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Message-Id: <5e2f9342-4ee9-4b30-9dcf-393e57e0f7c6@app.fastmail.com>
Date: Tue, 05 Mar 2024 10:04:45 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Mina Almasry" <almasrymina@...gle.com>, Netdev <netdev@...r.kernel.org>,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
 linux-parisc@...r.kernel.org, sparclinux@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, Linux-Arch <linux-arch@...r.kernel.org>,
 bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org
Cc: "David S . Miller" <davem@...emloft.net>,
 "Eric Dumazet" <edumazet@...gle.com>, "Jakub Kicinski" <kuba@...nel.org>,
 "Paolo Abeni" <pabeni@...hat.com>, "Jonathan Corbet" <corbet@....net>,
 "Richard Henderson" <richard.henderson@...aro.org>,
 "Ivan Kokshaysky" <ink@...assic.park.msu.ru>,
 "Matt Turner" <mattst88@...il.com>,
 "Thomas Bogendoerfer" <tsbogend@...ha.franken.de>,
 "James E . J . Bottomley" <James.Bottomley@...senPartnership.com>,
 "Helge Deller" <deller@....de>, "Andreas Larsson" <andreas@...sler.com>,
 "Jesper Dangaard Brouer" <hawk@...nel.org>,
 "Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
 "Steven Rostedt" <rostedt@...dmis.org>,
 "Masami Hiramatsu" <mhiramat@...nel.org>,
 "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>,
 "Alexei Starovoitov" <ast@...nel.org>,
 "Daniel Borkmann" <daniel@...earbox.net>,
 "Andrii Nakryiko" <andrii@...nel.org>,
 "Martin KaFai Lau" <martin.lau@...ux.dev>,
 "Eduard Zingerman" <eddyz87@...il.com>, "Song Liu" <song@...nel.org>,
 "Yonghong Song" <yonghong.song@...ux.dev>,
 "John Fastabend" <john.fastabend@...il.com>,
 "KP Singh" <kpsingh@...nel.org>, "Stanislav Fomichev" <sdf@...gle.com>,
 "Hao Luo" <haoluo@...gle.com>, "Jiri Olsa" <jolsa@...nel.org>,
 "David Ahern" <dsahern@...nel.org>,
 "Willem de Bruijn" <willemdebruijn.kernel@...il.com>,
 shuah <shuah@...nel.org>, "Sumit Semwal" <sumit.semwal@...aro.org>,
 Christian König <christian.koenig@....com>,
 "Pavel Begunkov" <asml.silence@...il.com>, "David Wei" <dw@...idwei.uk>,
 "Jason Gunthorpe" <jgg@...pe.ca>,
 "Yunsheng Lin" <linyunsheng@...wei.com>,
 "Shailend Chand" <shailend@...gle.com>,
 "Harshitha Ramamurthy" <hramamurthy@...gle.com>,
 "Shakeel Butt" <shakeelb@...gle.com>,
 "Jeroen de Borst" <jeroendb@...gle.com>,
 "Praveen Kaligineedi" <pkaligineedi@...gle.com>,
 "Willem de Bruijn" <willemb@...gle.com>,
 "Kaiyuan Zhang" <kaiyuanz@...gle.com>
Subject: Re: [RFC PATCH net-next v6 05/15] netdev: support binding dma-buf to netdevice

On Tue, Mar 5, 2024, at 03:01, Mina Almasry wrote:

> +int netdev_bind_dmabuf(struct net_device *dev, unsigned int dmabuf_fd,
> +		       struct netdev_dmabuf_binding **out)
> +{
> +	struct netdev_dmabuf_binding *binding;
> +	static u32 id_alloc_next;
> +	struct scatterlist *sg;
> +	struct dma_buf *dmabuf;
> +	unsigned int sg_idx, i;
> +	unsigned long virtual;
> +	int err;
> +
> +	if (!capable(CAP_NET_ADMIN))
> +		return -EPERM;
> +
> +	dmabuf = dma_buf_get(dmabuf_fd);
> +	if (IS_ERR_OR_NULL(dmabuf))
> +		return -EBADFD;

You should never need to use IS_ERR_OR_NULL() for a properly
defined kernel interface. This one should always return an
error or a valid pointer, so don't check for NULL.

> +	binding->attachment = dma_buf_attach(binding->dmabuf, dev->dev.parent);
> +	if (IS_ERR(binding->attachment)) {
> +		err = PTR_ERR(binding->attachment);
> +		goto err_free_id;
> +	}
> +
> +	binding->sgt =
> +		dma_buf_map_attachment(binding->attachment, DMA_BIDIRECTIONAL);
> +	if (IS_ERR(binding->sgt)) {
> +		err = PTR_ERR(binding->sgt);
> +		goto err_detach;
> +	}

Should there be a check to verify that this buffer
is suitable for network data?

In general, dmabuf allows buffers that are uncached or reside
in MMIO space of another device, but I think this would break
when you get an skb with those buffers and try to parse the
data inside of the kernel on architectures where MMIO space
is not a normal pointer or unaligned access is disallowed on
uncached data.

        Arnd

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