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TCP window shifts unsinged long
On Sat, Aug 09, 2025 at 03:01:36PM +0200, Mark Kettenis wrote:
> > Date: Tue, 5 Aug 2025 14:15:32 +0200
> > From: Alexander Bluhm <bluhm@openbsd.org>
> >
> > Hi,
> >
> > Coverity complains about signed integer shift in
> > tiwin = th->th_win << tp->snd_scale
> > I think we are safe as TCP_MAX_WINSHIFT 14 prevents us from using
> > potential sign bits.
> >
> > However, the type of the TCP window is inconsistent. I want to
> > convert it to long. Maybe u_long would be even better, but let's
> > start with the integer size. Converting to unsigned everywhere can
> > be done later.
> >
> > ok?
>
> Sorry, not that isn't right. It makes no sense to use long for this
> type, since on 32-bit systems that is the same as int. Where did this
> go wrong? This doesn't seem to be an area where we want different
> beheviour between 32-bit and 64-bit architectures.
In tcp_input long tiwin was introduced here:
commit 2d8be1d57460e127f5505122950d3cb8919e6263
Author: Andrew Cherenson <andrew>
Date: Fri Jan 8 18:42:28 1993 -0800
(By Sklower) checkpoint the current state of Cherenson's work.
SCCS-vsn: 7.30
and tcp_output has long win since:
commit 1ba008c794aee88f6e22ac3474b056a69e00f641
Author: Mike Karels <karels>
Date: Mon Feb 29 17:17:18 1988 -0800
use just one mbuf when sending small data segments;
truncate sbspace to our max window
SCCS-vsn: 7.14
Note that this diff is not about making the window size 64 bit in
general. There are some type conversion that are inconsistent.
When done in u_long they look less dangerous. My diff also removes
some ugly casts.
Wondow size calculations are more or less u_long or long for 35
years. I will not switch back to int, now that we have 64 bit
architectures.
Getting consistent u_long makes reasoning about integer conversions
easier. I am planning to convert the remaining long window sizes
to u_long in a separate diff.
bluhm
> > Index: netinet/tcp.h
> > ===================================================================
> > RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/tcp.h,v
> > diff -u -p -r1.24 tcp.h
> > --- netinet/tcp.h 19 May 2023 01:04:39 -0000 1.24
> > +++ netinet/tcp.h 2 Aug 2025 22:25:50 -0000
> > @@ -113,7 +113,7 @@ struct tcphdr {
> > */
> > #define TCP_MSS 512
> >
> > -#define TCP_MAXWIN 65535 /* largest value for (unscaled) window */
> > +#define TCP_MAXWIN 65535UL /* largest value for (unscaled) window */
> >
> > #define TCP_MAX_WINSHIFT 14 /* maximum window shift */
> >
> > Index: netinet/tcp_input.c
> > ===================================================================
> > RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/tcp_input.c,v
> > diff -u -p -r1.457 tcp_input.c
> > --- netinet/tcp_input.c 24 Jul 2025 21:34:07 -0000 1.457
> > +++ netinet/tcp_input.c 2 Aug 2025 22:28:22 -0000
> > @@ -690,7 +690,7 @@ findpcb:
> >
> > /* Unscale the window into a 32-bit value. */
> > if ((tiflags & TH_SYN) == 0)
> > - tiwin = th->th_win << tp->snd_scale;
> > + tiwin = (u_long)th->th_win << tp->snd_scale;
> > else
> > tiwin = th->th_win;
> >
> > @@ -1156,12 +1156,12 @@ findpcb:
> > * but not less than advertised window.
