From 4b8f7a2110d80fdad0f09f376dbfd2204e2e8f8e Mon Sep 17 00:00:00 2001
From: cinap_lenrek <cinap_lenrek@felloff.net>
Date: Thu, 7 Mar 2019 22:39:50 +0100
Subject: [PATCH] devip: ignore the evil bit in fragment info field

using ~IP_DF mask to select offset and "more fragments" bits
includes the evil bit 15. so instead define a constant IP_FO
for the fragment offset bits and use (IP_MF|IP_FO). that way
the evil bit gets ignored and doesnt cause any useless calls
to ipreassemble().
---
 sys/src/9/ip/icmp.c | 2 +-
 sys/src/9/ip/ip.c   | 8 ++++----
 sys/src/9/ip/ip.h   | 1 +
 3 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/sys/src/9/ip/icmp.c b/sys/src/9/ip/icmp.c
index 90c289f4b..5a2a810e8 100644
--- a/sys/src/9/ip/icmp.c
+++ b/sys/src/9/ip/icmp.c
@@ -401,7 +401,7 @@ icmpiput(Proto *icmp, Ipifc*, Block *bp)
 			goto raise;
 		}
 		p = (Icmp *)bp->rp;
-		if((nhgets(p->frag) & ~(IP_DF|IP_MF)) == 0){
+		if((nhgets(p->frag) & IP_FO) == 0){
 			pr = Fsrcvpcolx(icmp->f, p->proto);
 			if(pr != nil && pr->advise != nil) {
 				(*pr->advise)(pr, bp, msg);
diff --git a/sys/src/9/ip/ip.c b/sys/src/9/ip/ip.c
index ace707632..0752cb6f0 100644
--- a/sys/src/9/ip/ip.c
+++ b/sys/src/9/ip/ip.c
@@ -333,7 +333,7 @@ ipiput4(Fs *f, Ipifc *ifc, Block *bp)
 		/* reassemble if the interface expects it */
 		if(nifc->reassemble){
 			frag = nhgets(h->frag);
-			if(frag & ~IP_DF) {
+			if(frag & (IP_MF|IP_FO)) {
 				bp = ip4reassemble(ip, frag, bp);
 				if(bp == nil)
 					return;
@@ -360,7 +360,7 @@ ipiput4(Fs *f, Ipifc *ifc, Block *bp)
 	}
 
 	frag = nhgets(h->frag);
-	if(frag & ~IP_DF) {
+	if(frag & (IP_MF|IP_FO)) {
 		bp = ip4reassemble(ip, frag, bp);
 		if(bp == nil)
 			return;
@@ -436,7 +436,7 @@ ip4reassemble(IP *ip, int offset, Block *bp)
 	 *  and get rid of any fragments that might go
 	 *  with it.
 	 */
-	if((offset & ~IP_DF) == 0) {
+	if((offset & (IP_MF|IP_FO)) == 0) {
 		if(f != nil) {
 			ip->stats[ReasmFails]++;
 			ipfragfree4(ip, f);
@@ -451,7 +451,7 @@ ip4reassemble(IP *ip, int offset, Block *bp)
 	}
 
 	fp = (Ipfrag*)bp->base;
-	fp->foff = (offset & 0x1fff)<<3;
+	fp->foff = (offset & IP_FO)<<3;
 	fp->flen = fragsize;
 
 	/* First fragment allocates a reassembly queue */
diff --git a/sys/src/9/ip/ip.h b/sys/src/9/ip/ip.h
index 1d7b369d9..8354abbe8 100644
--- a/sys/src/9/ip/ip.h
+++ b/sys/src/9/ip/ip.h
@@ -57,6 +57,7 @@ enum
 	IP_HLEN4=	5,		/* v4: Header length in words */
 	IP_DF=		0x4000,		/* v4: Don't fragment */
 	IP_MF=		0x2000,		/* v4: More fragments */
+	IP_FO=		0x1fff,		/* v4: Fragment offset */
 	IP4HDR=		IP_HLEN4<<2,	/* sizeof(Ip4hdr) */
 	IP_MAX=		64*1024,	/* Max. Internet packet size, v4 & v6 */