net: l2: ppp: Explicitly negotiate ACCM

Many cellular modems attempt to negotiate an ACCM value of 0x00000000.
While the PPP driver rejects this by default, it does not propose an
alternative. As a result, some modems default to using 0x00000000 after
LCP negotiation. Because the PPP driver expects all control characters
to be escaped, this causes issues during decoding. This change
negotiates an ACCM value of 0xffffffff to ensure compatibility with such
modems.

Signed-off-by: Vytautas Virvičius <vytautas@virvicius.dev>

Author: vytvir

include/zephyr/net/ppp.h

@@ -378,6 +378,8 @@ struct lcp_options {
 };
 
 #if defined(CONFIG_NET_L2_PPP_OPTION_MRU)
+#define LCP_NUM_MY_OPTIONS	2
+#else
 #define LCP_NUM_MY_OPTIONS	1
 #endif
 
@@ -422,9 +424,9 @@ struct ppp_context {
 
 		/** Magic-Number value */
 		uint32_t magic;
-#if defined(CONFIG_NET_L2_PPP_OPTION_MRU)
+
+		/** Options data */
 		struct ppp_my_option_data my_options_data[LCP_NUM_MY_OPTIONS];
-#endif
 	} lcp;
 
 #if defined(CONFIG_NET_IPV4)

subsys/net/l2/ppp/lcp.c

@@ -268,15 +268,79 @@ static int lcp_nak_mru(struct ppp_context *ctx, struct net_pkt *pkt,
 
 	return 0;
 }
+#endif
+
+#define ASYNC_MAP_OPTION_LEN 6
+
+static int lcp_add_async_map(struct ppp_context *ctx, struct net_pkt *pkt)
+{
+	net_pkt_write_u8(pkt, ASYNC_MAP_OPTION_LEN);
+	return net_pkt_write_be32(pkt, ctx->lcp.my_options.async_map);
+}
+
+static int lcp_ack_async_map(struct ppp_context *ctx, struct net_pkt *pkt,
+			     uint8_t oplen)
+{
+	int ret;
+	uint32_t async_map;
+
+	/* Handle ACK : */
+	if (oplen != sizeof(async_map)) {
+		return -EINVAL;
+	}
+
+	ret = net_pkt_read(pkt, &async_map, sizeof(async_map));
+	if (ret) {
+		return ret;
+	}
+	if (async_map != ctx->lcp.my_options.async_map) {
+		/* Didn't acked our ASYNC_MAP: */
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
+static int lcp_nak_async_map(struct ppp_context *ctx, struct net_pkt *pkt,
+			     uint8_t oplen)
+{
+	int ret;
+	uint16_t async_map;
+
+	/* Handle NAK: accept only equal to ours */
+	if (oplen != sizeof(async_map)) {
+		return -EINVAL;
+	}
+
+	ret = net_pkt_read(pkt, &async_map, sizeof(async_map));
+	if (ret) {
+		return ret;
+	}
+
+	if (async_map != ctx->lcp.my_options.async_map) {
+		return -EINVAL;
+	}
+
+	return 0;
+}
+
 
 static const struct ppp_my_option_info lcp_my_options[] = {
+#if defined(CONFIG_NET_L2_PPP_OPTION_MRU)
 	PPP_MY_OPTION(LCP_OPTION_MRU, lcp_add_mru, lcp_ack_mru, lcp_nak_mru),
+#endif
+	PPP_MY_OPTION(LCP_OPTION_ASYNC_CTRL_CHAR_MAP, lcp_add_async_map,
+			lcp_ack_async_map, lcp_nak_async_map),
 };
 BUILD_ASSERT(ARRAY_SIZE(lcp_my_options) == LCP_NUM_MY_OPTIONS);
 
 static struct net_pkt *lcp_config_info_add(struct ppp_fsm *fsm)
 {
-	return ppp_my_options_add(fsm, MRU_OPTION_LEN);
+#if defined(CONFIG_NET_L2_PPP_OPTION_MRU)
+	return ppp_my_options_add(fsm, MRU_OPTION_LEN + ASYNC_MAP_OPTION_LEN);
+#else
+	return ppp_my_options_add(fsm, ASYNC_MAP_OPTION_LEN);
+#endif
 }
 
 static int lcp_config_info_nack(struct ppp_fsm *fsm, struct net_pkt *pkt,
@@ -297,7 +361,6 @@ static int lcp_config_info_nack(struct ppp_fsm *fsm, struct net_pkt *pkt,
 
 	return 0;
 }
-#endif
 
 static void lcp_init(struct ppp_context *ctx)
 {
@@ -311,8 +374,8 @@ static void lcp_init(struct ppp_context *ctx)
 	ppp_fsm_name_set(&ctx->lcp.fsm, ppp_proto2str(PPP_LCP));
 
 	ctx->lcp.my_options.mru = net_if_get_mtu(ctx->iface);
+	ctx->lcp.my_options.async_map = 0xffffffff;
 
-#if defined(CONFIG_NET_L2_PPP_OPTION_MRU)
 	ctx->lcp.fsm.my_options.info = lcp_my_options;
 	ctx->lcp.fsm.my_options.data = ctx->lcp.my_options_data;
 	ctx->lcp.fsm.my_options.count = ARRAY_SIZE(lcp_my_options);
@@ -321,7 +384,6 @@ static void lcp_init(struct ppp_context *ctx)
 	ctx->lcp.fsm.cb.config_info_req = lcp_config_info_req;
 	ctx->lcp.fsm.cb.config_info_nack = lcp_config_info_nack;
 	ctx->lcp.fsm.cb.config_info_rej = ppp_my_options_parse_conf_rej;
-#endif
 
 	ctx->lcp.fsm.cb.up = lcp_up;
 	ctx->lcp.fsm.cb.down = lcp_down;