linux-kernel - Re: Documenting ptrace access mode checking

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Date:	Fri, 24 Jun 2016 10:18:21 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	mtk.manpages@...il.com, Oleg Nesterov <oleg@...hat.com>,
	Jann Horn <jann@...jh.net>, James Morris <jmorris@...ei.org>,
	linux-man <linux-man@...r.kernel.org>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	lkml <linux-kernel@...r.kernel.org>,
	Kees Cook <keescook@...omium.org>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: Documenting ptrace access mode checking

On 06/23/2016 08:56 PM, Eric W. Biederman wrote:
> "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com> writes:
>
>> Hi Oleg,
>>
>> On 06/22/2016 11:51 PM, Oleg Nesterov wrote:
>>> On 06/21, Eric W. Biederman wrote:
>>>>
>>>> Adding Oleg just because he seems to do most of the ptrace related
>>>> maintenance these days.
>>>
>>> so I have to admit that I never even tried to actually understand
>>> ptrace_may_access ;)
>>>
>>>> We certainly need something that gives a high level view so people
>>>> reading the man page can know what to expect.   If you get down into the
>>>> weeds we run the danger of people beginning to think they can depend
>>>> upon bugs in the implementation.
>>>
>>> Personally I agree. I think "man ptrace" shouldn't not tell too much
>>> about kernel internals.
>>
>> See my other replies on this topic. Somehow, we need a way of
>> describing the behavior that user-space sees. I think it's
>> inevitable that that means talking about what;s going on
>> "under the hood".
>>
>> Regarding Eric's point that "we run the danger of people beginning
>> to think they can depend upon bugs in the implementation": when it
>> comes to breaking the ABI, the presence or absence of documentation
>> doesn't save us on that point (Linus has a few times made his position
>> wrt to documentation clear).
>
> Which are interesting in this respect as a bug in the implementation
> that is a security issue can and will be changed, even if userspace
> breaks.  Breaking userspace is not desirable but when there is no other
> reasonable choice it will happen.

Yes, good point.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/