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/lwip/netif_eth.c

@@ -138,6 +138,52 @@ net_eth_attach(struct device *dev)
     return nifs++;
 }
 
+// An interface whose device has gone away.
+//
+// The missing half of net_eth_attach, and it went missing because nothing had
+// ever taken a device back. /dev/pci does: the kernel's e1000 driver hands the
+// NIC to a process, and until this existed the interface stayed in lwIP --
+// up, holding an address, holding the route to its subnet, and attached to a
+// driver that now refuses every frame. Packets went to it and stopped, which
+// looks exactly like a driver that is not working.
+//
+// The slot is left occupied rather than compacted. Interface indices are
+// visible in /proc/netif and in the names the stack has already handed out,
+// and renumbering them because a device left would be worse than a gap.
+int
+net_eth_detach(struct device *dev)
+{
+    int i;
+
+    for (i = 0; i < nifs; i++) {
+        if (!ifs[i].used || ifs[i].dev != dev)
+            continue;
+
+        // Down first, then out. netif_remove() does stop DHCP and send the
+        // release, but a netif removed while still up leaves lwIP's default
+        // route pointing at freed storage if this was interface 0.
+        netif_set_link_down(&ifs[i].netif);
+        netif_set_down(&ifs[i].netif);
+        if (netif_default == &ifs[i].netif)
+            netif_set_default(NULL);
+        netif_remove(&ifs[i].netif);
+
+        ifs[i].used = 0;
+        ifs[i].dev  = 0;
+
+        // Something else becomes the default, if anything can. A machine with
+        // a second interface should not lose its route because the first one
+        // was handed to a driver in a process.
+        if (netif_default == NULL) {
+            int j;
+            for (j = 0; j < nifs; j++)
+                if (ifs[j].used) { netif_set_default(&ifs[j].netif); break; }
+        }
+        return i;
+    }
+    return -1;
+}
+
 int
 net_eth_count(void)
 {