[PATCH v2] tcp: Avoid comparison of expressions with different signedness in RTT_SET()

6 Mar 2026

With gcc 14.2, building against musl 1.2.5 (slightly outdated Alpine
on x86_64):

tcp.c: In function 'tcp_update_seqack_wnd':
util.h:40:39: warning: comparison of integer expressions of different signedness: 'unsigned int' and 'int' [-Wsign-compare]
   40 | #define MIN(x, y)               (((x) < (y)) ? (x) : (y))
      |                                       ^
tcp_conn.h:63:26: note: in expansion of macro 'MIN'
   63 |         (conn->rtt_exp = MIN(RTT_EXP_MAX, ilog2(MAX(1, rtt / RTT_STORE_MIN))))
      |                          ^~~
tcp.c:1234:17: note: in expansion of macro 'RTT_SET'
 1234 |                 RTT_SET(conn, tinfo->tcpi_rtt);
      |                 ^~~~~~~
util.h:40:54: warning: operand of '?:' changes signedness from 'int' to 'unsigned int' due to unsignedness of other operand [-Wsign-compare]
   40 | #define MIN(x, y)               (((x) < (y)) ? (x) : (y))
      |                                                      ^~~
tcp_conn.h:63:26: note: in expansion of macro 'MIN'
   63 |         (conn->rtt_exp = MIN(RTT_EXP_MAX, ilog2(MAX(1, rtt / RTT_STORE_MIN))))
      |                          ^~~
tcp.c:1234:17: note: in expansion of macro 'RTT_SET'
 1234 |                 RTT_SET(conn, tinfo->tcpi_rtt);
      |                 ^~~~~~~

for some reason, that's not reported by gcc with glibc.

Cast the result of ilog2() to unsigned before using it, and introduce
0 as lower bound, to make it obvious that we expect the argument to be
always valid, the way we're using it.

Suggested-by: David Gibson 
Fixes: 000601ba86da ("tcp: Adaptive interval based on RTT for socket-side acknowledgement checks")
Signed-off-by: Stefano Brivio 
---
v2: Apply lower bound to the result of ilog2() instead of its argument,
    as suggested by David

 tcp_conn.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tcp_conn.h b/tcp_conn.h
index b7b85c1..6985426 100644
--- a/tcp_conn.h
+++ b/tcp_conn.h
@@ -60,7 +60,8 @@ struct tcp_tap_conn {
 #define RTT_STORE_MIN			100 /* us, minimum representable */
 #define RTT_STORE_MAX			((long)(RTT_STORE_MIN << RTT_EXP_MAX))
 #define RTT_SET(conn, rtt)						\
-	(conn->rtt_exp = MIN(RTT_EXP_MAX, ilog2(MAX(1, rtt / RTT_STORE_MIN))))
+	(conn->rtt_exp = MIN(RTT_EXP_MAX,				\
+			     (unsigned)MAX(0, ilog2(rtt / RTT_STORE_MIN))))
 #define RTT_GET(conn)			(RTT_STORE_MIN << conn->rtt_exp)
 
 	bool		tap_inactive	:1;
-- 
2.43.0

    