linux-kernel - Re: [PATCH] x86: defconfig: Enable CONFIG

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Date:	Mon, 01 Dec 2014 01:41:13 +0100
From:	Richard Weinberger <richard@....at>
To:	Dave Chinner <david@...morbit.com>
CC:	x86@...nel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, gregkh@...uxfoundation.org,
	rafael.j.wysocki@...el.com, pebolle@...cali.nl,
	linux-kernel@...r.kernel.org,
	"systemd-devel@...ts.freedesktop.org" 
	<systemd-devel@...ts.freedesktop.org>,
	Lennart Poettering <lennart@...ttering.net>
Subject: Re: [PATCH] x86: defconfig: Enable CONFIG_FHANDLE

Am 01.12.2014 um 01:18 schrieb Dave Chinner:
> On Sun, Nov 30, 2014 at 10:08:01PM +0100, Richard Weinberger wrote:
>> Am 30.11.2014 um 21:54 schrieb Dave Chinner:
>>> On Wed, Nov 26, 2014 at 12:36:52AM +0100, Richard Weinberger wrote:
>>>> systemd has a hard dependency on CONFIG_FHANDLE.
>>>> If you run systemd with CONFIG_FHANDLE=n it will somehow
>>>> boot but fail to spawn a getty or other basic services.
>>>> As systemd is now used by most x86 distributions it
>>>> makes sense to enabled this by default and save kernel
>>>> hackers a lot of value debugging time.
>>>
>>> The bigger question to me is this: why does systemd need to
>>> store/open by handle rather than just opening paths directly when
>>> needed? This interface is intended for stable, pathless access to
>>> inodes across unmount/mount contexts (e.g. userspace NFS servers,
>>> filesystem backup programs, etc) so I'm curious as to the problem
>>> systemd is solving using this interface. I just can't see the
>>> problem being solved here, and why path based security checks on
>>> every open() aren't necessary...
>>
>> Digging inter systemd source shows that they are using name_to_handle_at()
>> to get the mount id of a given path.
> 
> From the name_to_handle_at() man page:
> 
>     The mount_id argument returns an identifier for the filesystem
>     mount that corresponds to pathname.  This corresponds to the
>     first  field in  one  of  the records in /proc/self/mountinfo.
>     Opening the pathname in the fifth field of that record yields a
>     file descriptor for the mount point; that file descriptor can be
>     used in a subsequent call to open_by_handle_at().
> 
> So why do they need CONFIG_FHANDLE to get the mount id in userspace?
> Indeed, what do they even need the mount id for?
> 
>> The actual struct file_handle result is always ignored.
> 
> That sounds like a classic case of interface abuse. i.e. using an
> interface for something it was not designed or intended for....

CC'ing systemd folks.

Lennart, can you please explain why you need CONFIG_FHANDLE for systemd?
Maybe I'm reading the source horrible wrong.

Thanks,
//richard
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