linux-kernel - Re: [PATCH v2] x86: fix early boot crash on gcc-10

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Date:   Fri, 17 Apr 2020 10:42:24 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Jakub Jelinek <jakub@...hat.com>
Cc:     Sergei Trofimovich <slyfox@...too.org>,
        Michael Matz <matz@...e.de>, 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
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10

On Fri, Apr 17, 2020 at 10:07:26AM +0200, Jakub Jelinek wrote:
> If you want minimal changes, you can as I said earlier either
> mark cpu_startup_entry noreturn (in the declaration in some header so that
> smpboot.c sees it), or you could add something after the cpu_startup_entry
> call to ensure it is not tail call optimized (e.g. just
> 	/* Prevent tail call to cpu_startup_entry because the stack
> 	   protector guard has been changed in the middle of this function
> 	   and must not be checked before tail calling another function.  */
> 	asm ("");

That sounds ok-ish to me too.

I know you probably can't tell the future :) but what stops gcc from
doing the tail-call optimization in the future?

Or are optimization decisions behind an inline asm a no-no and will
pretty much always stay that way?

And I hope the clang folks don't come around and say, err, nope, we're
much more aggressive here.

However, if we do it with the explicit disabling with
-fno-stack-protector for only this compilation unit, then it is

1. clear why we're doing this
2. no compiler would break it

So I'm still gravitating a bit towards the explicit thing...

Thx.

-- 
Regards/Gruss,
    Boris.

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