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

On Fri, Mar 06, 2026 at 08:02:35AM +0100, Stefano Brivio wrote:

With gcc 14.2, building against musl 1.2.5 (slightly outdated Alpine on x86_64):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

Reviewed-by: David Gibson

--- 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

-- David Gibson (he or they) | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you, not the other way | around. http://www.ozlabs.org/~dgibson