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

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Date:   Wed, 15 Apr 2020 23:19:30 +0100
From:   Sergei Trofimovich <slyfox@...too.org>
To:     Michael Matz <matz@...e.de>
Cc:     Borislav Petkov <bp@...en8.de>, Jakub Jelinek <jakub@...hat.com>,
        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 Wed, 15 Apr 2020 14:53:45 +0000 (UTC)
Michael Matz <matz@...e.de> wrote:

> Hello,
> 
> On Wed, 15 Apr 2020, Borislav Petkov wrote:
> 
> > On Tue, Apr 14, 2020 at 01:50:29PM +0000, Michael Matz wrote:  
> > > So this part expects that the caller (!) of trace_hardirqs_on was compiled
> > > with a frame pointer (in %ebp).  
> > 
> > /me looks at the .s file...
> > 
> > options passed comment at the top has -fno-omit-frame-pointer
> >   
> > > Obviously that's not the case as you traced above. Is start_secondary
> > > the immediate caller in the above case?  
> > 
> > Yes, start_secondary() is the function which is marked as
> > __attribute__((optimize("-fno-stack-protector"))) and it does:
> > 
> > # arch/x86/kernel/smpboot.c:264:        local_irq_enable();
> >         call    trace_hardirqs_on       #
> > 
> > (the local_irq_enable() is a macro which has the call to
> > trace_hardirqs_on().
> >   
> > > Look at it's disassembly.  If it doesn't have the usual push
> > > %ebp/mov%esp,%ebp prologue it probably doesn't use a frame pointer.  
> > 
> > Here's the preamble:
> > 
> >         .text
> >         .p2align 4
> >         .type   start_secondary, @function
> > start_secondary:
> >         pushl   %esi    #
> >         pushl   %ebx    #  
> 
> Right.  So meanwhile it became clear: the optimize function attribute 
> doesn't work cumulative but rather replaces all cmdline args (the 
> optimization ones, but that roughly translates to -fxxx options).  In this 
> case an 'optimize("-fno-stack-protector")' also disables the crucial 
> -fno-omit-frame-pointer, reverting to the compilers default, which, 
> depending on version, is also to omit the frame pointer on 32bit.  You 
> could fix that by adding ',-fno-omit-frame-pointer' to the attribute 
> string.  But that quickly gets out of hand, considering all the options 
> you carefully need to set in Makefiles to get the right behaviour.  (Note 
> that e.g. the optimization level is reset to -O0 as well!).
> 
> (I'll admit that I was somewhat surprised by this behaviour, even though 
> it makes sense in the abstract; resetting to a clean slate and 
> everything).
> 
> I think in its current form the optimize attribute is not useful for the 
> purposes you need, and you're better off to disable the stack protector 
> for the whole compilation unit from the Makefile.
> 
> (That attribute is also documented as "not suitable in production code", 
> so go figure ;-) )
> 
> I think it will be possible to make that attribute a bit more useful 
> in the future, but for the time being I think you'll just want to live 
> without it.

Ah, that makes sense. Borislav, should I send a fix forward against
x86 tree to move -fno-stack-protector as it was in v1 patch?
Or you'll revert v2 and apply v1 ~as is? Or should I send those myself?

-- 

  Sergei