Re: [PATCH] netfilter: TCPMSS: fix dropped packets when MSS option is unaligned
From: kernel test robot
Date: Thu May 28 2026 - 14:16:17 EST
Hi Kacper,
kernel test robot noticed the following build warnings:
[auto build test WARNING on nf-next/main]
[also build test WARNING on netfilter-nf/main linus/master v7.1-rc5 next-20260527]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Kacper-Kokot/netfilter-TCPMSS-fix-dropped-packets-when-MSS-option-is-unaligned/20260526-041308
base: https://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf-next.git main
patch link: https://lore.kernel.org/r/20260525201116.407338-2-kacper.kokot.44%40gmail.com
patch subject: [PATCH] netfilter: TCPMSS: fix dropped packets when MSS option is unaligned
config: s390-allmodconfig (https://download.01.org/0day-ci/archive/20260529/202605290221.PE1wkPWQ-lkp@xxxxxxxxx/config)
compiler: clang version 18.1.8 (https://github.com/llvm/llvm-project 3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260529/202605290221.PE1wkPWQ-lkp@xxxxxxxxx/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@xxxxxxxxx>
| Closes: https://lore.kernel.org/oe-kbuild-all/202605290221.PE1wkPWQ-lkp@xxxxxxxxx/
All warnings (new ones prefixed by >>):
>> net/netfilter/xt_TCPMSS.c:140:45: warning: & has lower precedence than !=; != will be evaluated first [-Wparentheses]
140 | if (((char *)&opt[i + 2] - (char *)tcph) & 0x1 != 0) {
| ^~~~~~~~~~
net/netfilter/xt_TCPMSS.c:140:45: note: place parentheses around the '!=' expression to silence this warning
140 | if (((char *)&opt[i + 2] - (char *)tcph) & 0x1 != 0) {
| ^
| ( )
net/netfilter/xt_TCPMSS.c:140:45: note: place parentheses around the & expression to evaluate it first
140 | if (((char *)&opt[i + 2] - (char *)tcph) & 0x1 != 0) {
| ^
| ( )
1 warning generated.
vim +140 net/netfilter/xt_TCPMSS.c
69
70 static int
71 tcpmss_mangle_packet(struct sk_buff *skb,
72 const struct xt_action_param *par,
73 unsigned int family,
74 unsigned int tcphoff,
75 unsigned int minlen)
76 {
77 const struct xt_tcpmss_info *info = par->targinfo;
78 struct tcphdr *tcph;
79 int len, tcp_hdrlen;
80 unsigned int i;
81 __be16 oldval;
82 u16 newmss;
83 u8 *opt;
84
85 /* This is a fragment, no TCP header is available */
86 if (par->fragoff != 0)
87 return 0;
88
89 if (skb_ensure_writable(skb, skb->len))
90 return -1;
91
92 len = skb->len - tcphoff;
93 if (len < (int)sizeof(struct tcphdr))
94 return -1;
95
96 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
97 tcp_hdrlen = tcph->doff * 4;
98
99 if (len < tcp_hdrlen || tcp_hdrlen < sizeof(struct tcphdr))
100 return -1;
101
102 if (info->mss == XT_TCPMSS_CLAMP_PMTU) {
103 struct net *net = xt_net(par);
104 unsigned int in_mtu = tcpmss_reverse_mtu(net, skb, family);
105 unsigned int min_mtu = min(dst_mtu(skb_dst(skb)), in_mtu);
106
107 if (min_mtu <= minlen) {
108 net_err_ratelimited("unknown or invalid path-MTU (%u)\n",
109 min_mtu);
110 return -1;
111 }
112 newmss = min_mtu - minlen;
113 } else
114 newmss = info->mss;
115
116 opt = (u_int8_t *)tcph;
117 for (i = sizeof(struct tcphdr); i <= tcp_hdrlen - TCPOLEN_MSS; i += optlen(opt, i)) {
118 if (opt[i] == TCPOPT_MSS && opt[i+1] == TCPOLEN_MSS) {
119 u_int16_t oldmss;
120 u16 csum_oldmss, csum_newmss;
121
122 oldmss = (opt[i+2] << 8) | opt[i+3];
123
124 /* Never increase MSS, even when setting it, as
125 * doing so results in problems for hosts that rely
126 * on MSS being set correctly.
127 */
128 if (oldmss <= newmss)
129 return 0;
130
131 opt[i+2] = (newmss & 0xff00) >> 8;
132 opt[i+3] = newmss & 0x00ff;
133
134 csum_oldmss = htons(oldmss);
135 csum_newmss = htons(newmss);
136
137 /* MSS may be unaligned; fix up the incremental checksum
138 * to avoid an invalid checksum and a dropped packet.
139 */
> 140 if (((char *)&opt[i + 2] - (char *)tcph) & 0x1 != 0) {
141 csum_oldmss = swab16(csum_oldmss);
142 csum_newmss = swab16(csum_newmss);
143 }
144
145 inet_proto_csum_replace2(&tcph->check, skb,
146 csum_oldmss, csum_newmss,
147 false);
148 return 0;
149 }
150 }
151
152 /* There is data after the header so the option can't be added
153 * without moving it, and doing so may make the SYN packet
154 * itself too large. Accept the packet unmodified instead.
155 */
156 if (len > tcp_hdrlen)
157 return 0;
158
159 /* tcph->doff has 4 bits, do not wrap it to 0 */
160 if (tcp_hdrlen >= 15 * 4)
161 return 0;
162
163 /*
164 * MSS Option not found ?! add it..
165 */
166 if (skb_tailroom(skb) < TCPOLEN_MSS) {
167 if (pskb_expand_head(skb, 0,
168 TCPOLEN_MSS - skb_tailroom(skb),
169 GFP_ATOMIC))
170 return -1;
171 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
172 }
173
174 skb_put(skb, TCPOLEN_MSS);
175
176 /*
177 * IPv4: RFC 1122 states "If an MSS option is not received at
178 * connection setup, TCP MUST assume a default send MSS of 536".
179 * IPv6: RFC 2460 states IPv6 has a minimum MTU of 1280 and a minimum
180 * length IPv6 header of 60, ergo the default MSS value is 1220
181 * Since no MSS was provided, we must use the default values
182 */
183 if (xt_family(par) == NFPROTO_IPV4)
184 newmss = min(newmss, (u16)536);
185 else
186 newmss = min(newmss, (u16)1220);
187
188 opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
189 memmove(opt + TCPOLEN_MSS, opt, len - sizeof(struct tcphdr));
190
191 inet_proto_csum_replace2(&tcph->check, skb,
192 htons(len), htons(len + TCPOLEN_MSS), true);
193 opt[0] = TCPOPT_MSS;
194 opt[1] = TCPOLEN_MSS;
195 opt[2] = (newmss & 0xff00) >> 8;
196 opt[3] = newmss & 0x00ff;
197
198 inet_proto_csum_replace4(&tcph->check, skb, 0, *((__be32 *)opt), false);
199
200 oldval = ((__be16 *)tcph)[6];
201 tcph->doff += TCPOLEN_MSS/4;
202 inet_proto_csum_replace2(&tcph->check, skb,
203 oldval, ((__be16 *)tcph)[6], false);
204 return TCPOLEN_MSS;
205 }
206
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