linux-kernel - Re: [PATCH 1/2] fs/ceph/debugfs: make all files world-readable

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Date:   Tue, 26 Sep 2023 08:09:20 +0200
From:   Max Kellermann <max.kellermann@...os.com>
To:     Xiubo Li <xiubli@...hat.com>
Cc:     Ilya Dryomov <idryomov@...il.com>,
        Jeff Layton <jlayton@...nel.org>, ceph-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] fs/ceph/debugfs: make all files world-readable

On Mon, Sep 25, 2023 at 7:18 AM Xiubo Li <xiubli@...hat.com> wrote:
> I am not sure whether will this make sense. Because the 'debug' under
> '/sys/kernel/' is also only accessible by privileged process.

Not exactly correct. It is by default accessible to processes who have
CAP_DAC_OVERRIDE and additionally it is accessible to (unprivileged)
processes running as uid=0 (those two traits usually overlap).
But we don't want to run kernel-exporter as uid=0 and neither do we
want to give it CAP_DAC_OVERRIDE; both would be too much, it would
affect much more than just (read) access to debugfs.
Instead, we mount debugfs with "gid=X,mode=0710". That way, we can
give (unprivileged) processes which are member of a certain group
access to debugfs, and we put our kernel-exporter process in that
group.

We can use these mount options to change debugfs defaults, but if a
debugfs implementor (such as cephfs) decides to override these global
debugfs settings by passing stricter file permissions, we can't easily
override that. And that is what my patch is about: restore the ability
to override debugfs permissions with a mount option, as debugfs was
designed.

Max