linux-kernel - Re: Revised statx(2) man page for review [and AT_EMPTY

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Message-ID: <20170426155259.GY29622@ZenIV.linux.org.uk>
Date:   Wed, 26 Apr 2017 16:53:00 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     David Howells <dhowells@...hat.com>
Cc:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
        Jeff Layton <jlayton@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-man <linux-man@...r.kernel.org>
Subject: Re: Revised statx(2) man page for review [and AT_EMPTY_PATH question]

On Wed, Apr 26, 2017 at 12:35:08PM +0100, David Howells wrote:

> AT_EMPTY_PATH wasn't there back in 2010.  I could eliminate the:
> 
> 	statx(fd, NULL, 0, ...);
> 
> option in favour of:
> 
> 	statx(fd, "", AT_EMPTY_PATH, ...);
> 
> Any thoughts either way, Al?
> 
> It would seem that AT_EMPTY_PATH should be redundant, though, since you can
> just set the pathname pointer to NULL.

NULL pathname pointer means an error for a lot of existing syscalls, so
if you want to turn them into wrappers for ...at() ones at libc level,
you'd need to do special-casing of NULL both kernel-side and in libc wrappers.

Requiring "" + AT_EMPTY_PATH means a single dereference of userland pointer.  
OTOH, that's not a terrible burden...