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Date:   Mon, 16 Mar 2020 14:42:34 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Jakub Jelinek <jakub@redhat.com>
Cc:     Sergei Trofimovich <slyfox@gentoo.org>,
        linux-kernel@vger.kernel.org, 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: <20200316134234.GE12561@hirez.programming.kicks-ass.net>
References: <20200314164451.346497-1-slyfox@gentoo.org>
 <20200316130414.GC12561@hirez.programming.kicks-ass.net>
 <20200316132648.GM2156@tucnak>
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On Mon, Mar 16, 2020 at 02:26:48PM +0100, Jakub Jelinek wrote:
> On Mon, Mar 16, 2020 at 02:04:14PM +0100, Peter Zijlstra wrote:
> > > 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
> 
> My bet is different inlining decisions.
> If somebody hands me over the preprocessed source + gcc command line, I can
> have a look in detail (which exact change and why).
> 
> > rather GCC10 add a function attrbute to kill this:
> > 
> >   __attribute__((no_stack_protect))
> 
> There is no such attribute,

Right I know, I looked for it recently :/ But since this is new in 10
and 10 isn't released yet, I figured someone can add the attribute
before it does get released.

> only __attribute__((stack_protect)) which is
> meant mainly for -fstack-protector-explicit and does the opposite, or
> __attribute__((optimize ("no-stack-protector"))) (which will work only
> in GCC7+, since https://gcc.gnu.org/PR71585 changes).

Cute..

> Or of course you could add noinline attribute to whatever got inlined
> and contains some array or addressable variable that whatever
> -fstack-protector* mode kernel uses triggers it.  With -fstack-protector-all
> it would never work even in the past I believe.

I don't think the kernel supports -fstack-protector-all, but I could be
mistaken.