Subject: Re: [PATCH, take2] netfilter : struct xt_table_info diet

From: Patrick McHardy <[email-masked]>
Date: Thu, 15 Nov 2007 13:41:54 +0100

Eric Dumazet wrote:
> [PATCH] netfilter : struct xt_table_info diet
> 
> Instead of using a big array of NR_CPUS entries, we can compute the size
> needed at runtime, using nr_cpu_ids
> 
> This should save some ram (especially on David's machines where
> NR_CPUS=4096 :
> 32 KB can be saved per table, and 64KB for dynamically allocated ones
> (because
> of slab/slub alignements) )
> 
> In particular, the 'bootstrap' tables are not any more static (in data
> section) but on stack as their size is now very small.
> 
> This also should reduce the size used on stack in compat functions
> (get_info() declares an automatic variable, that could be bigger than
> kernel
> stack size for big NR_CPUS)

I fixed a compilation error with CONFIG_COMPAT and applied it, thanks
Eric. One question though:

> +#define XT_TABLE_INFO_SZ (offsetof(struct xt_table_info, entries) \
> +			  + nr_cpu_ids * sizeof(char *))

>  	/* overflow check */
> -	if (tmp.size >= (INT_MAX - sizeof(struct xt_table_info)) / NR_CPUS -
> -			SMP_CACHE_BYTES)
> +	if (tmp.size >= INT_MAX / num_possible_cpus())
>  		return -ENOMEM;

We need to make sure offsetof(struct xt_table_info, entries) +
nr_cpu_ids * sizeof(char *) doesn't overflow, so why doesn't it
use nr_cpu_ids here as well?