linux-kernel - Re: 4.14.9 doesn't boot (regression)

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Date:   Fri, 29 Dec 2017 17:10:35 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Toralf Förster <toralf.foerster@....de>,
        Alexander Tsoy <alexander@...y.me>,
        stable <stable@...r.kernel.org>,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        jpoimboe@...hat.com
Subject: Re: 4.14.9 doesn't boot (regression)

> On Dec 29, 2017, at 3:53 PM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Fri, Dec 29, 2017 at 2:30 PM, Toralf Förster <toralf.foerster@....de> wrote:
>> 
>> The bad news - the issue is not solved with the changed cflags.
>> The good news - I could compile eventually a working config for my desktop  (works fine with 4.14.10 with generic CPU) having a higher screen resolution during boot.
>> 
>> So I made a "make distclean", followed by a "sudo zcat /proc/config.gz > .config", changed the .config to use MCORE2 instead of GENERIC and defined the string "-local" to ensure that the modules directory is really unique.
>> Then I run "time make -j4 && sudo make modules_install && sudo cp arch/x86_64/boot/bzImage /boot/vmlinuz-0 && sudo grub-mkconfig -o /boot/grub/grub.cfg", booted and made 3 fotos which were uploaded to [1], look for IMG_*
> 
> Ok, so what does seem to be consistent for everybody is that
> double-fault in the NMI backtrace.
> 
> So the fact that the NMI always hits on a double-fault does make me
> suspect that it's a infinite stream of double-faults, and that is
> presumably also what causes the RCU timeout.
> 
> And as I pointed out elsewhere (damn two threads), I think that it
> would help to simply catch the *first* double-fault.
> 
> And I *think* that the only thing that can make a double-fault
> silently be re-tried is the CONFIG_X86_ESPFIX64 case, so if you can
> build a failing kernel with the CONFIG_X86_ESPFIX64 case disabled in
> arch/x86/kernel/traps.c do_double_fault(), that would be interesting.

Double faults use IST, so a double fault that double faults will effectively just start over rather than eventually running out of stack and triple faulting.

But check out the registers. We have RSP = ...28fd8 and CR2 = ...27f08. IOW the double fault stack is ...28000 - ...28fff and we're somehow getting a failed page fault a couple hundred bytes below the bottom of the IST stack.  IOW, I think we're just stuck in a neverending loop of stack overflows.

(Also, Josh, the oops code should have printed the contents of the struct pt_regs at the top of the DF stack.  Any idea why it didn't?)

Toralf, can you send the complete output of:

objdump -dr arch/x86/kernel/traps.o

From the build tree of a nonworking kernel?

Also, you wouldn't happen to be using Gentoo perchance?  I already have two reports of a Gentoo system miscompiling the vDSO due to Gentoo enabling -fstack-check and GCC generating stack check code that is highly suboptimal, actively incorrect, and doesn't even manage to check the stack in a particularly helpful way.

If this is indeed what's going on, I'm going to try to come up with a patch to outright fail the build on these buggy systems.  We could probably fudge the build options to avoid the problem, but Gentoo really just needs fix its toolchain.