_rest_init at the
> end of start_kernel does not get tail-call optimized by gcc-10, but I
> don't see anything that actually prevents that from happening. We should
> add the asm("") there as well I think, unless the compiler guys see
> something about this function that will always prevent the optimization?

Hmm, that's what I was afraid of - having to sprinkle this around. Yah, let's
wait for compiler guys to have a look here and then maybe I'll convert that
thing to a macro called

	compiler_prevent_tail_call_opt()

or so, so that it can be sprinkled around. ;-\

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette