Re: [PATCH 4.4 00/37] 4.4.110-stable review

[Hugh Dickins]

On Thu, Jan 4, 2018 at 12:43 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>> On Jan 4, 2018, at 12:29 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>>> On Thu, Jan 4, 2018 at 12:16 PM, Thomas Voegtle <tv@xxxxxxxx> wrote:
>>>
>>> Attached a screenshot.
>>> Is that useful? Are there some debug options I can add?
>>
>> Not much of an oops, because the SIGSEGV happens in user space. The
>> only reason you get any kernel stack printout at all is because 'init'
>> dying will make the kernel print that out.
>>
>> The segfault address for init looks like the fixmap area to me (first
>> byte in the last page of the fixmap?). "Error 5" means that it's a
>> user-space read that got a protection fault. So it's not a LDT of GDT
>> update or anything like that, it's a normal access from user space (or
>> a qemu emulation bug, but that sounds unlikely).
>>
>> Is that the vsyscall page?
>>
>> Adding Luto to the participants. I think he noticed one of the
>> vsyscall patches missing earlier in the 4.9 series. Maybe the 4.4
>> series had something similar..
>>
>
> That's almost certainly it.

I'm hopeless on the FIXMAP arithmetic, but I'm pretty sure that
ffffffffff5ff000 is either VSYSCALL page or PVCLOCK page (I think it
was VVAR page when init segfaulted on it in my 3.2).

I'll forward Borislav's suggested 4.4 VSYSCALL patch from the kaiser
backports ml to Thomas, to see if that sorts his crash (forwarding in
the hope that gmail doesn't mess up the patch).

Seems odd that 4.4 should be broken but 4.9 not broken here, I'd
expect them to be equally known broken with respect to VSYSCALL; but
perhaps it's a matter of userspace trying different fallbacks
according to what kernel supports, and only hitting this on 4.4.

Hugh

>
> I'll try to find some time today  Thomnor tomorrow to add a proper selftest.
>
>>              Linus