netdev - Re: [PATCH iproute2] ip: Simplify executing ip cmd within namespace

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Message-ID: <5489E421.4020003@gmail.com>
Date:	Thu, 11 Dec 2014 16:36:17 -0200
From:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:	Jiri Benc <jbenc@...hat.com>
CC:	vadim4j@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH iproute2] ip: Simplify executing ip cmd within namespace

On 11-12-2014 16:08, Jiri Benc wrote:
> On Thu, 11 Dec 2014 15:33:34 -0200, Marcelo Ricardo Leitner wrote:
>> In that case, it would be interesting to also accelerate the original use
>> case, no? So all usages we currently have will benefit from this speed up
>> without a change.
>>
>> if (command to be executed == myself)
>>     switch namespace, continue without fork/exec..
>
> It's never good idea to do such tricks behind the user's back. This
> particular case could easily break for users wanting to execute a
> different ip binary (for whatever reason).

Then the if() above wouldn't match. That if means to check 
/proc/self/exe against the result of the path expansion. If that fails, 
continue with the normal path. If it matches, it is the same binary, and 
no need to re-exec itself.

> All programs should do what they are told to do, not try to outsmart
> the user.

It's not outsmarting, it's just not being dumb and doing it the proper 
way. Bash itself does this twist a lot. If you just type 'echo hi', it 
won't execute /bin/echo but use a built-in version. But if you write 
"/bin/echo hi", it will use /bin/echo..

We could use the same idea. "ip netns exec ip" -> ellipse it and avoid 
the fork/exec. But if it's cmd != "ip", execute it..

Now consider other applications that are user of this command. They will 
have to implement something like:

if (this ip command has --netns argument) {
    cmd="ip --netns ..."
} else {
    cmd="ip netns exec ..."
}

which is ugly and inconvenient.

   Marcelo

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