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conf: Use 0 instead of -1 as "unassigned" mtu value

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On the command line -m 0 means "don't assign an MTU" (letting the guest use
its default.  However, internally we use (c->mtu == -1) to represent that
state.  We use (c->mtu == 0) to represent "the user didn't specify on the
command line, so use the default" - but this is only used during conf(),
never afterwards.

This is unnecessarily confusing.  We can instead just initialise c->mtu to
its default (65520) before parsing options and use 0 on both the command
line and internally to represent the "don't assign" special case.  This
ensures that c->mtu is always 0..65535, so we can store it in a uint16_t
which is more natural.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
This commit is contained in:
David Gibson 2025-02-19 14:14:28 +11:00 • committed by Stefano Brivio
commit 1cc5d4c9fe
6 changed files with 8 additions and 14 deletions

2
tcp.c
View file

@ -1139,7 +1139,7 @@ int tcp_prepare_flags(const struct ctx *c, struct tcp_tap_conn *conn,
if (flags & SYN) {
int mss;
if (c->mtu == -1) {
if (!c->mtu) {
mss = tinfo.tcpi_snd_mss;
} else {
mss = c->mtu - sizeof(struct tcphdr);