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

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

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On Wed, Mar 04, 2026 at 05:32:32PM +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 With gcc 14.2, building against musl 1.2.5 (slightly outdated Alpine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, as it's always
> positive the way we're using it. Should this ever break this for
> whatever unlikely reason, RTT_EXP_MAX is the fallback value we want to
> use anyway.
> 
> Fixes: 000601ba86da ("tcp: Adaptive interval based on RTT for socket-side acknowledgement checks")
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

This is correct, so

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

It's kind of inelegant, though - ilog2() only returns a signed to
report an error case, which we avoid with the MAX inside the argument.
That's not a super obvious connection.  I guess we could do:

	(unsigned)MAX(0, ilog2(rtt / RTT_STORE_MIN))

which should be equivalent and makes it slightly more obvious that the
cast is safe.  Not sure if it's really an improvement, though.

This is the only user of ilog2(), so we could also consider replacing
it with a version that explicitly clamps its argument to >= 1 and
returns unsigned.

-- 
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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