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

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Message-ID: <20201118232259.GC15137@breakpoint.cc>
Date:   Thu, 19 Nov 2020 00:22:59 +0100
From:   Florian Westphal <fw@...len.de>
To:     Pablo Neira Ayuso <pablo@...filter.org>
Cc:     netfilter-devel@...r.kernel.org, davem@...emloft.net,
        netdev@...r.kernel.org, kuba@...nel.org, razor@...ckwall.org,
        tobias@...dekranz.com, jeremy@...zel.net
Subject: Re: [PATCH net-next,v4 3/9] net: resolve forwarding path from
 virtual netdevice and HW destination address

Pablo Neira Ayuso <pablo@...filter.org> 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?