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Fix Q-Block re-entrancy in rate-limiting pacing #1973

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Fix Q-Block re-entrancy in rate-limiting pacing #1973

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269f33b
Fix Q-Block re-entrancy in rate-limiting pacing
yavuzhaliloglu Apr 30, 2026
b63a789
Potential fix for pull request finding
yavuzhaliloglu Apr 30, 2026
80416e1
Save and restore is_rate_limiting in pacing loop to prevent re-entran…
yavuzhaliloglu Apr 30, 2026
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9 changes: 9 additions & 0 deletions include/coap3/coap_session_internal.h
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Original file line number Diff line number Diff line change
Expand Up @@ -261,6 +261,15 @@ struct coap_session_t {
uint64_t rl_ticks_per_packet; /**< If not 0, rate limit NON to ticks per packet */
coap_tick_t last_tx; /**< Last time a ratelimited packet is sent */
uint8_t is_rate_limiting; /**< Currently NON rate limiting */
uint8_t is_doing_q_block; /**< Q-Block send/response processing is in
progress on this session (e.g. 4.08
retransmit, 2.31 Continue triggering
next MAX_PAYLOADS set, or timer-driven
Q-Block restart sends). Forces
coap_send_internal() to still apply
pacing even though is_rate_limiting is
set by an outer wait loop in a
recursive io_process. */
uint32_t ping_failed; /**< Ping failure count */
};

Expand Down
62 changes: 54 additions & 8 deletions src/coap_block.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2245,6 +2245,16 @@ coap_block_check_q_block1_xmit(coap_session_t *session, coap_tick_t now, coap_ti
int ret = 0;

*tim_rem = COAP_MAX_DELAY_TICKS;
if (session->is_doing_q_block) {
/*
* A Q-Block send is already in flight on this session (e.g. an outer
* coap_send_q_blocks() iterating with rate-limit pacing, or a 4.08 /
* 2.31 response handler). Skip the timer-driven next-MAX_PAYLOAD-set
* push so we do not start an overlapping coap_send_q_blocks() for the
* same lg_xmit, which would re-send already-queued blocks.
*/
return ret;
}
LL_FOREACH_SAFE(session->lg_xmit, lg_xmit, q) {
coap_tick_t non_timeout = lg_xmit->non_timeout_random_ticks;

Expand All @@ -2265,7 +2275,9 @@ coap_block_check_q_block1_xmit(coap_session_t *session, coap_tick_t now, coap_ti
block.num = (uint32_t)(lg_xmit->offset / chunk);
block.m = lg_xmit->offset + chunk < lg_xmit->data_info->length;
block.szx = lg_xmit->blk_size;
session->is_doing_q_block = 1;
coap_send_q_blocks(session, lg_xmit, block, lg_xmit->sent_pdu, COAP_SEND_SKIP_PDU);
session->is_doing_q_block = 0;
if (*tim_rem > non_timeout) {
*tim_rem = non_timeout;
ret = 1;
Expand Down Expand Up @@ -2309,6 +2321,11 @@ coap_block_check_q_block2_xmit(coap_session_t *session, coap_tick_t now, coap_ti
int ret = 0;

*tim_rem = (coap_tick_t)-1;
if (session->is_doing_q_block) {
/* See coap_block_check_q_block1_xmit() — skip while a Q-Block send is
already in flight on this session to avoid overlapping sends. */
return ret;
}
LL_FOREACH_SAFE(session->lg_xmit, lg_xmit, q) {
coap_tick_t non_timeout = lg_xmit->non_timeout_random_ticks;

Expand All @@ -2329,8 +2346,11 @@ coap_block_check_q_block2_xmit(coap_session_t *session, coap_tick_t now, coap_ti
block.num = (uint32_t)(lg_xmit->offset / chunk);
block.m = lg_xmit->offset + chunk < lg_xmit->data_info->length;
block.szx = lg_xmit->blk_size;
if (block.num == (uint32_t)lg_xmit->last_block)
if (block.num == (uint32_t)lg_xmit->last_block) {
session->is_doing_q_block = 1;
coap_send_q_blocks(session, lg_xmit, block, lg_xmit->sent_pdu, COAP_SEND_SKIP_PDU);
session->is_doing_q_block = 0;
}
if (*tim_rem > non_timeout) {
*tim_rem = non_timeout;
ret = 1;
Expand Down Expand Up @@ -3908,8 +3928,14 @@ coap_handle_response_send_block(coap_session_t *session, coap_pdu_t *sent,
#if COAP_Q_BLOCK_SUPPORT
if (lg_xmit->option == COAP_OPTION_Q_BLOCK1 &&
pdu->type == COAP_MESSAGE_NON) {
if (coap_send_q_block1(session, block, pdu,
COAP_SEND_INC_PDU) == COAP_INVALID_MID)
coap_mid_t q_mid;
/* Save/restore — see 4.08 handler below for rationale. */
uint8_t saved_q_block = session->is_doing_q_block;

