Revert "Merge branch 'ipv6-overflow-arith'"

Linus dislikes these changes. To not hold up the net-merge let's revert
it for now and fix the bug like Linus suggested.

This reverts commit ec3661b4, reversing
changes made to c80dbe04.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NHannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/compiler-gcc.h

@@ -237,10 +237,6 @@
 #define KASAN_ABI_VERSION 3
 #endif
 
-#if GCC_VERSION >= 50000
-#define CC_HAVE_BUILTIN_OVERFLOW
-#endif
-
 #endif	/* gcc version >= 40000 specific checks */
 
 #if !defined(__noclone)

include/linux/overflow-arith.h

-#pragma once
-
-#include <linux/kernel.h>
-
-#ifdef CC_HAVE_BUILTIN_OVERFLOW
-
-#define overflow_usub __builtin_usub_overflow
-
-#else
-
-static inline bool overflow_usub(unsigned int a, unsigned int b,
-				 unsigned int *res)
-{
-	*res = a - b;
-	return *res > a ? true : false;
-}
-
-#endif

net/ipv6/ip6_output.c

@@ -28,7 +28,6 @@
 
 #include <linux/errno.h>
 #include <linux/kernel.h>
-#include <linux/overflow-arith.h>
 #include <linux/string.h>
 #include <linux/socket.h>
 #include <linux/net.h>
@@ -585,10 +584,7 @@ int ip6_fragment(struct sock *sk, struct sk_buff *skb,
 		if (np->frag_size)
 			mtu = np->frag_size;
 	}
-
-	if (overflow_usub(mtu, hlen + sizeof(struct frag_hdr), &mtu) ||
-	    mtu <= 7)
-		goto fail_toobig;
+	mtu -= hlen + sizeof(struct frag_hdr);
 
 	frag_id = ipv6_select_ident(net, &ipv6_hdr(skb)->daddr,
 				    &ipv6_hdr(skb)->saddr);