linux-kernel - Re: [PATCH 16/16] vfs: return -EOPNOTSUPP for fscaps from vfs

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Date:   Thu, 30 Nov 2023 10:40:57 -0600
From:   "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
To:     Amir Goldstein <amir73il@...il.com>
Cc:     Christian Brauner <brauner@...nel.org>,
        Serge Hallyn <serge@...lyn.com>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...hat.com>,
        James Morris <jmorris@...ei.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org, audit@...r.kernel.org,
        linux-unionfs@...r.kernel.org
Subject: Re: [PATCH 16/16] vfs: return -EOPNOTSUPP for fscaps from
 vfs_*xattr()

On Thu, Nov 30, 2023 at 08:10:15AM +0200, Amir Goldstein wrote:
> On Wed, Nov 29, 2023 at 11:51 PM Seth Forshee (DigitalOcean)
> <sforshee@...nel.org> wrote:
> >
> > Now that the new vfs-level interfaces are fully supported and all code
> > has been converted to use them, stop permitting use of the top-level vfs
> > xattr interfaces for capabilities xattrs. Unlike with ACLs we still need
> > to be able to work with fscaps xattrs using lower-level interfaces in a
> > handful of places, so only use of the top-level xattr interfaces is
> > restricted.
> 
> Can you explain why?
> Is there an inherent difference between ACLs and fscaps in that respect
> or is it just a matter of more work that needs to be done?

There are a number of differences. ACLs have caching, require additional
permission checks, and require a lot of filesystem-specific handling.
fscaps are simpler by comparison, and most filesystems can rely on a
common implementation that just converts to/from raw disk xattrs.

So at minimum I think the lowest level interfaces,
__vfs_{get,set,remove}xattr(), need to continue to allow fscaps, and
that's where ACL xattrs are blocked. Allowing some of the others to
still work with them is a matter of convenience (e.g. using
vfs_getxattr_alloc()) and trying to reduce code duplication. But as you
pointed out I did miss at least duplicating fsnotify_xattr(), so I'm
going to have another look at how I implemented these.

> 
> >
> > Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@...nel.org>
> > ---
> >  fs/xattr.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/fs/xattr.c b/fs/xattr.c
> > index 372644b15457..4b779779ad8c 100644
> > --- a/fs/xattr.c
> > +++ b/fs/xattr.c
> > @@ -540,6 +540,9 @@ vfs_setxattr(struct mnt_idmap *idmap, struct dentry *dentry,
> >         const void  *orig_value = value;
> >         int error;
> >
> > +       if (!strcmp(name, XATTR_NAME_CAPS))
> > +               return -EOPNOTSUPP;
> > +
> 
> It this is really not expected, then it should be an assert and
> please use an inline helper like is_posix_acl_xattr():
> 
> if (WARN_ON_ONCE(is_fscaps_xattr(name)))

Ack, makes sense.