netfilter: nf_osf: add nf_osf_find()

This new function returns the OS genre as a string. Plan is to use to
from the new nft_osf extension.

Note that this doesn't yet support ttl options, but it could be easily
extended to do so.
Tested-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/linux/netfilter/nf_osf.h

+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _NFOSF_H
+#define _NFOSF_H
+
 #include <uapi/linux/netfilter/nf_osf.h>
 
 /* Initial window size option state machine: multiple of mss, mtu or
@@ -31,3 +35,8 @@ bool nf_osf_match(const struct sk_buff *skb, u_int8_t family,
 		  int hooknum, struct net_device *in, struct net_device *out,
 		  const struct nf_osf_info *info, struct net *net,
 		  const struct list_head *nf_osf_fingers);
+
+const char *nf_osf_find(const struct sk_buff *skb,
+                        const struct list_head *nf_osf_fingers);
+
+#endif /* _NFOSF_H */

net/netfilter/nf_osf.c

@@ -249,4 +249,34 @@ nf_osf_match(const struct sk_buff *skb, u_int8_t family,
 }
 EXPORT_SYMBOL_GPL(nf_osf_match);
 
+const char *nf_osf_find(const struct sk_buff *skb,
+			const struct list_head *nf_osf_fingers)
+{
+	const struct iphdr *ip = ip_hdr(skb);
+	const struct nf_osf_user_finger *f;
+	unsigned char opts[MAX_IPOPTLEN];
+	const struct nf_osf_finger *kf;
+	struct nf_osf_hdr_ctx ctx;
+	const struct tcphdr *tcp;
+	const char *genre = NULL;
+
+	memset(&ctx, 0, sizeof(ctx));
+
+	tcp = nf_osf_hdr_ctx_init(&ctx, skb, ip, opts);
+	if (!tcp)
+		return false;
+
+	list_for_each_entry_rcu(kf, &nf_osf_fingers[ctx.df], finger_entry) {
+		f = &kf->finger;
+		if (!nf_osf_match_one(skb, f, -1, &ctx))
+			continue;
+
+		genre = f->genre;
+		break;
+	}
+
+	return genre;
+}
+EXPORT_SYMBOL_GPL(nf_osf_find);
+
 MODULE_LICENSE("GPL");