[PATCH] uml: fix critical IP checksum corruption

From: Lars Ellenberg <Lars.Ellenberg@linbit.com>

Add a memory barrier to the assembly checksum code - the code was copied
straight from the i386 one, and the patch resyncs the code with the
original.  I'll check if the original code can be included directly (i.e.
"#include") after 2.6.9.

Without this patch, every 2.6 UML release corrupts the checksum of every
UDP fragmented packet with size >= MTU (verified by various people, we all
agree on this issue; nobody reported "Works fine here").  The corrupted
packets are not accepted, thus blocking any kind of communication with
large-sized UDP packets.

In fact, I've even dissected the UML -> host traffic before and after this
patch with Ethereal - and it always reported an incorrect checksum for
fragmented UDP packets before and always correct after applying the patch.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade_spam@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/include/sysdep-i386/checksum.h

@@ -103,7 +103,8 @@ static inline unsigned short ip_fast_csum(unsigned char * iph,
 	   are modified, we must also specify them as outputs, or gcc
 	   will assume they contain their original values. */
 	: "=r" (sum), "=r" (iph), "=r" (ihl)
-	: "1" (iph), "2" (ihl));
+	: "1" (iph), "2" (ihl)
+	: "memory");
 	return(sum);
 }