BUG/MINOR: quic: too short PADDING frame for too short packets

This bug arrvived with this commit:

    MINOR: quic: centralize padding for HP sampling on packet building

What was missed is the fact that at the centralization point for the
PADDING frame to add for too short packet, <len> payload length  already includes
<*pn_len> the packet number field length value.

So when computing the length of the PADDING frame, the packet field length must
not be considered and added to the payload length (<len>).

This bug leaded too short PADDING frame to too short packets. This was the case,
most of times with Application level packets with a 1-byte packet number field
followed by a 1-byte PING frame. A 1-byte PADDING frame was added in this case
in place of a correct 2-bytes PADDINF frame. The header packet protection of
such packet could not be removed by the clients as for instance for ngtcp2 with
such traces:

    I00001828 0x5a135c81e803f092c74bac64a85513b657 pkt could not decrypt packet number

No need to backport.

Author: haproxyFred

include/haproxy/quic_conn-t.h

@@ -145,9 +145,6 @@ enum quic_pkt_type {
 #define QUIC_PACKET_PNL_BITMASK      0x03
 #define QUIC_PACKET_PN_MAXLEN        4
 
-/* TLS algo supported by QUIC uses a 16-bytes sample for HP. */
-#define QUIC_HP_SAMPLE_LEN           16
-
 /*
  *  0                   1                   2                   3
  *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

src/quic_tx.c

@@ -1990,12 +1990,16 @@ static int qc_do_build_pkt(unsigned char *pos, const unsigned char *end,
 	 */
 
 	/* Add padding if packet is too small for HP sampling as specified
-	 * above. QUIC TLS algos relies on 16 bytes sample extracted 4 bytes
-	 * after PN offset. Thus, pn and payload must be at least 4 bytes long,
-	 * so that the sample will be extracted as the AEAD tag.
+	 * above. QUIC TLS algos relies on 16 bytes sample extracted
+	 * QUIC_PACKET_PN_MAXLEN(4) bytes after the PN offset Thus, pn and payload
+	 * must be at least QUIC_PACKET_PN_MAXLEN(4) bytes long, so that the sample
+	 * will be extracted as the AEAD tag.
+	 *
+	 * Note that from here, <len> includes <*pn_len>, the total frame lenghts,
+	 * and QUIC_TLS_TAG_LEN(16).
 	 */
-	if (*pn_len + len < QUIC_PACKET_PN_MAXLEN + QUIC_HP_SAMPLE_LEN) {
-		padding_len = QUIC_PACKET_PN_MAXLEN + QUIC_HP_SAMPLE_LEN - (*pn_len + len);
+	if (len < QUIC_PACKET_PN_MAXLEN + QUIC_TLS_TAG_LEN) {
+		padding_len = QUIC_PACKET_PN_MAXLEN + QUIC_TLS_TAG_LEN - len;
 		TRACE_PRINTF(TRACE_LEVEL_DEVELOPER, QUIC_EV_CONN_PHPKTS, qc, 0, 0, 0,
 		             "adding padding pn=%llu padding_len=%zu *pn_len=%zu"
 		             " len=%zu len_frms=%zu",