BUG/MINOR: quic: Wrong connection ID to thread ID association
To work, quic_pin_cid_to_tid() must set cid[0] to a value with <target_id>
as <global.nbthread> modulo. For each integer n, (n - (n % m)) + d has always
d as modulo m (with d < m).
So, this statement seemed correct:
cid[0] = cid[0] - (cid[0] % global.nbthread) + target_tid;
except when n wraps or when another modulo is applied to the addition result.
Here, for 8bit modulo arithmetic, if m does not divides 256, this cannot
works for values which wraps when we increment them by d.
For instance n=255 m=3 and d=1 the formula result is 0 (should be d).
To fix this, we first limit c[0] to 255 - <target_id> to prevent c[0] from wrapping.
Thank you to @esb for having reported this issue in GH #1855.
Must be backported to 2.6
diff --git a/include/haproxy/xprt_quic.h b/include/haproxy/xprt_quic.h
index e84959f..5899238 100644
--- a/include/haproxy/xprt_quic.h
+++ b/include/haproxy/xprt_quic.h
@@ -225,6 +225,7 @@
*/
static inline void quic_pin_cid_to_tid(unsigned char *cid, int target_tid)
{
+ cid[0] = MIN(cid[0], 255 - target_tid);
cid[0] = cid[0] - (cid[0] % global.nbthread) + target_tid;
}