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

include/socket.h

@@ -22,6 +22,32 @@
 #define SOCK_STREAM     1
 #define SOCK_DGRAM      2
 
+// Flags ORed into socket()'s and accept4()'s `type`/`flags` argument. Linux's
+// values, because a musl program passes Linux's constants.
+#define SOCK_NONBLOCK   0x800      // O_NONBLOCK
+#define SOCK_CLOEXEC    0x80000    // O_CLOEXEC
+
+// setsockopt/getsockopt levels and names, Linux's numbering.
+//
+// Only what is listed here is accepted; anything else is answered with
+// ENOPROTOOPT rather than a success it did not earn. See the comment above
+// sys_setsockopt for why that is worth the risk of failing a caller that never
+// checked before.
+#define SOL_SOCKET      1
+#define IPPROTO_ICMP    1
+#define IPPROTO_TCP     6
+
+#define SO_REUSEADDR    2
+#define SO_TYPE         3
+#define SO_ERROR        4
+#define SO_BROADCAST    6
+#define SO_SNDBUF       7
+#define SO_RCVBUF       8
+#define SO_KEEPALIVE    9
+#define SO_ACCEPTCONN   30
+
+#define TCP_NODELAY     1
+
 #define UNIX_PATH_MAX   108
 
 #define SHUT_RD         0
@@ -93,6 +119,15 @@ struct socket {
   // Connections waiting to be accepted, on a listener.
   struct socket *queue[NBACKLOG];
   int   nqueue;
+
+  // Socket options that are genuinely honoured. Kept here rather than only in
+  // lwIP's pcb because getsockopt has to answer for an AF_UNIX socket too, and
+  // because a flag set before bind() has to survive until there is a pcb to
+  // apply it to.
+  int   o_reuseaddr;
+  int   o_keepalive;
+  int   o_broadcast;
+  int   o_nodelay;
 };
 
 #endif