linux-kernel - Re: [PATCH] nextfd(2)

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Message-ID: <4F85E045.9070706@zytor.com>
Date:	Wed, 11 Apr 2012 12:49:25 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alexey Dobriyan <adobriyan@...il.com>,
	akpm@...ux-foundation.org, viro@...iv.linux.org.uk,
	drepper@...il.com, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] nextfd(2)

On 04/11/2012 12:46 PM, KOSAKI Motohiro wrote:
>>
>> a) inherently racy in a multithreaded environment;
> 
> I would say two things. 1) I know and I agree we _can_ misuse the interface.
> but many already existed interface also can be misused. 2) As I
> already explained
> this can be used correctly.
> 
> So, I have a question. Why do you bother a possibility of misuse? Of
> if you didn't point out misuse, can you please point out a real world
> use case of multi threads + fd interation?
> 

This were brought up in the POSIX discussion as part of why these
interfaces were considered undesirable.

> 
>> b) unsafe because there might be file descriptors used by libc itself.
> 
> I agree this. Even though almost developer don't use libc message catalogue and
> we can avoid such issue by using nextfd() + fcntl(O_CLOEXEC).
> 

No, that's exactly the point that we cannot.

> 
> Yeah, I don't think fdwalk() is problematic. It's an option if I
> understand Alexey's mail
> correctly. but I disagree almost all developers should fix a design
> and rewrite their
> applications. In theory, they can avoid glibc or they can rewrite all
> of their code or
> avoid linux. but there is one problem. unrealistic.
> 

The problem -- as was brought up in the POSIX discussion -- is that you
actually end up breaking *properly functioning programs*.

	-hpa
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