linux-kernel - Re: [PATCH] ipv4: fib_rules: Fix possible infinite loop in fib_empty

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Message-Id: <20181228.211503.1983080144355610600.davem@davemloft.net>
Date:   Fri, 28 Dec 2018 21:15:03 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     yuehaibing@...wei.com
Cc:     kuznet@....inr.ac.ru, yoshfuji@...ux-ipv6.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] ipv4: fib_rules: Fix possible infinite loop in
 fib_empty_table

From: YueHaibing <yuehaibing@...wei.com>
Date: Wed, 26 Dec 2018 16:34:20 +0800

> diff --git a/net/ipv4/fib_rules.c b/net/ipv4/fib_rules.c
> index f8eb78d..1567e12 100644
> --- a/net/ipv4/fib_rules.c
> +++ b/net/ipv4/fib_rules.c
> @@ -200,9 +200,13 @@ static struct fib_table *fib_empty_table(struct net *net)
>  {
>  	u32 id;
>  
> -	for (id = 1; id <= RT_TABLE_MAX; id++)
> +	for (id = 1; id <= RT_TABLE_MAX; id++) {
>  		if (!fib_get_table(net, id))
>  			return fib_new_table(net, id);
> +
> +		if (id == RT_TABLE_MAX)
> +			break;
> +	}
>  	return NULL;
>  }

The loop now has two exit conditions, one of which (by your analysis
is completely impossible).

Please clean this up into a loop with better structure and no
impossible tests.  One approach could be simply:

	id = 1;
	while (1) {
	...
		if (id++ == RT_TABLE_MAX)
			break;
	}

Or even:

	id = 0;
	while (++id) {
	...
	}