linux-kernel - Re: [PATCH 3/4] inotify_user: add system call inotify_add_watch

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Message-ID: <CAKPOu+9yAKCtrRZsZPFDtM6RP6Ev_-2x84WYCLf_SPHJcr3Faw@mail.gmail.com>
Date:   Mon, 18 Sep 2023 21:45:16 +0200
From:   Max Kellermann <max.kellermann@...os.com>
To:     Jan Kara <jack@...e.cz>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        amir73il@...il.com
Subject: Re: [PATCH 3/4] inotify_user: add system call inotify_add_watch_at()

On Mon, Sep 18, 2023 at 2:40 PM Jan Kara <jack@...e.cz> wrote:
> Is there any problem with using fanotify for you?

Turns out fanotify is unusable for me, unfortunately.
I have been using inotify to get notifications of cgroup events, but
the cgroup filesystem appears to be unsupported by fanotify: all
attempts to use fanotify_mark() on cgroup event files fail with
ENODEV. I think that comes from fanotify_test_fsid(). Filesystems
without a fsid work just fine with inotify, but fail with fanotify.

Since fanotify lacks important features, is it really a good idea to
feature-freeze inotify?

(By the way, what was not documented is that fanotify_init() can only
be used by unprivileged processes if the FAN_REPORT_FID flag was
specified. I had to read the kernel sources to figure that out - I
have no idea why this limitation exists - the code comment in the
kernel source doesn't explain it.)