Algorithm modules can define their own private congestion signal types in the

top 8 bits of the 32 bit signal bit field space for internal use. These private
signals should not be leaked outside of a module.

Given that many algorithm modules use the NewReno hook functions to simplify
their implementation, the obvious place such a leak would show up is in the
NewReno cong_signal hook function.

- Show the full number of significant bits in the signal type definitions in
  <netinet/cc.h>.

- Add a bitmask to simplify figuring out if a given signal is in the private or
  public bit range.

- Add a sanity check in newreno_cong_signal() to ensure private signals are not
  being leaked into the hook function.

Sponsored by:	FreeBSD Foundation
Discussed with:	David Hayes <dahayes at swin edu au>
MFC after:	1 week
X-MFC with:	r215166

sys/netinet/cc.h

@@ -101,10 +101,12 @@ struct cc_var {
  * bits (0x01000000 - 0x80000000) are reserved for CC algos to declare their own
  * congestion signal types.
  */
-#define	CC_ECN		0x000001/* ECN marked packet received. */
-#define	CC_RTO		0x000002/* RTO fired. */
-#define	CC_RTO_ERR	0x000004/* RTO fired in error. */
-#define	CC_NDUPACK	0x000008/* Threshold of dupack's reached. */
+#define	CC_ECN		0x00000001	/* ECN marked packet received. */
+#define	CC_RTO		0x00000002	/* RTO fired. */
+#define	CC_RTO_ERR	0x00000004	/* RTO fired in error. */
+#define	CC_NDUPACK	0x00000008	/* Threshold of dupack's reached. */
+
+#define	CC_SIGPRIVMASK	0xFF000000	/* Mask to check if sig is private. */
 
 /*
  * Structure to hold data and function pointers that together represent a

sys/netinet/cc/cc_newreno.c

@@ -182,6 +182,10 @@ newreno_cong_signal(struct cc_var *ccv, uint32_t type)
 {
 	u_int win;
 
+	/* Catch algos which mistakenly leak private signal types. */
+	KASSERT((type & CC_SIGPRIVMASK) == 0,
+	    ("%s: congestion signal type 0x%08x is private\n", __func__, type));
+
 	win = max(CCV(ccv, snd_cwnd) / 2 / CCV(ccv, t_maxseg), 2) *
 	    CCV(ccv, t_maxseg);