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BUG/MINOR: quic: fix possible integer wrap around in cubic window calculation

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Avoid loss of precision when computing K cubic value.
Same issue when computing the congestion window value from cubic increase function
formula with possible integer varaiable wrap around.

Depends on this commit:

	MINOR: quic: Code clarifications for QUIC CUBIC (RFC 9438)

Must be backported as far as 2.6.
This commit is contained in:
Frederic Lecaille 2024-01-31 18:21:34 +01:00
parent 88d13caa38
commit 19a66b290e
1 changed files with 6 additions and 2 deletions
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8
src/quic_cc_cubic.c
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@ -245,7 +245,7 @@ static inline void quic_cubic_update(struct quic_cc *cc, uint32_t acked)
* Note that K is stored in 1024th of a second.
*/
c->K = cubic_root((c->last_w_max - path->cwnd) *
(CUBIC_ONE_SCALED - CUBIC_BETA_SCALED) / CUBIC_C_SCALED / path->mtu) << TIME_SCALE_FACTOR_SHIFT;
((CUBIC_ONE_SCALED - CUBIC_BETA_SCALED) << TIME_SCALE_FACTOR_SHIFT) / (CUBIC_C_SCALED * path->mtu));
c->W_target = c->last_w_max;
}
@ -273,7 +273,11 @@ static inline void quic_cubic_update(struct quic_cc *cc, uint32_t acked)
}
/* Compute W_cubic_t at t time. */
W_cubic_t = path->mtu * ((CUBIC_C_SCALED * t * t * t) >> (CUBIC_SCALE_FACTOR_SHIFT + 3 * TIME_SCALE_FACTOR_SHIFT));
W_cubic_t = CUBIC_C_SCALED * path->mtu;
W_cubic_t = (W_cubic_t * t) >> TIME_SCALE_FACTOR_SHIFT;
W_cubic_t = (W_cubic_t * t) >> TIME_SCALE_FACTOR_SHIFT;
W_cubic_t = (W_cubic_t * t) >> TIME_SCALE_FACTOR_SHIFT;
W_cubic_t >>= CUBIC_SCALE_FACTOR_SHIFT;
if (elapsed_time < c->K)
target = c->W_target - W_cubic_t;
else