Subject: Re: [criu] 1M guard page ruined restore
From: Dmitry Safonov <dsafonov@virtuozzo.com>
To: Oleg Nesterov <oleg@redhat.com>, Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Hugh Dickins <hughd@google.com>, Andrey Vagin <avagin@openvz.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Pavel Emelyanov <xemul@virtuozzo.com>,
        Andrew Morton <akpm@linuxfoundation.org>,
        Adrian Reber <areber@redhat.com>
References: <20170620075206.GB1909@uranus.lan>
 <alpine.LSU.2.11.1706200259080.11599@eggly.anvils>
 <20170621152256.GC31050@uranus> <20170621155730.GA32554@redhat.com>
 <20170621160410.GF31050@uranus> <20170621170129.GA32752@redhat.com>
 <a747d0be-ddf9-d338-06de-223337a03a67@virtuozzo.com>
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Date: Wed, 21 Jun 2017 20:19:34 +0300
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On 06/21/2017 08:15 PM, Dmitry Safonov wrote:
> On 06/21/2017 08:01 PM, Oleg Nesterov wrote:
>> On 06/21, Cyrill Gorcunov wrote:
>>>
>>> On Wed, Jun 21, 2017 at 05:57:30PM +0200, Oleg Nesterov wrote:
>>>>>
>>>>>     p = fake_grow_down;
>>>>>     *p-- = 'c';
>>>>
>>>> I guess this works? I mean, *p-- = 'c' should not fail...
>>>
>>> It fails.
>>
>> Hmm. Impossible ;) could you add the additional printf's to re-check?
>>
>>> Here is the complete code. It supposed to _extend_ stack but it fails
>>> on the latest master + Hugh's [PATCH] mm: fix new crash in 
>>> unmapped_area_topdown()
>>> ---
>>> [root@fc2 criu]# ~/st2
>>> start_addr 7fe6162a8000
>>> start_addr 7fe6163d9000
>>> Segmentation fault (core dumped)
>>> ---
>>> #include <stdio.h>
>>> #include <stdlib.h>
>>> #include <errno.h>
>>> #include <stdlib.h>
>>> #include <string.h>
>>> #include <unistd.h>
>>>
>>> #include <sys/mman.h>
>>>
>>> #define PAGE_SIZE 4096
>>>
>>> int main(int argc, char **argv)
>>> {
>>>     char *start_addr, *start_addr1, *fake_grow_down, *test_addr, 
>>> *grow_down;
>>>     volatile char *p;
>>>
>>>     start_addr = mmap(NULL, PAGE_SIZE * 512, PROT_READ | PROT_WRITE, 
>>> MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
>>>     if (start_addr == MAP_FAILED) {
>>>         printf("Can't mal a new region");
>>>         return 1;
>>>     }
>>>     printf("start_addr %lx\n", start_addr);
>>>     munmap(start_addr, PAGE_SIZE * 512);
>>>
>>>     start_addr += PAGE_SIZE * 300;
>>>
>>>     fake_grow_down = mmap(start_addr + PAGE_SIZE * 5, PAGE_SIZE,
>>>              PROT_READ | PROT_WRITE,
>>>              MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED | MAP_GROWSDOWN, 
>>> -1, 0);
>>>     if (fake_grow_down == MAP_FAILED) {
>>>         printf("Can't mal a new region");
>>>         return 1;
>>>     }
>>>     printf("start_addr %lx\n", fake_grow_down);
>>>
>>>     p = fake_grow_down;
>>>     *p-- = 'c';
>>
>> once again, I can't believe this STORE can fail...
>>
>>>     *p = 'b';
>>
>> Ah. I forgot about another kernel "feature" ;) not related to the 
>> recent guard
>> page changes...
>>
>> Could you test the patch below?
> 
> Well, for me only the second store caused segfault.
> With your patch it passes..

The only question I have - how is it connected to guard page?
Before the former patch the test has passed. Why?
And this thing seems to be in kernel since 2009.

> 
>>
>>
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index 8ad91a0..edc5d68 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -1416,7 +1416,7 @@ __do_page_fault(struct pt_regs *regs, unsigned 
>> long error_code,
>>            * and pusha to work. ("enter $65535, $31" pushes
>>            * 32 pointers and then decrements %sp by 65535.)
>>            */
>> -        if (unlikely(address + 65536 + 32 * sizeof(unsigned long) < 
>> regs->sp)) {
>> +if (0)        if (unlikely(address + 65536 + 32 * sizeof(unsigned 
>> long) < regs->sp)) {
>>               bad_area(regs, error_code, address);
>>               return;
>>           }
>>
> 


-- 
              Dmitry