linux-kernel - Re: [PATCH 03/10] ptrace: implement PTRACE

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Date:	Wed, 18 May 2011 12:44:58 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	oleg@...hat.com, jan.kratochvil@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, indan@....nu, bdonlan@...il.com
Subject: Re: [PATCH 03/10] ptrace: implement PTRACE_SEIZE

On Wed, May 18, 2011 at 11:55 AM, Tejun Heo <tj@...nel.org> wrote:
> Hello, Denys.
>
> On Wed, May 18, 2011 at 02:40:56AM +0200, Denys Vlasenko wrote:
>> This makes PTRACE_EVENT_STOP similar to other PTRACE_EVENTs.
>> The only difference is that it can't be set by PTRACE_SETOPTIONS
>> as other events do, but activated implicitly by PTRACE_SEIZE.
>
> Also by PTRACE_INTERRUPT and group stop.

"Activating" meaning "enabling", not "causing". Only PTRACE_SEIZE
enables PTRACE_EVENT_STOP.

>> This made me thinking.
>>
>> How about making API even more similar to existing one?
>>
>> Create PTRACE_O_TRACESTOP, make it settable by PTRACE_SETOPTIONS too.
>>
>> Make PTRACE_SEIZE take the mask of PTRACE_O_xyz flags
>> as data argument.
>> If PTRACE_O_TRACESTOP is set, it works as you described above.
>> If PTRACE_O_TRACESTOP is not set, then it works as good old PTRACE_ATTACH.
>> In both cases, immediately at attach it sets opts a-la PTRACE_SETOPTIONS.
>>
>> We can even avoid introducing PTRACE_SEIZE at all, because
>> currently PTRACE_ATTACH ignores its data argument.
>>
>> I know, I know, "this changes API", but did we ever promise
>> that PTRACE_ATTACH with nonzero data arg is a valid usage?
>> Also, I perused first 10 pages of google code search results
>> and I see that everybody passes 0 or NULL.
>
> But as SEIZE introduces behavior differences throughout ptrace
> operation,
>
> Similar issue with PTRACE_O_TRACESTOP.  It won't only enable TRACESTOP
> it will change other behaviors too

All these differences revolve around making handling of stops
and SIGCONT better. It seems fitting to the option name.

PTRACE_SEIZE sets a flag somewhere "please convert
group-stops into PTRACE_EVENT_STOPs".

But, we *already have* ptrace op which performs this action
of modifying (or adding) stops - it's PTRACE_SETOPTIONS.
With PTRACE_O_TRACESYSGOOD it modifies syscall-stops.
With PTRACE_O_TRACEEXEC - post-execve stop.
With PTRACE_O_TRACEFORK it adds a stop after fork.

Since we have this API, why not use it for the very similar
concept of modifying group-stops too?

> I think it's actually beneficial to use a distinctively new
> request.  It's not like it costs anything or we're short on request
> number space.

"Entities must not be multiplied beyond necessity"?

-- 
vda
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