Subject: Re: [PATCH net-next,v4 3/9] net: resolve forwarding path from virtual netdevice and HW destination address

From: Florian Westphal <[email-masked]>
Date: Thu, 19 Nov 2020 00:22:59 +0100

Pablo Neira Ayuso <[email-masked]> wrote:
> +#define NET_DEVICE_PATH_STACK_MAX	5
> +
> +struct net_device_path_stack {
> +	int			num_paths;
> +	struct net_device_path	path[NET_DEVICE_PATH_STACK_MAX];
> +};

[..]

> +int dev_fill_forward_path(const struct net_device *dev, const u8 *daddr,
> +			  struct net_device_path_stack *stack)
> +{
> +	const struct net_device *last_dev;
> +	struct net_device_path_ctx ctx = {
> +		.dev	= dev,
> +		.daddr	= daddr,
> +	};
> +	struct net_device_path *path;
> +	int ret = 0, k;
> +
> +	stack->num_paths = 0;
> +	while (ctx.dev && ctx.dev->netdev_ops->ndo_fill_forward_path) {
> +		last_dev = ctx.dev;
> +		k = stack->num_paths++;
> +		if (WARN_ON_ONCE(k >= NET_DEVICE_PATH_STACK_MAX))
> +			return -1;

This guarantees k < NET_DEVICE_PATH_STACK_MAX, so we can fill
entire path[].

> +		path = &stack->path[k];
> +		memset(path, 0, sizeof(struct net_device_path));
> +
> +		ret = ctx.dev->netdev_ops->ndo_fill_forward_path(&ctx, path);
> +		if (ret < 0)
> +			return -1;
> +
> +		if (WARN_ON_ONCE(last_dev == ctx.dev))
> +			return -1;
> +	}

... but this means that stack->num_paths == NET_DEVICE_PATH_STACK_MAX
is possible, with k being last element.

> +	path = &stack->path[stack->num_paths++];

... so this may result in a off by one?