[CCID 3]: Avoid `division by zero' errors
Several places of the code divide by the current RTT value. A division-by-zero
error results if this value is 0.
To protect against this,
* DCCP_BUG_ON conditions are added throughout the tx code;
* in ccid3_hc_tx_packet_recv, a code block is shifted, to avoid not updating
p and x_recv in case the RTT estimation results in an error;
Since this could probably be done in a smarter way, a FIXME is added. Lastly,
some minor code comment updates.
Signed-off-by: Gerrit Renker <gerrit@xxxxxxxxxxxxxx>
---
net/dccp/ccids/ccid3.c | 53 ++++++++++++++++++++-----------------
net/dccp/ccids/ccid3.h | 2 -
net/dccp/ccids/lib/tfrc_equation.c | 7 +---
3 files changed, 33 insertions(+), 29 deletions(-)
--- a/net/dccp/ccids/ccid3.c
+++ b/net/dccp/ccids/ccid3.c
@@ -117,7 +117,7 @@ static inline void ccid3_update_send_tim
TFRC_OPSYS_HALF_TIME_GRAN);
}
/*
- * Update X by
+ * Update X by (this implements step (4) of [RFC 3448, 4.3]):
* If (p > 0)
* x_calc = calcX(s, R, p);
* X = max(min(X_calc, 2 * X_recv), s / t_mbi);
@@ -125,12 +125,17 @@ static inline void ccid3_update_send_tim
* If (now - tld >= R)
* X = max(min(2 * X, 2 * X_recv), s / R);
* tld = now;
+ *
+ * If X has changed, we also update the scheduled send time t_now,
+ * the inter-packet interval t_ipi, and the delta value.
*/
static void ccid3_hc_tx_update_x(struct sock *sk)
{
struct ccid3_hc_tx_sock *hctx = ccid3_hc_tx_sk(sk);
const __u32 old_x = hctx->ccid3hctx_x;
+ DCCP_BUG_ON(hctx->ccid3hctx_rtt == 0);
+
/* To avoid large error in calcX */
if (hctx->ccid3hctx_p >= TFRC_SMALLEST_P) {
hctx->ccid3hctx_x_calc = tfrc_calc_x(hctx->ccid3hctx_s,
@@ -421,7 +426,6 @@ static void ccid3_hc_tx_packet_recv(stru
unsigned long next_tmout;
u32 t_elapsed;
u32 pinv;
- u32 x_recv;
u32 r_sample;
BUG_ON(hctx == NULL);
@@ -434,15 +438,11 @@ static void ccid3_hc_tx_packet_recv(stru
opt_recv = &hctx->ccid3hctx_options_received;
t_elapsed = dp->dccps_options_received.dccpor_elapsed_time * 10;
- x_recv = opt_recv->ccid3or_receive_rate;
pinv = opt_recv->ccid3or_loss_event_rate;
switch (hctx->ccid3hctx_state) {
case TFRC_SSTATE_NO_FBACK:
case TFRC_SSTATE_FBACK:
- /* Calculate new round trip sample by
- * R_sample = (now - t_recvdata) - t_delay */
- /* get t_recvdata from history */
packet = dccp_tx_hist_find_entry(&hctx->ccid3hctx_hist,
DCCP_SKB_CB(skb)->dccpd_ack_seq);
if (unlikely(packet == NULL)) {
@@ -453,7 +453,25 @@ static void ccid3_hc_tx_packet_recv(stru
return;
}
- /* Update RTT */
+ /* Update receive rate */
+ hctx->ccid3hctx_x_recv = opt_recv->ccid3or_receive_rate;
+
+ /* Update loss event rate */
+ if (pinv == ~0 || pinv == 0)
+ hctx->ccid3hctx_p = 0;
+ else {
+ hctx->ccid3hctx_p = 1000000 / pinv;
+
+ if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
+ hctx->ccid3hctx_p = TFRC_SMALLEST_P;
+ ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
+ dccp_role(sk), sk);
+ }
+ }
+
+ /* Calculate new round trip sample by
+ * R_sample = (t_now - t_recvdata) - t_delay
+ * t_recvdata is taken from packet history */
dccp_timestamp(sk, &now);
r_sample = timeval_delta(&now, &packet->dccphtx_tstamp);
if (unlikely(r_sample <= t_elapsed))
@@ -462,7 +480,7 @@ static void ccid3_hc_tx_packet_recv(stru
else
r_sample -= t_elapsed;
- /* Update RTT estimate by
+ /* Update RTT estimate by (step (2) of [RFC 3448, 4.3]):
* If (No feedback recv)
* R = R_sample;
* Else
@@ -481,21 +499,10 @@ static void ccid3_hc_tx_packet_recv(stru
"r_sample=%us\n", dccp_role(sk), sk,
hctx->ccid3hctx_rtt, r_sample);
- /* Update receive rate */
- hctx->ccid3hctx_x_recv = x_recv;/* X_recv in bytes per sec */
-
- /* Update loss event rate */
- if (pinv == ~0 || pinv == 0)
- hctx->ccid3hctx_p = 0;
- else {
- hctx->ccid3hctx_p = 1000000 / pinv;
-
- if (hctx->ccid3hctx_p < TFRC_SMALLEST_P) {
- hctx->ccid3hctx_p = TFRC_SMALLEST_P;
- ccid3_pr_debug("%s, sk=%p, Smallest p used!\n",
- dccp_role(sk), sk);
- }
- }
+ /* Avoid `division-by zero' errors.
+ * FIXME: We need a smarter and more robust way to
+ * protect against this possibility */
+ DCCP_BUG_ON(hctx->ccid3hctx_rtt == 0);
/* unschedule no feedback timer */
sk_stop_timer(sk, &hctx->ccid3hctx_no_feedback_timer);
--- a/net/dccp/ccids/lib/tfrc_equation.c
+++ b/net/dccp/ccids/lib/tfrc_equation.c
@@ -578,6 +578,8 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
u32 f;
u64 tmp1, tmp2;
+ DCCP_BUG_ON(R == 0); /* RTT can't be zero or else divide by zero */
+
if (p < TFRC_CALC_X_SPLIT)
index = (p / (TFRC_CALC_X_SPLIT / TFRC_CALC_X_ARRSIZE)) - 1;
else
@@ -587,11 +589,6 @@ u32 tfrc_calc_x(u16 s, u32 R, u32 p)
/* p should be 0 unless there is a bug in my code */
index = 0;
- if (R == 0) {
- DCCP_WARN("RTT==0, setting to 1\n");
- R = 1; /* RTT can't be zero or else divide by zero */
- }
-
BUG_ON(index >= TFRC_CALC_X_ARRSIZE);
if (p >= TFRC_CALC_X_SPLIT)
--- a/net/dccp/ccids/ccid3.h
+++ b/net/dccp/ccids/ccid3.h
@@ -78,7 +78,7 @@ enum ccid3_hc_tx_states {
/** struct ccid3_hc_tx_sock - CCID3 sender half-connection socket
*
* @ccid3hctx_x - Current sending rate
- * @ccid3hctx_x_recv - Receive rate
+ * @ccid3hctx_x_recv - Receive rate in bytes per second
* @ccid3hctx_x_calc - Calculated send rate (RFC 3448, 3.1)
* @ccid3hctx_rtt - Estimate of current round trip time in usecs
* @ccid3hctx_p - Current loss event rate (0-1) scaled by 1000000
-
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