linux-kernel - Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and env_start|end in mm

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Date:   Tue, 27 Mar 2018 14:44:09 -0400
From:   Yang Shi <yang.shi@...ux.alibaba.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     Matthew Wilcox <willy@...radead.org>, adobriyan@...il.com,
        mhocko@...nel.org, mguzik@...hat.com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [v2 PATCH] mm: introduce arg_lock to protect arg_start|end and
 env_start|end in mm_struct



On 3/27/18 3:32 AM, Cyrill Gorcunov wrote:
> On Mon, Mar 26, 2018 at 05:59:49PM -0400, Yang Shi wrote:
>>> Say we've two syscalls running prctl_set_mm_map in parallel, and imagine
>>> one have @start_brk = 20 @brk = 10 and second caller has @start_brk = 30
>>> and @brk = 20. Since now the call is guarded by _read_ the both calls
>>> unlocked and due to OO engine it may happen then when both finish
>>> we have @start_brk = 30 and @brk = 10. In turn "write" semaphore
>>> has been take to have consistent data on exit, either you have [20;10]
>>> or [30;20] assigned not something mixed.
>>>
>>> That said I think using read-lock here would be a bug.
>> Yes it sounds so. However, it was down_read before
>> ddf1d398e517e660207e2c807f76a90df543a217 ("prctl: take mmap sem for writing
>> to protect against others"). And, that commit is for fixing the concurrent
>> writing to arg_* and env_*. I just checked that commit, but omitted the brk
>> part. The potential issue mentioned by you should exist before that commit,
>> but might be just not discovered or very rare to hit.
>>
>> I will change it back to down_write.
> down_read before was a bug ;) And it was not discovered earlier simply
> because not that many users of this interface exist, namely only criu
> as far as I know by now.

Thanks for confirming this. I assumed so :-)