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

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Date:   Thu, 30 Nov 2023 08:10:15 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     "Seth Forshee (DigitalOcean)" <sforshee@...nel.org>
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 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?

>
> 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)))

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.