Bug #792 avoid assertion and other fixes

SLL (Linux cooked packets v1) caused a crash due to an overly aggressive assert.

While here, fixed an issue where resultant packets were corrupt (wrong size, incorrect protocol)

docs/CHANGELOG

@@ -16,7 +16,9 @@
     - configure.ac: unify search dirs for pcap and add lib32 (#819)
     - CVE-2023-4256 double free in tcprewrite DLT_JUNIPER_ETHER (#813 #851)
     - dlt_jnpr_ether_cleanup: check config before cleanup (#812 #851)
-    - nanosecond timestamps (#796)
+    - SEGV on invalid Juniper Ethernet header length (#811)
+    - nanosecond timestamps support (#796)
+    - Linux cooked packet fatal error (#792)
     - low PPS values run at full speed after several days (#779)
     - create DLT_LINUX_SLL2 plugin (#727)
 

src/tcpedit/plugins/dlt_en10mb/en10mb.c

@@ -519,9 +519,9 @@ dlt_en10mb_encode(tcpeditdlt_t *ctx, u_char *packet, int pktlen, tcpr_dir_t dir)
     }
 
     /* newl2len for some other DLT -> ethernet */
-    else if (config->vlan == TCPEDIT_VLAN_ADD) {
-        /* if add a vlan then 18, */
-        newl2len = TCPR_802_1Q_H;
+    else {
+        newl2len = config->vlan == TCPEDIT_VLAN_ADD ? TCPR_802_1Q_H : TCPR_802_3_H;
+        oldl2len = ctx->l2len;
     }
 
     if ((uint32_t)pktlen < newl2len || pktlen + newl2len - ctx->l2len > MAXPACKET) {
@@ -555,7 +555,6 @@ dlt_en10mb_encode(tcpeditdlt_t *ctx, u_char *packet, int pktlen, tcpr_dir_t dir)
 
     /* update the total packet length */
     pktlen += (int)(newl2len - oldl2len);
-    ctx->l2len += (int)(newl2len - oldl2len);
 
     /* set the src & dst address as the first 12 bytes */
     eth = (struct tcpr_ethernet_hdr *)(packet + ctx->l2offset);
@@ -665,6 +664,11 @@ dlt_en10mb_encode(tcpeditdlt_t *ctx, u_char *packet, int pktlen, tcpr_dir_t dir)
         }
     }
 
+    if (newl2len == TCPR_802_3_H) {
+        /* all we need for 802.3 is the proto */
+        eth->ether_type = ctx->proto;
+    }
+
     if (config->vlan == TCPEDIT_VLAN_ADD || (config->vlan == TCPEDIT_VLAN_OFF && extra->vlan)) {
         vlan_hdr = (struct tcpr_802_1q_hdr *)(packet + extra->vlan_offset);
         if (config->vlan == TCPEDIT_VLAN_ADD) {
@@ -812,7 +816,7 @@ dlt_en10mb_merge_layer3(tcpeditdlt_t *ctx, u_char *packet, int pktlen, u_char *i
     if (l2len == -1 || pktlen < l2len)
         return NULL;
 
-    assert(ctx->decoded_extra_size == sizeof(*extra));
+    assert(ctx->decoded_extra_size >= sizeof(*extra));
     extra = (en10mb_extra_t *)ctx->decoded_extra;
     eth = (struct tcpr_ethernet_hdr *)(packet + ctx->l2offset);
     assert(eth);