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Date:   Mon, 16 Mar 2020 14:04:14 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sergei Trofimovich <slyfox@gentoo.org>
Cc:     linux-kernel@vger.kernel.org, Jakub Jelinek <jakub@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>, x86@kernel.org
Subject: Re: [PATCH] x86: fix early boot crash on gcc-10
Message-ID: <20200316130414.GC12561@hirez.programming.kicks-ass.net>
References: <20200314164451.346497-1-slyfox@gentoo.org>
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On Sat, Mar 14, 2020 at 04:44:51PM +0000, Sergei Trofimovich wrote:
> The change fixes boot failure on physical machine where kernel
> is built with gcc-10 with stack-protector enabled by default:

> This happens because `start_secondary()` is responsible for setting
> up initial stack canary value in `smpboot.c`, but nothing prevents
> gcc from inserting stack canary into `start_secondary()` itself
> before `boot_init_stack_canary()` call.

> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index 9b294c13809a..da9f4ea9bf4c 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -11,6 +11,12 @@ extra-y	+= vmlinux.lds
>  
>  CPPFLAGS_vmlinux.lds += -U$(UTS_MACHINE)
>  
> +# smpboot's init_secondary initializes stack canary.
> +# Make sure we don't emit stack checks before it's
> +# initialized.
> +nostackp := $(call cc-option, -fno-stack-protector)
> +CFLAGS_smpboot.o := $(nostackp)

What makes GCC10 insert this while GCC9 does not. Also, I would much
rather GCC10 add a function attrbute to kill this:

  __attribute__((no_stack_protect))

Then we can explicitly clear this one function and keep it on for the
rest of the file.