Re: [PATCH] x86: defconfig: Enable CONFIG_FHANDLE

[Dave Chinner <david@fromorbit.com>]

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....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com