net/pkt: Add a debug option to track free pkt access more easily

Now that net_pkt are accessed through a common r/w API, using below a
net_pkt_cursor, let's have an option that will reset this cursor once
the net_pkt is freed.

Result is instead of segfaulting on r/w access, these operations will
bail out properly. Subsequent, and logical (unless you have a leak
which is another issue) net_pkt_unref will tell you who/where the pkt
was freed. Without it, you will get a segfault for instance, but that
won't tell you the exact reason. This options can help you then.

Signed-off-by: Tomasz Bursztyka <[email protected]>

Author: Tomasz Bursztyka

subsys/net/ip/Kconfig.debug

@@ -36,6 +36,17 @@ config NET_DEBUG_NET_PKT_EXTERNALS
 	  This value is used when allocating space for tracking the
 	  memory allocations.
 
+config NET_DEBUG_NET_PKT_NON_FRAGILE_ACCESS
+	bool "Reduce r/w fragility by resetting the packet cursor when freed"
+	default n
+	select NET_DEBUG_NET_PKT_ALLOC
+	help
+	  This MUST not be used unless you have an hard to catch bug. This will
+	  reset the pkt cursor when it's freed, so any subsequent r/w operations
+	  will not segfault, but just bail out and hopefuly it will enable you
+	  to know who/where the packet was freed already. Do not set this, by
+	  any means, unless you are actively debugging.
+
 if !NET_RAW_MODE
 
 module = NET_CORE

subsys/net/ip/net_pkt.c

@@ -817,6 +817,11 @@ void net_pkt_unref(struct net_pkt *pkt)
 		net_pkt_frag_unref(pkt->frags);
 	}
 
+	if (IS_ENABLED(CONFIG_NET_DEBUG_NET_PKT_NON_FRAGILE_ACCESS)) {
+		pkt->buffer = NULL;
+		net_pkt_cursor_init(pkt);
+	}
+
 	k_mem_slab_free(pkt->slab, (void **)&pkt);
 }