Re: [PATCH 1/6] Netfilter: Merge ipt_LOG and ip6_LOG into xt_LOG

From: Jan Engelhardt
Date: Wed Feb 01 2012 - 09:47:47 EST

On Wednesday 2012-02-01 10:14, richard -rw- weinberger wrote:
>> Do you have the patchset as a retrievable git tree, btw?
>Only for kernel stuff:
>git:// xt_LOG
>If you want it for the user space part too, I can setup something...

That would be welcome.

>>>[PATCH 6/6] iptables: xt_LOG: Add ring buffer support
>>>in libxt_LOG.c:
>>>+.size = XT_ALIGN(sizeof(struct xt_log_info_v1) + sizeof(void *)),
>> Why the extra void*?
>The kernel space part of xt_log_info_v1 differs from the user space struct,
>it contains an extra pointer to the ring buffer context.
>Iptables must not see this extra pointer because the kernel writes to it.

I don't see that extra pointer (outside xt_log_info_v1) that you talk about.

+struct xt_log_info_v1 {
+ unsigned char level;
+ unsigned char logflags;
+ char prefix[30];
+ char ring_name[30];
+ unsigned long ring_size;
+ struct xt_LOG_ring_ctx *rctx;

This is the struct as you defined it. First, you must not use
unsigned long (due to its flexible size, which is an ABI breaker; cf.
"Writing Netfilter Modules" PDF), and the pointer must be tagged for
8-alignment as well for the same reason.

Second, the kernel will reject a ruleset if (kernel) .targetsize != (user)
.size. This is likely to occur, since you essentially have
.targetsize=72 in xt_LOG.c and .size=72+sizeof(void*)
in libxt_LOG.

Check commit 8a252e8941cd50e4390e6fedf280ba1c7ff9ac43 for
trailing whitespace, git log -p shows them.

+ if (par->family == NFPROTO_IPV4)
+ m = ipt_log_packet(NFPROTO_IPV4, par->hooknum, skb, par->in,
+ par->out, &li, loginfo->prefix);
+ else if (par->family == NFPROTO_IPV6)
+ m = ip6t_log_packet(NFPROTO_IPV6, par->hooknum, skb, par->in,
+ par->out, &li, loginfo->prefix);
+ else
+ BUG();

Please avoid BUG. Either do nothing in the 'else' case, or print
a courtesy message with some unkn_log_packet() or
WARN_ON_ONCE(). The same goes for xt_LOG_ring_exit, which I vote
for being reduced to WARN_ON as well. If the condition triggers,
one will have a little memory leak, but at least the machine
still runs.
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