session->is_doing_q_block = 1;
q_mid = coap_send_q_block1(session, block, pdu, COAP_SEND_INC_PDU);
session->is_doing_q_block = saved_q_block;
if (q_mid == COAP_INVALID_MID)
goto fail_body;
return 1;
} else if (coap_send_internal(session, pdu, NULL) == COAP_INVALID_MID)
Expand Down Expand Up @@ -3990,15 +4016,25 @@ coap_handle_response_send_block(coap_session_t *session, coap_pdu_t *sent,
uint64_t token;
uint8_t ltoken[8];
size_t ltoken_length;
/*
* Save/restore is_doing_q_block: this handler can run nested
* inside an outer coap_send_q_blocks() (timer-driven next
* MAX_PAYLOAD set, etc.) that already set the flag. Plain
* set-then-clear would zero the outer's flag on our exit and
* let coap_block_check_q_block1_xmit() spawn an overlapping
* coap_send_q_blocks() while the outer is still iterating.
*/
uint8_t saved_q_block = session->is_doing_q_block;

session->is_doing_q_block = 1;
for (i = 0; (bp < data + length) &&
i < COAP_MAX_PAYLOADS(session); i++) {
if ((*bp & 0xc0) != 0x00) /* uint(value) */
goto fail_cbor;
goto fail_cbor_q_block;
block.num = derive_cbor_value(&bp, data + length - bp);
coap_log_debug("Q-Block1: Missing block %d\n", block.num);
if (block.num > (1 << 20) -1)
goto fail_cbor;
goto fail_cbor_q_block;
block.m = (block.num + 1) * chunk < lg_xmit->data_info->length;
block.szx = lg_xmit->blk_size;

Expand All @@ -4007,8 +4043,10 @@ coap_handle_response_send_block(coap_session_t *session, coap_pdu_t *sent,
ltoken_length = coap_encode_var_safe8(ltoken, sizeof(token), token);
pdu = coap_pdu_duplicate_lkd(lg_xmit->sent_pdu, session, ltoken_length,
ltoken, NULL, COAP_BOOL_FALSE);
if (!pdu)
if (!pdu) {
session->is_doing_q_block = saved_q_block;
goto fail_body;
}

coap_update_option(pdu, lg_xmit->option,
coap_encode_var_safe(buf, sizeof(buf),
Expand All @@ -4021,12 +4059,20 @@ coap_handle_response_send_block(coap_session_t *session, coap_pdu_t *sent,
lg_xmit->data_info->length,
lg_xmit->data_info->data,
block.num,
block.szx))
block.szx)) {
session->is_doing_q_block = saved_q_block;
goto fail_body;
if (coap_send_internal(session, pdu, NULL) == COAP_INVALID_MID)
}
if (coap_send_internal(session, pdu, NULL) == COAP_INVALID_MID) {
session->is_doing_q_block = saved_q_block;
goto fail_body;
}
}
session->is_doing_q_block = saved_q_block;
return 1;
fail_cbor_q_block:
session->is_doing_q_block = saved_q_block;
goto fail_cbor;
}
fail_cbor:
coap_log_info("Invalid application/missing-blocks+cbor-seq\n");
Expand Down
5 changes: 3 additions & 2 deletions src/coap_net.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2044,7 +2044,7 @@ coap_send_internal(coap_session_t *session, coap_pdu_t *pdu, coap_pdu_t *request
if (pdu->type == COAP_MESSAGE_NON && session->rl_ticks_per_packet) {
coap_tick_t now;

if (!session->is_rate_limiting) {
if (!session->is_rate_limiting || session->is_doing_q_block) {
coap_ticks(&now);
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while (1) {
uint32_t timeout_ms;
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Expand All @@ -2058,9 +2058,10 @@ coap_send_internal(coap_session_t *session, coap_pdu_t *pdu, coap_pdu_t *request
if (timeout_ms == 0) {
timeout_ms = COAP_IO_NO_WAIT;
}
uint8_t prev_rate_limiting = session->is_rate_limiting;
session->is_rate_limiting = 1;
coap_io_process_lkd(session->context, timeout_ms);
session->is_rate_limiting = 0;
session->is_rate_limiting = prev_rate_limiting;
coap_ticks(&now);
}
session->last_tx = now;
Expand Down
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