QUIC TXP: Handle padding correctly for ACK_ONLY archetype

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from #22615)

(cherry picked from commit e1c15a8)

ssl/quic/quic_txp.c

@@ -828,35 +828,51 @@ int ossl_quic_tx_packetiser_generate(OSSL_QUIC_TX_PACKETISER *txp,
     if (need_padding) {
         size_t total_dgram_size = 0;
         const size_t min_dpl = QUIC_MIN_INITIAL_DGRAM_LEN;
-        uint32_t first_el = QUIC_ENC_LEVEL_NUM;
+        uint32_t pad_el = QUIC_ENC_LEVEL_NUM;
 
         for (enc_level = QUIC_ENC_LEVEL_INITIAL;
              enc_level < QUIC_ENC_LEVEL_NUM;
              ++enc_level)
             if (pkt[enc_level].h_valid && pkt[enc_level].h.bytes_appended > 0) {
-                if (first_el == QUIC_ENC_LEVEL_NUM)
-                    first_el = enc_level;
+                if (pad_el == QUIC_ENC_LEVEL_NUM
+                    /*
+                     * We might not be able to add padding, for example if we
+                     * are using the ACK_ONLY archetype.
+                     */
+                    && pkt[enc_level].geom.adata.allow_padding
+                    && !pkt[enc_level].h.done_implicit)
+                    pad_el = enc_level;
 
                 txp_pkt_postgen_update_pkt_overhead(&pkt[enc_level], txp);
                 total_dgram_size += pkt[enc_level].geom.pkt_overhead
                     + pkt[enc_level].h.bytes_appended;
             }
 
-        if (first_el != QUIC_ENC_LEVEL_NUM
-            && total_dgram_size < min_dpl) {
+        if (pad_el != QUIC_ENC_LEVEL_NUM && total_dgram_size < min_dpl) {
             size_t deficit = min_dpl - total_dgram_size;
 
-            if (!ossl_assert(!pkt[first_el].h.done_implicit))
+            if (!txp_pkt_append_padding(&pkt[pad_el], txp, deficit))
                 goto out;
 
-            if (!txp_pkt_append_padding(&pkt[first_el], txp, deficit))
-                goto out;
+            total_dgram_size += deficit;
 
             /*
              * Padding frames make a packet ineligible for being a non-inflight
              * packet.
              */
-            pkt[first_el].tpkt->ackm_pkt.is_inflight = 1;
+            pkt[pad_el].tpkt->ackm_pkt.is_inflight = 1;
+        }
+
+        /*
+         * If we have failed to make a datagram of adequate size, for example
+         * because we have a padding requirement but are using the ACK_ONLY
+         * archetype (because we are CC limited), which precludes us from
+         * sending padding, give up on generating the datagram - there is
+         * nothing we can do.
+         */
+        if (total_dgram_size < min_dpl) {
+            res = 1;
+            goto out;
         }
     }
 
@@ -875,11 +891,17 @@ int ossl_quic_tx_packetiser_generate(OSSL_QUIC_TX_PACKETISER *txp,
 
         rc = txp_pkt_commit(txp, &pkt[enc_level], archetype,
                             &txpim_pkt_reffed);
-        if (rc)
+        if (rc) {
             status->sent_ack_eliciting
                 = status->sent_ack_eliciting
                 || pkt[enc_level].tpkt->ackm_pkt.is_ack_eliciting;
 
+            if (enc_level == QUIC_ENC_LEVEL_HANDSHAKE)
+                status->sent_handshake
+                    = (pkt[enc_level].h_valid
+                       && pkt[enc_level].h.bytes_appended > 0);
+        }
+
         if (txpim_pkt_reffed)
             pkt[enc_level].tpkt = NULL; /* don't free */
 
@@ -889,10 +911,6 @@ int ossl_quic_tx_packetiser_generate(OSSL_QUIC_TX_PACKETISER *txp,
         ++pkts_done;
     }
 
-    status->sent_handshake
-        = (pkt[QUIC_ENC_LEVEL_HANDSHAKE].h_valid
-           && pkt[QUIC_ENC_LEVEL_HANDSHAKE].h.bytes_appended > 0);
-
     /* Flush & Cleanup */
     res = 1;
 out: