linux-kernel - Re: [PATCH v2 bpf-next 2/4] landlock: Use path_walk

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Message-ID: <CAPhsuW7MtxryseFsHF2xqBFS2UWammJatjf8UxBhytgn_nA4=g@mail.gmail.com>
Date: Thu, 5 Jun 2025 09:47:36 -0700
From: Song Liu <song@...nel.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org, 
	kernel-team@...a.com, andrii@...nel.org, eddyz87@...il.com, ast@...nel.org, 
	daniel@...earbox.net, martin.lau@...ux.dev, viro@...iv.linux.org.uk, 
	brauner@...nel.org, jack@...e.cz, kpsingh@...nel.org, 
	mattbobrowski@...gle.com, amir73il@...il.com, repnop@...gle.com, 
	jlayton@...nel.org, josef@...icpanda.com, gnoack@...gle.com, m@...wtm.org
Subject: Re: [PATCH v2 bpf-next 2/4] landlock: Use path_walk_parent()

On Wed, Jun 4, 2025 at 12:37 PM Song Liu <song@...nel.org> wrote:
>
> On Tue, Jun 3, 2025 at 6:46 AM Mickaël Salaün <mic@...ikod.net> wrote:
> >
> > Landlock tests with hostfs fail:
> >
> > ok 126 layout3_fs.hostfs.tag_inode_file
> > #  RUN           layout3_fs.hostfs.release_inodes ...
> > # fs_test.c:5555:release_inodes:Expected EACCES (13) == test_open(TMP_DIR, O_RDONLY) (0)
> >
> > This specific test checks that an access to a (denied) mount point over
> > an allowed directory is indeed denied.

I just realized this only fails on hostfs. AFAICT, hostfs is only used
by um. Do we really need this to behave the same on um+hostfs?

Thanks,
Song

>
> I am having trouble understanding the test. It appears to me
> the newly mounted tmpfs on /tmp is allowed, but accesses to
> / and thus mount point /tmp is denied? What would the walk in
> is_access_to_paths_allowed look like?
>
> > It's not clear to me the origin of the issue, but it seems to be related
> > to choose_mountpoint().
> >
> > You can run these tests with `check-linux.sh build kselftest` from
> > https://github.com/landlock-lsm/landlock-test-tools
>
> How should I debug this test? printk doesn't seem to work.
>
> Thanks,
> Song