net: the socket layer stops saying yes to things it did not do, and ping works

setsockopt was `return 0;` and getsockopt wrote a zero for whatever was asked.
The comment said it was so a server setting SO_REUSEADDR and treating failure as
fatal would not fail. That trade is the wrong way round: a caller told "no"
adapts, a caller told "yes" changes its behaviour on the strength of it -- sets
SO_RCVTIMEO and then blocks forever in a read it believes will time out, sets
TCP_NODELAY and reports latency numbers that measure Nagle. The failure is
silent, remote from the call, and indistinguishable from a bug in the stack.

Six options are honoured now and everything else is ENOPROTOOPT. SO_REUSEADDR,
SO_KEEPALIVE, SO_BROADCAST and TCP_NODELAY reach lwIP's pcb; SO_SNDBUF/SO_RCVBUF
are accepted and report the real size on read. SO_REUSEADDR is implemented rather
than refused precisely because of the original comment's concern -- it is the one
option a server really does set and really does check.

accept4 becomes its own table entry rather than an alias for accept. It has to
be: accept takes three arguments, so a shared handler reading a fourth register
reads whatever the caller last left there and invents a SOCK_NONBLOCK out of
stack litter. SOCK_CLOEXEC and SOCK_NONBLOCK are now honoured by socket(),
socketpair() and accept4() -- a dropped CLOEXEC leaks a listening socket across
every exec, and a dropped NONBLOCK has the caller block forever in what it
believes is a poll loop.

ICMP ping sockets: socket(AF_INET, SOCK_DGRAM, IPPROTO_ICMP), on lwIP's raw API,
with the callback stripping the variable-length IP header. Only echo replies are
eaten, and that restriction is load-bearing -- raw_input runs before lwIP's own
icmp_input, so consuming every ICMP message would swallow the echo *requests* and
this machine would silently stop answering pings from outside.

Loopback, so 127.0.0.1 is a real destination. LWIP_NETIF_LOOPBACK is the half
that is easy to miss: without it a packet addressed to an interface's own address
goes out of the wire and never comes back. With NO_SYS=1 nothing drains the
loopback queue, so the network thread calls netif_poll_all().

cmd/ping is first-party rather than toybox's, and the reason is a limitation not
a preference: toybox reads replies with recvmsg(2) and wants the TTL through an
IP_RECVTTL control message, and SchOS has neither. recvmsg is item 1 of
docs/plan/foundations.md, where it is wanted for SCM_RIGHTS anyway, and should
not be rushed to make one applet run.

Two bugs found on the way. Every AF_INET socket failed with ENFILE until DHCP
bound, because inet_new() demanded an *address* before handing out a socket and
reported it as "file table overflow"; net_is_ready() (lwIP up) is now distinct
from net_is_up() (DHCP bound). And that bug was hiding a bad test: socktest
asserted "AF_INET is refused, not faked", true when written, false since AF_INET
landed, and green the whole time because the suite runs before DHCP binds and the
ENFILE made the refusal look real. Fixing the kernel is what turned it red.

Verified: ping to 127.0.0.1, localhost, the gateway, 1.1.1.1 and 9.9.9.9, all
0% loss; socktest ALL CHECKS PASSED with ten new checks, including a setsockopt
round trip and an unknown option that must be refused -- a permissive stub cannot
pass both at once.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2

kernel/net/net.c

@@ -55,12 +55,29 @@ static int net_started;
 // something true to say rather than reporting 0.0.0.0 forever.
 static int net_bound;
 
+// Set once lwIP is initialised and this thread is running its timers. Distinct
+// from net_bound, and the distinction is not academic: inet_new() used to
+// require an *address* before it would hand out a socket, so every AF_INET
+// socket() failed until DHCP answered -- reported as ENFILE, which sends you to
+// look at the file table. It is also simply wrong for 127.0.0.1, which needs no
+// lease and no NIC.
+//
+// Creating a socket is not the same as having somewhere to send: a bind or a
+// connect with no route fails on its own, with an error that says so.
+static int net_ready;
+
 int
 net_is_up(void)
 {
     return net_bound;
 }
 
+int
+net_is_ready(void)
+{
+    return net_ready;
+}
+
 static void
 net_thread(void)
 {
@@ -69,6 +86,9 @@ net_thread(void)
     // before there is a process context means doing it somewhere that cannot
     // sleep if the allocator ever needs to.
     lwip_sys_init();
+    // From here a socket can be created: the pools exist and the timers are
+    // about to run. An address may still be minutes away, or never arrive.
+    net_ready = 1;
 
     lwip_sys_lock();
     lwip_init();
@@ -111,6 +131,10 @@ net_thread(void)
         lwip_sys_lock();
         moved = net_eth_poll();
         sys_check_timeouts();
+        // Loopback has no driver and no interrupt: a packet sent to 127.0.0.1
+        // sits on the loopif's queue until something drains it. With NO_SYS=1
+        // that something is this thread.
+        netif_poll_all();
 
         // Report each address once, when it first arrives. Anyone who has spent
         // an evening wondering whether the guest has an address at all wants