linux-kernel - Re: [crash] PANIC: double fault, error

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Date:   Sat, 25 Nov 2017 10:39:49 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...capital.net>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [crash] PANIC: double fault, error_code: 0x0


* Ingo Molnar <mingo@...nel.org> wrote:

> > diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> > index 4ac952080869..ea992ca4e74f 100644
> > --- a/arch/x86/entry/entry_64.S
> > +++ b/arch/x86/entry/entry_64.S
> > @@ -547,6 +547,8 @@ END(irq_entries_start)
> >  	ud2
> >  .Lokay_\@:
> >  	addq $8, %rsp
> > +#else
> > +	.rep 64; nop; .endr
> 
> Also note that turning off CONFIG_UNWINDER_ORC also solves the crash. I did that 
> in an attempt to get a different backtrace.
> 
> So it's either unwinder related, or seemingly minor changes to code 
> alignment/placement will make the bug go away.

Ok, I think the Orc unwinder is innocent: I just forced a build with frame 
pointers but with ORC debuginfo and unwinder, and that is booting fine too.

So it's the specific code size and alignment present in the config I sent that is 
triggering the bug. Fudging that alignment/sizing with the workaround patch above 
makes the crash go away.

Thanks,

	Ingo