net: skb: add function name as part of reason

In addition to the line number ("__LINE__"), this is to make the
function name ("__func__", which is the caller of kfree_skb_reason())
as part of reason.

The (function, line) combination is able to uniquely represent where the
sk_buff is dropped.

The existing trace_kfree_skb() is able to trace the 'location'. While it
is fine to either echo 'stacktrace' to the trigger, or trace at
userspace via ebpf, the TP_printk will only print %p.

With the function name, the TP_printk will tell the function and the
line number that the sk_buff is dropped.

Cc: Joao Martins <joao.m.martins@oracle.com>
Cc: Joe Jin <joe.jin@oracle.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>

include/linux/skbuff.h

@@ -309,6 +309,8 @@ struct sk_buff;
 
 #define SKB_DROP_LINE_NONE	0
 #define SKB_DROP_LINE		__LINE__
+#define SKB_DROP_FUNC_NONE	"none"
+#define SKB_DROP_FUNC		__func__
 
 /* To allow 64K frame to be packed as single skb without frag_list we
  * require 64K/PAGE_SIZE pages plus 1 additional page to allow for
@@ -1090,15 +1092,16 @@ static inline bool skb_unref(struct sk_buff *skb)
 	return true;
 }
 
-void kfree_skb_reason(struct sk_buff *skb, unsigned int line);
+void kfree_skb_reason(struct sk_buff *skb, const char *func,
+		      unsigned int line);
 
 /**
  *	kfree_skb - free an sk_buff with 'NOT_SPECIFIED' reason
  *	@skb: buffer to free
  */
 static inline void kfree_skb(struct sk_buff *skb)
 {
-	kfree_skb_reason(skb, SKB_DROP_LINE_NONE);
+	kfree_skb_reason(skb, SKB_DROP_FUNC_NONE, SKB_DROP_LINE_NONE);
 }
 
 void skb_release_head_state(struct sk_buff *skb);

include/trace/events/skb.h

@@ -15,27 +15,29 @@
 TRACE_EVENT(kfree_skb,
 
 	TP_PROTO(struct sk_buff *skb, void *location,
-		 unsigned int line),
+		 const char *function, unsigned int line),
 
-	TP_ARGS(skb, location, line),
+	TP_ARGS(skb, location, function, line),
 
 	TP_STRUCT__entry(
 		__field(void *,		skbaddr)
 		__field(void *,		location)
 		__field(unsigned short,	protocol)
+		__string(function, function)
 		__field(unsigned int,	line)
 	),
 
 	TP_fast_assign(
 		__entry->skbaddr = skb;
 		__entry->location = location;
 		__entry->protocol = ntohs(skb->protocol);
+		__assign_str(function, function);
 		__entry->line = line;
 	),
 
-	TP_printk("skbaddr=%p protocol=%u location=%p line: %u",
+	TP_printk("skbaddr=%p protocol=%u location=%p function=%s line=%u",
 		  __entry->skbaddr, __entry->protocol, __entry->location,
-		  __entry->line)
+		  __get_str(function), __entry->line)
 );
 
 TRACE_EVENT(consume_skb,

net/core/dev.c

@@ -4900,6 +4900,7 @@ static __latent_entropy void net_tx_action(struct softirq_action *h)
 				trace_consume_skb(skb);
 			else
 				trace_kfree_skb(skb, net_tx_action,
+						SKB_DROP_FUNC,
 						SKB_DROP_LINE);
 
 			if (skb->fclone != SKB_FCLONE_UNAVAILABLE)

net/core/skbuff.c

@@ -761,17 +761,20 @@ EXPORT_SYMBOL(__kfree_skb);
 /**
  *	kfree_skb_reason - free an sk_buff with special reason
  *	@skb: buffer to free
+ *	@func: the function where this skb is dropped
  *	@line: the line where this skb is dropped
  *
  *	Drop a reference to the buffer and free it if the usage count has
- *	hit zero. Meanwhile, pass the drop line to 'kfree_skb' tracepoint.
+ *	hit zero. Meanwhile, pass the drop function and line to 'kfree_skb'
+ *	tracepoint.
  */
-void kfree_skb_reason(struct sk_buff *skb, unsigned int line)
+void kfree_skb_reason(struct sk_buff *skb, const char *func,
+		      unsigned int line)
 {
 	if (!skb_unref(skb))
 		return;
 
-	trace_kfree_skb(skb, __builtin_return_address(0), line);
+	trace_kfree_skb(skb, __builtin_return_address(0), func, line);
 	__kfree_skb(skb);
 }
 EXPORT_SYMBOL(kfree_skb_reason);

net/ipv4/tcp_ipv4.c

@@ -2149,7 +2149,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
 
 discard_it:
 	/* Discard frame. */
-	kfree_skb_reason(skb, drop_line);
+	kfree_skb_reason(skb, SKB_DROP_FUNC, drop_line);
 	return 0;
 
 discard_and_relse:

net/ipv4/udp.c

@@ -2477,7 +2477,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 	 * Hmm.  We got an UDP packet to a port to which we
 	 * don't wanna listen.  Ignore it.
 	 */
-	kfree_skb_reason(skb, drop_line);
+	kfree_skb_reason(skb, SKB_DROP_FUNC, drop_line);
 	return 0;
 
 short_packet:
@@ -2502,7 +2502,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 	__UDP_INC_STATS(net, UDP_MIB_CSUMERRORS, proto == IPPROTO_UDPLITE);
 drop:
 	__UDP_INC_STATS(net, UDP_MIB_INERRORS, proto == IPPROTO_UDPLITE);
-	kfree_skb_reason(skb, drop_line);
+	kfree_skb_reason(skb, SKB_DROP_FUNC, drop_line);
 	return 0;
 }