linux-kernel - Re: Why is CONFIG

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Date:	Fri, 10 Jun 2011 23:39:55 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jesper Juhl <jj@...osbits.net>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jonas Gorski <jonas.gorski@...il.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: Why is CONFIG_FHANDLE an option??

On Sat, Jun 11, 2011 at 12:14:02AM +0200, Jesper Juhl wrote:
> Hello
> 
> I just configured a new kernel based on a recent git checkout and when I 
> had copied in my old configuration and did a "make oldconfig"I was greeted 
> with
> 
>     open by fhandle syscalls (FHANDLE) [N/y/?] (NEW)
> 
> Ok, so I read the help text description and learn that it's about two new 
> syscalls - open_by_handle_at(2) and name_to_handle_at(2).
> 
> My first thought at this point was "why are new syscalls even an option"?
> 
> Syscalls are in my oppinion ABI - having optional syscalls is just about 
> as bad as removing a syscall. It basically means that users cannot know if 
> the syscall is there and will need to test (it's bad enough having to 
> check the kernel version, having to check for specific syscalls as well 
> is just, well, annoying at best). 
> 
> Why are we making these optional?

Why not?  Software needs to test *anyway*, since it might run on earlier
kernels.  And "does that syscall return -ENOSYS" is self-documenting,
while "is the version higher than $MAGIC_NUMBER" is *not*.  Especially since
there's such thing as backports.

If you need to check that syscall is there, _check_ _it_.  Don't breed
dependencies on version numbers.
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