Re: [PATCH v2 0/4] x86: sigcontext fixes, again

[Stas Sergeev <stsp@list.ru>]

29.10.2015 01:51, Toshi Kani пишет:
> On Wed, 2015-10-28 at 13:22 -0600, Toshi Kani wrote:
>> On Wed, 2015-10-28 at 10:34 -0600, Toshi Kani wrote:
>>> On Wed, 2015-10-28 at 12:53 +0300, Stas Sergeev wrote:
>>>> 28.10.2015 03:04, Toshi Kani пишет:
>>>>> On Wed, 2015-10-28 at 07:37 +0900, Linus Torvalds wrote:
>>>>>> On Tue, Oct 27, 2015 at 11:05 PM, Stas Sergeev <stsp@list.ru>
>>>>>> wrote:
>>>>>>> I can't easily post an Oops: under X it doesn't even appear -
>>>>>>> machine freezes immediately, and under non-KMS console it is
>>>>>>> possible to get one, but difficult to screen-shot (using bare
>>>>>>> metal, not VM). Also the Oops was seemingly unrelated.
>>>>>>> And if you run "dosemu -s" under non-KMS console, you'll also
>>>>>>> reproduce this one:
>>>>>>> https://bugzilla.kernel.org/show_bug.cgi?id=97321
>>>>>> Hmm. Andrew Morton responded to that initially, but then nothing
>>>>>> happened, and now it's been another six months. Andrew?
>>>>>>
>>>>>> The arch/x86/mm/pat.c error handling does seem to be suspect. This
>>>>>> is all code several years old, so none of this is new, and I think
>>>>>> Suresh is gone.  Adding a few other people with recent sign-offs to
>>>>>> that file, in the hope that somebody feels like they own it..
>>>>> In the case of PFNMAP, the range should always be mapped.  So, I
>>>>> wonder why follow_phys() failed with the !pte_present() check.
>>>>>
>>>>> Stas, do you have a test program that can reproduce 97321?
>>>> Get dosemu2 from here:
>>>> https://github.com/stsp/dosemu2/releases
>>>> or from git, or get dosemu1.
>>>> Then boot your kernel with "nomodeset=1" to get a text console.
>>>> Run
>>>>
>>>> dosemu -s
>>>>
>>>> and you'll get the bug.
>> I looked at the dosemu code and was able to reproduce the issue with a test
>> program.  This problem happens when mremap() to /dev/mem (or PFNMAP) is
>> called with MREMAP_FIXED.
>>
>> In this case, mremap calls move_vma(), which first calls move_page_tables()
>> to remap the translation and then calls do_munmap() to remove the original
>> mapping.  Hence, when untrack_pfn() is called from do_munmap(), the
>> original map is already removed, and follow_phys() fails with the
>>   !pte_present() check.
>>
>> I think there are a couple of issues:
>>   - If untrack_pfn() ignores an error from follow_phys() and skips
>> free_pfn_range(), PAT continues to track the original map that is removed.
>>   - untrack_pfn() calls free_pfn_range() to untrack a given free range.
>>   However, rbt_memtype_erase() requires the free range match exactly to the
>> tracked range.  This does not support mremap, which needs to free up part
>> of the tracked range.
>>   - PAT does not track a new translation specified by mremap() with MREMAP_F
>> IXED.
> Thinking further, I think the 1st and 3rd items are non-issues.  mremap remaps
> virtual address, but keeps the same cache type and pfns.  So, PAT does not have
> to change the tracked pfns in this case.  The 2nd item is still a problem,
> though.
Hello Toshi, thanks for your analysis.
Now as you do not seem to be preparing a fix, how
about attaching your test-case to the bug-report for
others to re-use? Or maybe you can even make it a
part of the kernel's test suit - I suppose this will help.