Re: [v3 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct

[Yang Shi <yang.shi@linux.alibaba.com>]

On 4/10/18 9:21 AM, Yang Shi wrote:
>
>
> On 4/10/18 5:28 AM, Cyrill Gorcunov wrote:
>> On Tue, Apr 10, 2018 at 01:10:01PM +0200, Michal Hocko wrote:
>>>> Because do_brk does vma manipulations, for this reason it's
>>>> running under down_write_killable(&mm->mmap_sem). Or you
>>>> mean something else?
>>> Yes, all we need the new lock for is to get a consistent view on brk
>>> values. I am simply asking whether there is something fundamentally
>>> wrong by doing the update inside the new lock while keeping the 
>>> original
>>> mmap_sem locking in the brk path. That would allow us to drop the
>>> mmap_sem lock in the proc path when looking at brk values.
>> Michal gimme some time. I guess  we might do so, but I need some
>> spare time to take more precise look into the code, hopefully today
>> evening. Also I've a suspicion that we've wracked check_data_rlimit
>> with this new lock in prctl. Need to verify it again.
>
> I see you guys points. We might be able to move the drop of mmap_sem 
> before setting mm->brk in sys_brk since mmap_sem should be used to 
> protect vma manipulation only, then protect the value modify with the 
> new arg_lock. Then we can eliminate mmap_sem stuff in prctl path, and 
> it also prevents from wrecking check_data_rlimit.
>
> At the first glance, it looks feasible to me. Will look into deeper 
> later.

A further look told me this might be *not* feasible.

It looks the new lock will not break check_data_rlimit since in my patch 
both start_brk and brk is protected by mmap_sem. The code flow might 
look like below:

CPU A                             CPU B
--------                       --------
prctl                               sys_brk
                                       down_write
check_data_rlimit           check_data_rlimit (need mm->start_brk)
                                       set brk
down_write                    up_write
set start_brk
set brk
up_write


If CPU A gets the mmap_sem first, it will set start_brk and brk, then 
CPU B will check with the new start_brk. And, prctl doesn't care if 
sys_brk is run before it since it gets the new start_brk and brk from 
parameter.

If we protect start_brk and brk with the new lock, sys_brk might get old 
start_brk, then sys_brk might break rlimit check silently, is that right?

So, it looks using new lock in prctl and keeping mmap_sem in brk path 
has race condition.

Thanks,
Yang

>
> Thanks,
> Yang
>
>