linux-kernel - Re: + c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-update-after-mm- num_exe_file

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Date:	Fri, 20 Apr 2012 02:28:30 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"matthltc@...ibm.com" <matthltc@...ibm.com>,
	"tj@...nel.org" <tj@...nel.org>,
	Pavel Emelianov <xemul@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: + c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-update-after-mm-
 num_exe_file_vmas-removal.patch added to -mm tree

Oleg Nesterov wrote:
> On 04/20, Cyrill Gorcunov wrote:
>>
>> Guys, while I more-less agree with Matt about single-shot behaviour
>>
>> [ let me copy my and his email
>>
>>    >>  With mm->exe_file this prctl option become a one-shot
>>    >>  only, and while at moment our user-space tool can perfectly
>>    >>  live with that I thought that there is no strict need to
>>    >>  limit the option this way from the very beginning.
>>    >>
>>    >  As far as backward compatibility, isn't it better to lift that restriction
>>    >  later rather than add it? I think the latter would very likely "break"
>>    >  things whereas the former would not.
>>    >
>>    >  I also prefer that restriction because it establishes a bound on how
>>    >  frequently the symlink can change. Keeping it a one-shot deal makes the
>>    >  values that show up in tools like top more reliable for admins.
>> ]
>>
>> I guess maybe it's time to drop one-shot requirement and as result
>> we can drop MMF_EXE_FILE_CHANGED bit completely,
>
> Plus perhaps we can remove this for_each_vma check?
>
>> making overall code
>> simplier?
>
> Personally I'd certainly prefer this ;)
>
>
>
> But let me repeat to avoid the confusion. I am fine either way,
> I am not going to discuss this again unless I see something which
> looks technically wrong. And the current MMF_EXE_FILE_CHANGED
> doesn't look right even if the problem is minor.

Yeah, whole this protection does not protect anything and can be easily bypassed.
For example task can re-execute itself and change exe-file again and again.

>
> Oleg.
>
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