dco: port core/context infrastructure needed for backport of commit 7791f53

Change ovpn_dco_init() to take a reference to struct context, add a
backlink from the platform DCO contexts to the owning openvpn context,
and store the top multi context in struct context for server mode.

This prepares the tree for the follow-up backend changes from commit
7791f53 ("dco: process messages immediately after read") where Linux and
FreeBSD process DCO notifications directly from their backend read
paths.

It is part of a reworked backport of PR #945 originally proposed by
Nikolai Shelekhov <nickshv13@icloud.com>.

The original commits in master are

    commit a699681bb8
    Author: Antonio Quartulli <antonio@mandelbit.com>
    Date:   Wed Jul 23 15:39:11 2025 +0200

	dco: only pass struct context to init function

    commit 7f5a6deae3
    Author: Antonio Quartulli <antonio@mandelbit.com>
    Date:   Wed Jul 23 08:10:25 2025 +0200

	multi: store multi_context address inside top instance

Change-Id: I974e10ec91a0b63f52387f1406ce1b49145eb0be
Signed-off-by: Ralf Lici <ralf@mandelbit.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1631
Message-Id: <20260420165923.14226-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg36691.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>

src/openvpn/dco.h

@@ -104,11 +104,10 @@ bool dco_check_pull_options(int msglevel, const struct options *o);
 /**
  * Initialize the DCO context
  *
- * @param mode      the instance operating mode (P2P or multi-peer)
- * @param dco       the context to initialize
+ * @param c         the main instance context
  * @return          true on success, false otherwise
  */
-bool ovpn_dco_init(int mode, dco_context_t *dco);
+bool ovpn_dco_init(struct context *c);
 
 /**
  * Open/create a DCO interface
@@ -284,7 +283,7 @@ dco_check_pull_options(int msglevel, const struct options *o)
 }
 
 static inline bool
-ovpn_dco_init(int mode, dco_context_t *dco)
+ovpn_dco_init(struct context *c)
 {
     return true;
 }

src/openvpn/dco_freebsd.c

@@ -220,9 +220,11 @@ close_fd(dco_context_t *dco)
 }
 
 bool
-ovpn_dco_init(int mode, dco_context_t *dco)
+ovpn_dco_init(struct context *c)
 {
-    if (open_fd(dco) < 0)
+    c->c1.tuntap->dco.c = c;
+
+    if (open_fd(&c->c1.tuntap->dco) < 0)
     {
         msg(M_ERR, "Failed to open socket");
         return false;

src/openvpn/dco_freebsd.h

@@ -57,6 +57,7 @@ typedef struct dco_context {
     int dco_message_peer_id;
     int dco_del_peer_reason;
     struct sockaddr_storage dco_float_peer_ss;
+    struct context *c;
     uint64_t dco_read_bytes;
     uint64_t dco_write_bytes;
 } dco_context_t;

src/openvpn/dco_linux.c

@@ -391,9 +391,11 @@ ovpn_dco_init_netlink(dco_context_t *dco)
 }
 
 bool
-ovpn_dco_init(int mode, dco_context_t *dco)
+ovpn_dco_init(struct context *c)
 {
-    switch (mode)
+    dco_context_t *dco = &c->c1.tuntap->dco;
+
+    switch (c->mode)
     {
         case CM_TOP:
             dco->ifmode = OVPN_MODE_MP;
@@ -407,6 +409,10 @@ ovpn_dco_init(int mode, dco_context_t *dco)
             ASSERT(false);
     }
 
+    /* store pointer to context as it may be required by message
+     * parsing routines
+     */
+    dco->c = c;
     ovpn_dco_init_netlink(dco);
     return true;
 }

src/openvpn/dco_linux.h

@@ -43,6 +43,8 @@ typedef struct
     struct nl_cb *nl_cb;
     int status;
 
+    struct context *c;
+
     enum ovpn_mode ifmode;
 
     int ovpn_dco_id;

src/openvpn/dco_win.c

@@ -53,7 +53,7 @@ create_dco_handle(const char *devname, struct gc_arena *gc)
 }
 
 bool
-ovpn_dco_init(int mode, dco_context_t *dco)
+ovpn_dco_init(struct context *c)
 {
     return true;
 }

src/openvpn/init.c

@@ -1882,7 +1882,7 @@ do_open_tun(struct context *c, int *error_flags)
 #endif
         if (dco_enabled(&c->options))
         {
-            ovpn_dco_init(c->mode, &c->c1.tuntap->dco);
+            ovpn_dco_init(c);
         }
 
         /* open the tun device */

src/openvpn/mtcp.c

@@ -792,6 +792,7 @@ tunnel_server_tcp(struct context *top)
     int status;
 
     top->mode = CM_TOP;
+    top->multi = &multi;
     context_clear_2(top);
 
     /* initialize top-tunnel instance */

src/openvpn/mudp.c

@@ -466,6 +466,7 @@ tunnel_server_udp(struct context *top)
     struct multi_context multi;
 
     top->mode = CM_TOP;
+    top->multi = &multi;
     context_clear_2(top);
 
     /* initialize top-tunnel instance */

src/openvpn/openvpn.h

@@ -492,6 +492,9 @@ struct context
                                  *   CM_P2P, \c CM_TOP, \c CM_TOP_CLONE,
                                  *   \c CM_CHILD_UDP, and \c CM_CHILD_TCP. */
 
+    struct multi_context *multi; /**< Pointer to the main P2MP context.
+                                  *   Non-NULL only when mode == CM_TOP. */
+
     struct gc_arena gc;         /**< Garbage collection arena for
                                  *   allocations done in the scope of this
                                  *   context structure. */