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

[David Gibson <david@gibson.dropbear.id.au>]

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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 With gcc 14.2, building against musl 1.2.5 (slightly outdated Alpine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 <david@gibson.dropbear.id.au>
> Fixes: 000601ba86da ("tcp: Adaptive interval based on RTT for socket-side acknowledgement checks")
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

> ---
> 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

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