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From:   David Howells <dhowells@redhat.com>
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To:     syzbot+62cbf263225ae13ff153@syzkaller.appspotmail.com
Cc:     dhowells@redhat.com, Eric Dumazet <edumazet@google.com>,
        bpf@vger.kernel.org, davem@davemloft.net, dsahern@kernel.org,
        kuba@kernel.org, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, pabeni@redhat.com,
        syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] [net?] WARNING in __ip6_append_data
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Date:   Mon, 18 Sep 2023 11:11:01 +0100
Message-ID: <3905313.1695031861@warthog.procyon.org.uk>
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#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.gi=
t 65d6e954e378

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 0665e8b09968..7978335c1fc4 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1517,8 +1517,10 @@ static int __ip6_append_data(struct sock *sk,
 		     rt->rt6i_nfheader_len;
 =

 	if (mtu <=3D fragheaderlen ||
-	    ((mtu - fragheaderlen) & ~7) + fragheaderlen <=3D sizeof(struct frag=
_hdr))
+	    ((mtu - fragheaderlen) & ~7) + fragheaderlen <=3D sizeof(struct frag=
_hdr)) {
+		printk("__%u__: EMSGSIZE\n", __LINE__);
 		goto emsgsize;
+	}
 =

 	maxfraglen =3D ((mtu - fragheaderlen) & ~7) + fragheaderlen -
 		     sizeof(struct frag_hdr);
@@ -1526,8 +1528,10 @@ static int __ip6_append_data(struct sock *sk,
 	/* as per RFC 7112 section 5, the entire IPv6 Header Chain must fit
 	 * the first fragment
 	 */
-	if (headersize + transhdrlen > mtu)
+	if (headersize + transhdrlen > mtu) {
+		printk("__%u__: EMSGSIZE\n", __LINE__);
 		goto emsgsize;
+	}
 =

 	if (cork->length + length > mtu - headersize && ipc6->dontfrag &&
 	    (sk->sk_protocol =3D=3D IPPROTO_UDP ||
@@ -1535,15 +1539,23 @@ static int __ip6_append_data(struct sock *sk,
 	     sk->sk_protocol =3D=3D IPPROTO_RAW)) {
 		ipv6_local_rxpmtu(sk, fl6, mtu - headersize +
 				sizeof(struct ipv6hdr));
+		printk("__%u__: EMSGSIZE\n", __LINE__);
 		goto emsgsize;
 	}
 =

-	if (ip6_sk_ignore_df(sk))
+	if (ip6_sk_ignore_df(sk)) {
 		maxnonfragsize =3D sizeof(struct ipv6hdr) + IPV6_MAXPLEN;
-	else
+		printk("MAXPLEN\n");
+	} else {
 		maxnonfragsize =3D mtu;
+		printk("mtu\n");
+	}
 =

+	printk("check %d %zd %d %d, %d %d\n",
+	       cork->length, length, maxnonfragsize, headersize,
+	       transhdrlen, mtu);
 	if (cork->length + length > maxnonfragsize - headersize) {
+		printk("__%u__: EMSGSIZE\n", __LINE__);
 emsgsize:
 		pmtu =3D max_t(int, mtu - headersize + sizeof(struct ipv6hdr), 0);
 		ipv6_local_error(sk, EMSGSIZE, fl6, pmtu);
@@ -1817,8 +1829,10 @@ static int __ip6_append_data(struct sock *sk,
 			if (!skb_can_coalesce(skb, i, pfrag->page,
 					      pfrag->offset)) {
 				err =3D -EMSGSIZE;
-				if (i =3D=3D MAX_SKB_FRAGS)
+				if (i =3D=3D MAX_SKB_FRAGS) {
+					printk("__%u__: EMSGSIZE\n", __LINE__);
 					goto error;
+				}
 =

 				__skb_fill_page_desc(skb, i, pfrag->page,
 						     pfrag->offset, 0);
diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index ed8ebb6f5909..daaaf60dce01 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -502,6 +502,8 @@ static int l2tp_ip6_sendmsg(struct sock *sk, struct ms=
ghdr *msg, size_t len)
 	int ulen;
 	int err;
 =

+	printk("%s()\n", __func__);
+
 	/* Rough check on arithmetic overflow,
 	 * better check is made in ip6_append_data().
 	 */