netdev - Re: [RFC PATCH net-next v6 05/15] netdev: support binding dma-buf to netdevice

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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>,
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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