linux-kernel - Re: [fuse-devel] [PATCH 01/25] VFS: move attr_kill logic from notify

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Date:	Tue, 07 Aug 2007 08:00:40 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	trond.myklebust@....uio.no
CC:	miklos@...redi.hu, jlayton@...hat.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	mikulas@...ax.karlin.mff.cuni.cz, zippel@...ux-m68k.org,
	joel.becker@...cle.com, wli@...omorphy.com, dhowells@...hat.com,
	bfennema@...con.csc.calpoly.edu
Subject: Re: [fuse-devel] [PATCH 01/25] VFS: move attr_kill logic from
	notify_change into helper function

(cutting out lists from CC)

> > > > Your patch is changing the API in a very unsafe way, since there will
> > > > be no error or warning on an unconverted fs.  And that could lead to
> > > > security holes.
> > > > 
> > > > If we would rename the setattr method to setattr_new as well as
> > > > changing it's behavior, that would be fine.  But I guess we do not
> > > > want to do that.
> > > 
> > > Which "unconverted fses"? If we're talking out of tree stuff, then too
> > > bad: it is _their_ responsibility to keep up with kernel changes.
> > 
> > It is usually a good idea to not change the semantics of an API in a
> > backward incompatible way without changing the syntax as well.
> 
> We're taking two setattr flags ATTR_KILL_SGID, and ATTR_KILL_SUID which
> have existed for several years in the VFS, and making them visible to
> the filesystems. Out-of-tree filesystems that care can check for them in
> a completely backward compatible way: you don't even need to add a
> #define.

Making flags visible is not the problem.

Making another flag invisible (ATTR_MODE) at the same time _is_.

> > This is true regardless of whether we care about out-of-tree code or
> > not (and we should care to some degree).  And especially true if the
> > change in question is security sensitive.
> 
> It is not true "regardless": the in-tree code is being converted.
> Out-of-tree code is the only "problem" here, and their only problem is
> that they may have to add support for the new flags if they also support
> suid/sgid mode bits.

Yes.  And there would be no problem with that, as long as it would be
breaking the API in a visible way.  It does not do that and that is
unsafe.

The other thing with this change is, that it's generally a good idea
to let the VFS do as much as possible, because then filesystem writers
won't get it wrong.

In this case the suid/sgid check needs to be omitted for _very_ few
filesystems (NFS and fuse).  So it makes sense to leave the check
outside filesystem code, and move it inside only when necessary.

> Are you advocating reserving a new filesystem bit every time we need to
> add an attribute flag?

No, just adding a flag does not constitute a backward incompatible
change.

Miklos
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