ldls: a second driver, and everything it found

e1000 -- Linux's, unmodified -- probes, opens, gets an address by DHCP and
carries ping traffic in a SchOS process. The kernel was driving that NIC and
gave it up, which makes it the first real handover as well as the second
driver.

None of what it needed was a missing feature. Every one was a call that
succeeded, returned zero and did nothing:

  _find_next_bit          "bit 0 is set"          -- for_each_cpu never ended
  netif_carrier_off       carrier already up      -- TCTL.EN never set
  __x86_indirect_thunk_r8 the jump discarded      -- 255 rx buffers, none made
  dma_map_page_attrs      bus address 0           -- NIC aimed at physical 0

so the rule from memcpy is now general: a stub returning zero is unsafe
wherever zero is a meaningful answer.

The loader grew ko_fixup_percpu(). The stack guard turned out to be one
per-CPU variable among several; `current` cannot be defused because the driver
dereferences it. Taking the %gs off instead leaves the same opcode reading the
same width from storage the host supplies -- a per-CPU variable in a
single-threaded host is just a variable. Eight bytes become seven and a nop,
and the displacement is not always last, so there is now just enough of an x86
decoder in ko.c to know how long the instruction is.

ldls/kpi/thunk.c is fifteen lines of assembly and the highest-value file here:
retpolines are control flow, and a stub cannot stand in for a jump.

The kernel side:

  - ioperm.c, the TSS I/O permission bitmap. A driver's outl is an instruction,
    not a call, and e1000's reset path picks it deliberately. IOPL=3 would hand
    over the PIC and the disk with it, so it is per-port. Writing it found that
    the I/O map base had been written at offset 64 rather than 102, leaving the
    CPU checking ports 0-831 against the TSS's own RSP0.
  - claim masks the device's IRQ line: 450,000 a second, otherwise.
  - net_detach(), the missing half of net_attach_late(). An interface left in
    the stack keeps its route and sends every frame to a driver that refuses
    it, which looks exactly like a userspace driver that does not work.
  - e1000_release() gives up the interface, not just the hardware.

8139too still passes 4/4; ldls-check still passes 27; four CPUs, no panic.

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

kernel/net/net.c

@@ -208,6 +208,42 @@ net_attach_late(struct device *d)
     return idx;
 }
 
+// Which interfaces have had their address reported. File scope rather than
+// local to net_thread(), because an interface that goes away has to be able to
+// be announced again if one comes back in its slot.
+static int announced[NETIF_MAX];
+
+static void
+net_unannounce(int idx)
+{
+    if (idx >= 0 && idx < NETIF_MAX)
+        announced[idx] = 0;
+}
+
+// The other direction: a device the kernel no longer drives.
+//
+// Called when /dev/pci hands a NIC to a process. Same rule about lwIP headers
+// as net_attach_late above -- the caller is a device file and must not see
+// them.
+int
+net_detach(struct device *d)
+{
+    int idx;
+
+    if (!net_ready)
+        return -1;
+
+    lwip_sys_lock();
+    idx = net_eth_detach(d);
+    lwip_sys_unlock();
+
+    if (idx >= 0) {
+        net_unannounce(idx);
+        cprintf("net: %s is gone; interface %d removed\n", d->name, idx);
+    }
+    return idx;
+}
+
 static void
 net_thread(void)
 {
@@ -253,7 +289,7 @@ net_thread(void)
             attached, attached == 1 ? "" : "s");
 
     // One bit per interface, so a second one binding later is announced too.
-    int announced[NETIF_MAX] = { 0 };
+
 
     for (;;) {
         int moved;
@@ -269,8 +305,14 @@ net_thread(void)
         // Report each address once, when it first arrives. Anyone who has spent
         // an evening wondering whether the guest has an address at all wants
         // this on the console rather than in a file they have to know about.
-        for (int i = 0; i < attached; i++) {
+        // Over every interface there is now, not over the ones there were at
+        // boot: an interface can arrive later. /dev/netdev makes one whenever a
+        // process becomes a network device, and until this counted them an
+        // address on `ldls0` was never reported at all.
+        for (int i = 0; i < net_eth_count() && i < NETIF_MAX; i++) {
             struct netif *nif = net_eth_netif(i);
+            if (nif == 0 || net_eth_device(i) == 0)
+                continue;
             if (announced[i] || ip4_addr_isany_val(*netif_ip4_addr(nif)))
                 continue;