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From:   =?UTF-8?Q?Christian_G=C3=B6ttsche?= <cgzones@googlemail.com>
Date:   Thu, 9 Jun 2022 11:14:45 +0200
Message-ID: <CAJ2a_DdpvaXxoWKJhVww3=xGcp5n4O3LZ+n5dZMh8pUb9WZM_w@mail.gmail.com>
Subject: Re: [RFC PATCH] f*xattr: allow O_PATH descriptors
To:     Amir Goldstein <amir73il@gmail.com>
Cc:     Andreas Dilger <adilger@dilger.ca>,
        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 Thu, 9 Jun 2022 at 06:36, Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Wed, Jun 8, 2022 at 9:01 PM Andreas Dilger <adilger@dilger.ca> wrote:
> >
> > On Jun 7, 2022, at 9:31 AM, Christian G=C3=B6ttsche <cgzones@googlemail=
.com> wrote:
> > >
> > > From: Miklos Szeredi <mszeredi@redhat.com>
> > >
> > > Support file descriptors obtained via O_PATH for extended attribute
> > > operations.
> > >
> > > Extended attributes are for example used by SELinux for the security
> > > context of file objects. To avoid time-of-check-time-of-use issues wh=
ile
> > > setting those contexts it is advisable to pin the file in question an=
d
> > > operate on a file descriptor instead of the path name. This can be
> > > emulated in userspace via /proc/self/fd/NN [1] but requires a procfs,
> > > which might not be mounted e.g. inside of chroots, see[2].
> >
> > Will this allow get/set xattrs directly on symlinks?  That is one probl=
em
> > that we have with some of the xattrs that are inherited on symlinks, bu=
t
> > there is no way to change them.  Allowing setxattr directly on a symlin=
k
> > would be very useful.
>
> It is possible.
> See: https://github.com/libfuse/libfuse/pull/514
>

Does it really? (It should not since xtattr(7) mentions some quota
related issues.)
In my tests setting extended attributes via O_PATH on symlinks fails
with ENOTSUP (even as root), except for special ones, like
"security.selinux".

> That's why Miklos withdrew this patch:
> https://lore.kernel.org/linux-fsdevel/CAOssrKeV7g0wPg4ozspG4R7a+5qARqWdG+=
GxWtXB-MCfbVM=3D9A@mail.gmail.com/
>
> Thanks,
> Amir.