Unaligend accesses nulldevname

Hi,

I found an excessive amount of unaligned accesses on my AXP workstation and tracked it down to ip_packet_match() in the ip_tables module. indev and outdev are not properly aligned if set to nulldevname in ipt_do_table().
This destroys the benefits of comparing names in units of (long) and on architectures with expensive unaligned accesses (such as ia64 or alpha), it even hurts a lot.

Find attached a patch against 2.6.0. A similar patch is needed for 2.4, also attached.

Please consider merging them.

Looking at ip_packet_match(), I have two more thoughts:
* It should not be inlined. It's too large to benefit from inlining, IMHO. (OTOH, it's only called from one place, so it does not really matter.)
* There's a comment about the compiler being able to unroll the 2/4 (64/32bit) iter loop which is not completely appropriate: We don't pass -funroll-loops, so gcc does not do it :-(
It would be beneficial though.

Regards,

Kurt Garloff  <[email protected]>                            Cologne, DE 
SUSE LINUX AG, Nuernberg, DE                          SUSE Labs (Head)

@@ -260,7 +260,7 @@
 	     struct ipt_table *table,
 	     void *userdata)
 {
-	static const char nulldevname[IFNAMSIZ];
+	static const char nulldevname[IFNAMSIZ] __attribute__((aligned(sizeof(long))));
 	u_int16_t offset;
 	struct iphdr *ip;
 	u_int16_t datalen;

@@ -259,7 +264,7 @@
 	     struct ipt_table *table,
 	     void *userdata)
 {
-	static const char nulldevname[IFNAMSIZ] = { 0 };
+	static const char nulldevname[IFNAMSIZ] __attribute__((aligned(sizeof(long)))) = { 0 };
 	u_int16_t offset;
 	struct iphdr *ip;
 	void *protohdr;

Thanks for submitting your patch, this is the same bug report as
https://bugzilla.netfilter.org/cgi-bin/bugzilla/show_bug.cgi?id=84

the fix is already in our patch-o-matic CVS tree, and I'm going to submit this to davem together with some other fixes I have pending.

That is the idea. It's just split in two functions to make it more readable. It's never intended to be called from somewhere else.

I'm not a compiler geek. Thanks for pointing this out, I will do some testing and look at the compiler output to see what is happening.

- Harald Welte <[email protected]>             http://www.netfilter.org/
============================================================================
  "Fragmentation is like classful addressing -- an interesting early
   architectural error that shows how much experimentation was going
   on while IP was being designed."                    -- Paul Vixie