tls: rx: add counter for NoPad violations

As discussed with Maxim add a counter for true NoPad violations.
This should help deployments catch unexpected padded records vs
just control records which always need re-encryption.

https: //lore.kernel.org/all/[e-mail]/
Signed-off-by: Jakub Kicinski <[e-mail]>

Documentation/networking/tls.rst

···
   number of RX records which had to be re-decrypted due to
   ``TLS_RX_EXPECT_NO_PAD`` mis-prediction. Note that this counter will
   also increment for non-data records.
+
+- ``TlsRxNoPadViolation`` -
+  number of data RX records which had to be re-decrypted due to
+  ``TLS_RX_EXPECT_NO_PAD`` mis-prediction.

include/uapi/linux/snmp.h

···
 	LINUX_MIB_TLSDECRYPTERROR,		/* TlsDecryptError */
 	LINUX_MIB_TLSRXDEVICERESYNC,		/* TlsRxDeviceResync */
 	LINUX_MIB_TLSDECRYPTRETRY,		/* TlsDecryptRetry */
+	LINUX_MIB_TLSRXNOPADVIOL,		/* TlsRxNoPadViolation */
 	__LINUX_MIB_TLSMAX
 };
 

net/tls/tls_proc.c

···
 	SNMP_MIB_ITEM("TlsDecryptError", LINUX_MIB_TLSDECRYPTERROR),
 	SNMP_MIB_ITEM("TlsRxDeviceResync", LINUX_MIB_TLSRXDEVICERESYNC),
 	SNMP_MIB_ITEM("TlsDecryptRetry", LINUX_MIB_TLSDECRYPTRETRY),
+	SNMP_MIB_ITEM("TlsRxNoPadViolation", LINUX_MIB_TLSRXNOPADVIOL),
 	SNMP_MIB_SENTINEL
 };
 

net/tls/tls_sw.c

···
 	if (unlikely(darg->zc && prot->version == TLS_1_3_VERSION &&
 		     darg->tail != TLS_RECORD_TYPE_DATA)) {
 		darg->zc = false;
+		if (!darg->tail)
+			TLS_INC_STATS(sock_net(sk), LINUX_MIB_TLSRXNOPADVIOL);
 		TLS_INC_STATS(sock_net(sk), LINUX_MIB_TLSDECRYPTRETRY);
 		return decrypt_skb_update(sk, skb, dest, darg);
 	}