fix: stacks at top of user VA in own 1 MiB band (guard, heap kept below), 8/16 pages default; kernel fault msg names pid, kind, flags stack overflow; flaky OVERFLOW + spawntest/test-qemu check it

crates/apps/flaky/src/lib.rs

@@ -16,6 +16,16 @@ pub mod op {
     pub const CRASH: usize = 2;
     /// Blocks for good inside the handler.
     pub const HANG: usize = 3;
+    /// Recurses until its stack runs out: the kernel says it overflowed.
+    pub const OVERFLOW: usize = 4;
+}
+
+/// Ever deeper, a KiB of stack a call, until there is none.
+#[allow(unconditional_recursion, reason = "running off the stack is what it is for")]
+fn deeper(n: usize) -> usize {
+    let frame = core::hint::black_box([n as u8; 1024]);
+    // Used after the call, so the call is not a jump.
+    deeper(n + 1) + frame[n % 1024] as usize
 }
 
 /// Everything goes through `handle_call`.
@@ -49,6 +59,9 @@ impl Server for Flaky {
         match words[0] {
             op::WHO => {}
             op::CRASH => panic!("asked to crash"),
+            op::OVERFLOW => {
+                core::hint::black_box(deeper(0));
+            }
             op::HANG => {
                 // A notification nobody signals.
                 if let Ok(never) = syscall::create_notification() {