> > */
> > {
> > - int win;
> > + long win;
> >
> > win = sbspace(&so->so_rcv);
> > if (win < 0)
> > win = 0;
> > - tp->rcv_wnd = imax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
> > + tp->rcv_wnd = lmax(win, (int)(tp->rcv_adv - tp->rcv_nxt));
> > }
> >
> > switch (tp->t_state) {
> > @@ -1551,7 +1551,7 @@ trimthenstep6:
> > (TF_RCVD_SCALE|TF_REQ_SCALE)) {
> > tp->snd_scale = tp->requested_s_scale;
> > tp->rcv_scale = tp->request_r_scale;
> > - tiwin = th->th_win << tp->snd_scale;
> > + tiwin = (u_long)th->th_win << tp->snd_scale;
> > }
> > tcp_flush_queue(tp);
> > tp->snd_wl1 = th->th_seq - 1;
> > @@ -1582,11 +1582,13 @@ trimthenstep6:
> > if (do_ecn && (tiflags & TH_ECE)) {
> > if ((tp->t_flags & TF_ECN_PERMIT) &&
> > SEQ_GEQ(tp->snd_una, tp->snd_last)) {
> > - u_int win;
> > + u_long win;
> >
> > - win = min(tp->snd_wnd, tp->snd_cwnd) / tp->t_maxseg;
> > + win = ulmin(tp->snd_wnd, tp->snd_cwnd) /
> > + tp->t_maxseg;
> > if (win > 1) {
> > - tp->snd_ssthresh = win / 2 * tp->t_maxseg;
> > + tp->snd_ssthresh = win / 2 *
> > + tp->t_maxseg;
> > tp->snd_cwnd = tp->snd_ssthresh;
> > tp->snd_last = tp->snd_max;
> > tp->t_flags |= TF_SEND_CWR;
> > Index: netinet/tcp_output.c
> > ===================================================================
> > RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/tcp_output.c,v
> > diff -u -p -r1.156 tcp_output.c
> > --- netinet/tcp_output.c 8 Jul 2025 00:47:41 -0000 1.156
> > +++ netinet/tcp_output.c 2 Aug 2025 22:23:04 -0000
> > @@ -418,7 +418,7 @@ again:
> > * taking into account that we are limited by
> > * TCP_MAXWIN << tp->rcv_scale.
> > */
> > - long adv = lmin(win, (long)TCP_MAXWIN << tp->rcv_scale) -
> > + long adv = lmin(win, TCP_MAXWIN << tp->rcv_scale) -
> > (tp->rcv_adv - tp->rcv_nxt);
> >
> > if (adv >= (long) (2 * tp->t_maxseg))
> > @@ -859,13 +859,13 @@ send:
> > */
> > if (win < (rcv_hiwat / 4) && win < (long)tp->t_maxseg)
> > win = 0;
> > - if (win > (long)TCP_MAXWIN << tp->rcv_scale)
> > - win = (long)TCP_MAXWIN << tp->rcv_scale;
> > - if (win < (long)(int32_t)(tp->rcv_adv - tp->rcv_nxt))
> > - win = (long)(int32_t)(tp->rcv_adv - tp->rcv_nxt);
> > + if (win > TCP_MAXWIN << tp->rcv_scale)
> > + win = TCP_MAXWIN << tp->rcv_scale;
> > + if (win < (int32_t)(tp->rcv_adv - tp->rcv_nxt))
> > + win = (int32_t)(tp->rcv_adv - tp->rcv_nxt);
> > if (flags & TH_RST)
> > win = 0;
> > - th->th_win = htons((u_int16_t) (win>>tp->rcv_scale));
> > + th->th_win = htons((u_int16_t)(win >> tp->rcv_scale));
> > if (th->th_win == 0)
> > tp->t_sndzerowin++;
> > if (SEQ_GT(tp->snd_up, tp->snd_nxt)) {
> > @@ -1164,7 +1164,7 @@ out:
> > * then remember the size of the advertised window.
> > * Any pending ACK has now been sent.
> > */
> > - if (win > 0 && SEQ_GT(tp->rcv_nxt+win, tp->rcv_adv))
> > + if (win > 0 && SEQ_GT(tp->rcv_nxt + win, tp->rcv_adv))
> > tp->rcv_adv = tp->rcv_nxt + win;
> > tp->last_ack_sent = tp->rcv_nxt;
> > tp->t_sndacktime = now;
> > Index: netinet/tcp_subr.c
> > ===================================================================
> > RCS file: /data/mirror/openbsd/cvs/src/sys/netinet/tcp_subr.c,v
> > diff -u -p -r1.216 tcp_subr.c
> > --- netinet/tcp_subr.c 18 Jul 2025 08:39:14 -0000 1.216
> > +++ netinet/tcp_subr.c 2 Aug 2025 22:11:34 -0000
> > @@ -294,7 +294,7 @@ tcp_respond(struct tcpcb *tp, caddr_t te
> > tcp_seq ack, tcp_seq seq, int flags, u_int rtableid, uint64_t now)
> > {
> > int tlen;
> > - int win = 0;
> > + long win = 0;
> > struct mbuf *m = NULL;
> > struct tcphdr *th;
> > struct ip *ip;
> >
> >
TCP window shifts unsinged long