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/types/src/layout.rs

@@ -0,0 +1,18 @@
+//! Where things sit in every program's address space, below 1 GiB (the
+//! RISC-V adapter's user window): the arguments page at `start_arg::
+//! ARGS_VA`, the image from 0x10000, the heap above it (wherever the
+//! kernel finds room, below `HEAP_END`), and the stack at the very top,
+//! in a band of its own. The band's pages below the stack are never
+//! mapped, so running off the stack faults there, and the kernel says so.
+
+/// The first thread's stack ends here (the top of the user window).
+pub const STACK_TOP: usize = 0x4000_0000;
+/// The band the stack grows down into: its unmapped rest is the guard.
+pub const STACK_BAND: usize = 0x10_0000;
+/// The heap stays below the stack's band.
+pub const HEAP_END: usize = STACK_TOP - STACK_BAND;
+
+/// Whether a fault at `va` is a stack running off its end.
+pub const fn is_stack_overflow(va: usize) -> bool {
+    va >= HEAP_END && va < STACK_TOP
+}

crates/types/src/lib.rs

@@ -12,6 +12,7 @@ pub mod gatekeeper;
 pub mod generation;
 pub mod id;
 pub mod input;
+pub mod layout;
 pub mod log;
 pub mod macros;
 pub mod mapping;

crates/types/src/spawn.rs

@@ -12,8 +12,8 @@
 use crate::budget::Limits;
 
 pub const SPAWN: usize = 0;
-/// Where init puts the child's stack top (its stack ends at the image).
-pub const STACK_TOP: usize = 0x10000;
+/// Where init puts the child's stack top (see `layout`).
+pub const STACK_TOP: usize = crate::layout::STACK_TOP;
 /// A slot word meaning "none".
 pub const NONE: usize = usize::MAX;
 

crates/types/src/start_arg.rs

@@ -14,7 +14,8 @@ const PERMISSIONS_SHIFT: u32 = 48;
 const SESSION_SHIFT: u32 = 56;
 
 /// Where the arguments page sits: the program's arguments, NUL separated,
-/// the first its name. Below the stack, which ends at the image.
+/// the first its name. Above the unmapped page 0, below the image (see
+/// `layout`).
 pub const ARGS_VA: usize = 0x1000;
 pub const ARGS_SIZE: usize = 4096;