netdev - Re: [PATCH v7 02/11] net: add ndo to setup/query xdp prog in adapter rx

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Date:	Mon, 11 Jul 2016 23:12:24 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	bblanco@...mgrid.com
Cc:	netdev@...r.kernel.org, jhs@...atatu.com,
	saeedm@....mellanox.co.il, kafai@...com, brouer@...hat.com,
	as754m@....com, alexei.starovoitov@...il.com, gerlitz.or@...il.com,
	john.fastabend@...il.com, hannes@...essinduktion.org,
	tgraf@...g.ch, tom@...bertland.com, daniel@...earbox.net
Subject: Re: [PATCH v7 02/11] net: add ndo to setup/query xdp prog in
 adapter rx

From: Brenden Blanco <bblanco@...mgrid.com>
Date: Mon, 11 Jul 2016 14:29:49 -0700

> +	if (fd >= 0) {
> +		prog = bpf_prog_get_type(fd, BPF_PROG_TYPE_XDP);
> +		if (IS_ERR(prog))
> +			return PTR_ERR(prog);
> +	}
> +
> +	xdp.command = XDP_SETUP_PROG;
> +	xdp.prog = prog;
> +	err = ops->ndo_xdp(dev, &xdp);
> +	if (err < 0 && prog)
> +		bpf_prog_put(prog);

I don't understand the reference counting on 'prog' here.

The enumeration documentation states that no matter what, the passed
in prog doesn't need to be mangaged by the ->ndo_xdp() method.

Therefore, if that is true, we must always put the 'prog' here if it
is non-NULL.  Regardless of the 'err' value.