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From:   =?UTF-8?Q?Christian_G=C3=B6ttsche?= <cgzones@googlemail.com>
Date:   Fri, 19 Aug 2022 20:05:31 +0200
Message-ID: <CAJ2a_DfkMvh7EdOA6k+omxhi18-oVbSXSGzXnpU1tXPD55B2qw@mail.gmail.com>
Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Amir Goldstein <amir73il@gmail.com>,
        Christian Brauner <brauner@kernel.org>,
        SElinux list <selinux@vger.kernel.org>,
        Miklos Szeredi <mszeredi@redhat.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-man <linux-man@vger.kernel.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Wed, 22 Jun 2022 at 04:57, Aleksa Sarai <cyphar@cyphar.com> wrote:
>
> On 2022-06-20, Amir Goldstein <amir73il@gmail.com> wrote:
> > To be a reasonable reaction to the currently broken API is
> > to either accept the patch as is or request that setxattrat()
> > will be added to provide the new functionality.
>
> Since the current functionality cannot be retroactively disabled as it
> is being used already through /proc/self/fd/$n, adding
> *xattrat(AT_EMPTY_PATH) doesn't really change what is currently possible
> by userspace.
>
> I would say we should add *xattrat(2) and then we can add an upgrade
> mask blocking it (and other operations) later.
>

It seems setxattrat() is the preferred way to continue.
fsetxattr() would have one advantage though (w.r.t. SELinux):

The steps to label a file are:
  1. get the type of the file (via stat(2) family)
  2. lookup the desired label from the label database via selabel_lookup(3)
  3. assign the retrieved label to the file

The label is sensitive to the file type, e.g.

    $ matchpathcon -m file /etc/shadow
    /etc/shadow     system_u:object_r:shadow_t:s0
    $ matchpathcon -m lnk_file /etc/shadow
    /etc/shadow     system_u:object_r:etc_t:s0

Using the *at() family the file type could change between step 1. and 3.,
which operating on an O_PATH file descriptor would prevent.