linux-kernel - Re: [RFC/PATCH v2 bpf-next fanotify 7/7] selftests/bpf: Add test for BPF based fanotify fastpath handler

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Message-ID: <CAADnVQK6YyPUzQoPKkXptLHoHXJZ50A8vNPfpDAk8Jc3Z6+iRw@mail.gmail.com>
Date: Thu, 14 Nov 2024 12:14:08 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Song Liu <song@...nel.org>
Cc: bpf <bpf@...r.kernel.org>, Linux-Fsdevel <linux-fsdevel@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, 
	LSM List <linux-security-module@...r.kernel.org>, Kernel Team <kernel-team@...a.com>, 
	Andrii Nakryiko <andrii@...nel.org>, Eddy Z <eddyz87@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, KP Singh <kpsingh@...nel.org>, 
	Matt Bobrowski <mattbobrowski@...gle.com>, Amir Goldstein <amir73il@...il.com>, repnop@...gle.com, 
	Jeff Layton <jlayton@...nel.org>, Josef Bacik <josef@...icpanda.com>, 
	Mickaël Salaün <mic@...ikod.net>, gnoack@...gle.com
Subject: Re: [RFC/PATCH v2 bpf-next fanotify 7/7] selftests/bpf: Add test for
 BPF based fanotify fastpath handler

On Thu, Nov 14, 2024 at 12:44 AM Song Liu <song@...nel.org> wrote:
>
> +
> +       if (bpf_is_subdir(dentry, v->dentry))
> +               ret = FAN_FP_RET_SEND_TO_USERSPACE;
> +       else
> +               ret = FAN_FP_RET_SKIP_EVENT;

It seems to me that all these patches and feature additions
to fanotify, new kfuncs, etc are done just to do the above
filtering by subdir ?

If so, just hard code this logic as an extra flag to fanotify ?
So it can filter all events by subdir.
bpf programmability makes sense when it needs to express
user space policy. Here it's just a filter by subdir.
bpf hammer doesn't look like the right tool for this use case.