Re: [PATCH 16/16] vfs: return -EOPNOTSUPP for fscaps from vfs_*xattr()

From: Amir Goldstein <amir73il@gmail.com>
To: "Seth Forshee (DigitalOcean)" <sforshee@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>,
	Serge Hallyn <serge@hallyn.com>,
	 Paul Moore <paul@paul-moore.com>, Eric Paris <eparis@redhat.com>,
	 James Morris <jmorris@namei.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	 Miklos Szeredi <miklos@szeredi.hu>,
	linux-kernel@vger.kernel.org,  linux-fsdevel@vger.kernel.org,
	linux-security-module@vger.kernel.org,  audit@vger.kernel.org,
	linux-unionfs@vger.kernel.org
Subject: Re: [PATCH 16/16] vfs: return -EOPNOTSUPP for fscaps from vfs_*xattr()
Date: Thu, 30 Nov 2023 08:10:15 +0200	[thread overview]
Message-ID: <CAOQ4uxhtJ89LknKjE=tiTgvZXbufmOaqHnhnrz348Ktq2H+yHA@mail.gmail.com> (raw)
In-Reply-To: <20231129-idmap-fscap-refactor-v1-16-da5a26058a5b@kernel.org>

On Wed, Nov 29, 2023 at 11:51 PM Seth Forshee (DigitalOcean)
<sforshee@kernel.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?

>
> Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.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)))

It wouldn't hurt to add those assertions to is_posix_acl_xattr()
cases as well, but that is unrelated to your change.

Thanks,
Amir.