Re: [NETFILTER][PATCH] Re: Question about the hashlimit network mask patch

[Patrick McHardy <kaber@trash.net>]

[-- Attachment #1: Type: text/plain, Size: 443 bytes --]

Jan Engelhardt wrote:
> [NETFILTER]: xt_hashlimit: add workaround for >>32 case
> 
> Hardware surprisingly does nothing when a 32-bit right-shift is
> to be done. Worse yet, compilers do not even work around it.

Thats because the C standard states that the result is undefined.
Anyways, I think this patch is slightly nicer because it
gets rid of the double negation and the %32 == 0 special-casing
for IPv6.

Do you want to add an ACKed-by?

[-- Attachment #2: x --]
[-- Type: text/plain, Size: 1553 bytes --]

commit 830213d52cb7a7e003335003bd56bf82d6153dcf
Author: Patrick McHardy <kaber@trash.net>
Date:   Tue Apr 1 14:48:04 2008 +0200

    [NETFILTER]: xt_hashlimit: fix mask calculation
    
    Shifts larger than the data type are undefined, don't try to shift
    an u32 by 32. Also remove some special-casing of bitmasks divisible
    by 32.
    
    Based on patch by Jan Engelhardt <jengelh@computergmbh.de>.

diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c
index dc29007..40d344b 100644
--- a/net/netfilter/xt_hashlimit.c
+++ b/net/netfilter/xt_hashlimit.c
@@ -466,38 +466,25 @@ static inline void rateinfo_recalc(struct dsthash_ent *dh, unsigned long now)
 
 static inline __be32 maskl(__be32 a, unsigned int l)
 {
-	return htonl(ntohl(a) & ~(~(u_int32_t)0 >> l));
+	return l ? htonl(ntohl(a) & ~0 << (32 - l)) : 0;
 }
 
 #if defined(CONFIG_IP6_NF_IPTABLES) || defined(CONFIG_IP6_NF_IPTABLES_MODULE)
 static void hashlimit_ipv6_mask(__be32 *i, unsigned int p)
 {
 	switch (p) {
-	case 0:
-		i[0] = i[1] = 0;
-		i[2] = i[3] = 0;
-		break;
-	case 1 ... 31:
+	case 0 ... 31:
 		i[0] = maskl(i[0], p);
 		i[1] = i[2] = i[3] = 0;
 		break;
-	case 32:
-		i[1] = i[2] = i[3] = 0;
-		break;
-	case 33 ... 63:
+	case 32 ... 63:
 		i[1] = maskl(i[1], p - 32);
 		i[2] = i[3] = 0;
 		break;
-	case 64:
-		i[2] = i[3] = 0;
-		break;
-	case 65 ... 95:
+	case 64 ... 95:
 		i[2] = maskl(i[2], p - 64);
 		i[3] = 0;
-	case 96:
-		i[3] = 0;
-		break;
-	case 97 ... 127:
+	case 96 ... 127:
 		i[3] = maskl(i[3], p - 96);
 		break;
 	case 128: