From e31902dc4cb1ceec375b34d925be48a16983a485 Mon Sep 17 00:00:00 2001
From: Sylvain Rochet <gradator@gradator.net>
Date: Sun, 24 Jan 2016 17:48:57 +0100
Subject: [PATCH] PPP, prepare for chained compressors (e.g. VJ+MPPE) requiring
 packet copy

lwIP TCP stack requires that we don't change the packet payload in netif
output functions in order to resend the pbuf if we don't receive a TCP
ACK in time, therefore we copy the whole packet before compression or
encryption.

It gets tricky because we should keep track of whether we previously
copied the buffer in PPP output netif function in order to free the
previous pbuf if necessary, BUT the first pbuf passed to the netif
output function must not be freed.

The worst case is VJ compression followed by MPPE encryption, in this
case we should free the packet allocated by VJ compressor after MPPE
encryption and we should free the packet allocated by MPPE encrypter
after calling the PPP low level output function.

Signed-off-by: Sylvain Rochet <gradator@gradator.net>
---
 src/netif/ppp/ppp.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

diff --git a/src/netif/ppp/ppp.c b/src/netif/ppp/ppp.c
index 30c6ffef..d0f4079c 100644
--- a/src/netif/ppp/ppp.c
+++ b/src/netif/ppp/ppp.c
@@ -456,10 +456,9 @@ static err_t ppp_netif_output_ip6(struct netif *netif, struct pbuf *pb, const ip
 #endif /* PPP_IPV6_SUPPORT */
 
 static err_t ppp_netif_output(struct netif *netif, struct pbuf *pb, u16_t protocol) {
-#if MPPE_SUPPORT
-  err_t err;
-#endif /* MPPE_SUPPORT */
   ppp_pcb *pcb = (ppp_pcb*)netif->state;
+  err_t err;
+  struct pbuf *fpb = NULL;
 
   /* Check that the link is up. */
   if (0
@@ -519,12 +518,17 @@ static err_t ppp_netif_output(struct netif *netif, struct pbuf *pb, u16_t protoc
       LINK_STATS_INC(link.memerr);
       LINK_STATS_INC(link.drop);
       MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
-      return err;
+      goto err;
     }
-
-    err = pcb->link_cb->netif_output(pcb, pcb->link_ctx_cb, pb, PPP_COMP);
-    pbuf_free(pb);
-    return err;
+    /* if VJ compressor returned a new allocated pbuf, free it */
+    if (fpb) {
+      pbuf_free(fpb);
+    }
+    /* mppe_compress() returns a new allocated pbuf, indicate we should free
+     * our duplicated pbuf later */
+    fpb = pb;
+    protocol = PPP_COMP;
+    break;
 #endif /* MPPE_SUPPORT */
   default:
     PPPDEBUG(LOG_ERR, ("ppp_netif_output[%d]: bad CCP transmit method\n", pcb->netif->num));
@@ -532,13 +536,19 @@ static err_t ppp_netif_output(struct netif *netif, struct pbuf *pb, u16_t protoc
   }
 #endif /* CCP_SUPPORT */
 
-  return pcb->link_cb->netif_output(pcb, pcb->link_ctx_cb, pb, protocol);
+  err = pcb->link_cb->netif_output(pcb, pcb->link_ctx_cb, pb, protocol);
+  goto err;
 
 err_rte_drop:
+  err = ERR_RTE;
   LINK_STATS_INC(link.rterr);
   LINK_STATS_INC(link.drop);
   MIB2_STATS_NETIF_INC(netif, ifoutdiscards);
-  return ERR_RTE;
+err:
+  if (fpb) {
+    pbuf_free(fpb);
+  }
+  return err;
 }
 
 /************************************/