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

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Date:   Mon, 16 Mar 2020 14:26:48 +0100
From:   Jakub Jelinek <jakub@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergei Trofimovich <slyfox@...too.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org
Subject: Re: [PATCH] x86: fix early boot crash on gcc-10

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, 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).
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.

	Jakub