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Date:   Wed, 22 Apr 2020 21:21:13 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Michael Matz <matz@suse.de>, Jakub Jelinek <jakub@redhat.com>,
        Sergei Trofimovich <slyfox@gentoo.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v2] x86: fix early boot crash on gcc-10
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On Wed, Apr 22, 2020 at 11:55:50AM -0700, Nick Desaulniers wrote:
> Can you add by whom?  It's not clear to me which function call in
> start_secondary modifies the stack protector guard.

How's that

        /*
         * Prevent tail call to cpu_startup_entry() because the stack protector
         * guard has been changed a couple of functions up, in
         * boot_init_stack_canary() and must not be checked before tail calling
         * another function.
         */
        asm ("");

?

> Another question.  Do we not want a stack protector at all in this
> function?  I'm not super familiar with how they work; do we not want
> them at all, or simply not to check the guard?

Not to check the guard. See the beginning of
arch/x86/include/asm/stackprotector.h about how they work.

> But if we're not going to check it, I think
> __attribute__((no_stack_protector)) applied to start_secondary might
> be a more precise fix.

No such attribute in gcc yet. But yes, this came up a bit upthread, you
can go back in time for details. :)

Thx.

-- 
Regards/Gruss,
    Boris.

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