(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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