linux-kernel - Re: [PATCH bpf-next 1/2] bpf: Add kfunc bpf_get_dentry

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Message-ID: <20240726053430.GB99483@ZenIV>
Date: Fri, 26 Jul 2024 06:34:30 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Song Liu <song@...nel.org>
Cc: bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	andrii@...nel.org, eddyz87@...il.com, ast@...nel.org,
	daniel@...earbox.net, martin.lau@...ux.dev, brauner@...nel.org,
	jack@...e.cz, kpsingh@...nel.org, mattbobrowski@...gle.com
Subject: Re: [PATCH bpf-next 1/2] bpf: Add kfunc bpf_get_dentry_xattr() to
 read xattr from dentry

On Thu, Jul 25, 2024 at 04:47:05PM -0700, Song Liu wrote:

> +__bpf_kfunc struct dentry *bpf_file_dentry(const struct file *file)
> +{
> +	/* file_dentry() does not hold reference to the dentry. We add a
> +	 * dget() here so that we can add KF_ACQUIRE flag to
> +	 * bpf_file_dentry().
> +	 */
> +	return dget(file_dentry(file));
> +}
> +
> +__bpf_kfunc struct dentry *bpf_dget_parent(struct dentry *dentry)
> +{
> +	return dget_parent(dentry);
> +}
> +
> +__bpf_kfunc void bpf_dput(struct dentry *dentry)
> +{
> +	return dput(dentry);
> +}

	If you keep a file reference, why bother grabbing dentry one?
If not, you have a very bad trouble if that opened file is the only
thing that keeps the filesystem busy.

	It's almost certainly a wrong interface; please, explain what
exactly are you trying to do here.