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

[Yang Shi]

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 AÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ CPU B
--------ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ --------
prctlÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ sys_brk
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ down_write
check_data_rlimitÂÂÂÂÂÂÂÂÂÂ check_data_rlimit (need mm->start_brk)
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ set brk
down_writeÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ 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