linux-kernel - Re: /etc/fstab.d yes or not

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Date:	Fri, 20 Jan 2012 17:08:02 +0100
From:	Kay Sievers <kay.sievers@...y.org>
To:	Roger Leigh <rleigh@...elibre.net>
Cc:	Masatake YAMATO <yamato@...hat.com>, kzak@...hat.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	util-linux@...r.kernel.org
Subject: Re: /etc/fstab.d yes or not

On Fri, Jan 20, 2012 at 16:57, Roger Leigh <rleigh@...elibre.net> wrote:
> On Fri, Jan 20, 2012 at 03:56:43PM +0100, Kay Sievers wrote:
>> On Fri, Jan 20, 2012 at 15:20, Masatake YAMATO <yamato@...hat.com> wrote:
>> >>
>> >>  I'd like to add support for /etc/fstab.d to libmount. The library is
>> >>  currently used by mount, umount and mount.nfs. The goal is to use it
>> >>  on more places.
>> >
>> > I'm working on systemd to support /etc/fstab.d.
>>
>> It's an old glibc API, and /etc/fstab is ABI, not a service config
>> file, which now can read more than one file. It's a very different
>> problem. It an ABI change, not a config extension.
>
> I think that's putting the problem a little too strongly.  The
> glibc API is generic: it can (and is) used to parse any file using
> the fstab(5) file format; it's not restricted to only /etc/mtab
> and /etc/fstab.  setmntent(3) accepts any filename.

That's right, but it's just not the way it's used. None of the old
tools is prepared to just wrap that in a readdir() loop.

> You are entirely correct that it may break the assumptions made by
> some software, but it's certainly not "ABI" or even "API" breakage.

It is surely the de-facto Linux/UNIX ABI for system mounts.

>> Tools rightfully expect that they find all system mounts in that file,
>> and not in some new split-up directory. In some cases, fstab is used
>> to 'check if the device is not a system volume', and that will just
>> break now,
>
> The number of tools which do this is tiny, and could (if we wished)
> be patched trivially to also look at the files in fstab.d.
>
> Alternatively, glibc itself could be updated so that if e.g.
> setmntent(3) is called with "/etc/fstab" as its filename argument,
> it will additionally process and include the contents of
> /etc/fstab.d as well.  This would have the advantage of transparency
> for all users of the glibc interface.

That's true, but good luck with glibc changes. :)

Kay
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