# rsdos Debug Notes — heap/cells corruption family (v0.5.7)

Handoff state: **HEAD = `ab633ce`**; the mystery below is SOLVED and the
verify rework is committed on top (see git log).

## TL;DR

- Real-hardware (OptiPlex, no serial) boots crash in the shell with heap
  corruption. Two **distinct** bug families:
  - **(A) scroll-crash family**: in-heap `ptr=0x404443e0` with zeroed header.
  - **(B) 0xAA family**: `0xaaaaaa00aaaaaa` = two `0x00AAAAAA` u32s = UEFI
    firmware memory-scrub fill. A dropped object's memory was never
    allocated/freed by us — it's firmware-fresh scrub.
- Everything converges on **the `vt` static `Vec<Cell>`s' VecData (ptr/cap/len)
  or their heap blocks being corrupted/stale** — the cells VecData ptr at shell
  entry was `heap+0x1D8630` (= cells region) in the last crash, and the dropped
  126-byte String (`cap 0x7e`) is the vt verify klog line itself.
- **SOLVED — the pre-verify `0x70020` "mystery" was a debug artifact, not
  corruption.** `BLANK = {ch:32, fg:7, bg:0}` = raw `0x70020`, and
  `vec![BLANK; rows*cols]` fills the whole grid with it at init.  The old
  content check compared cells against **0** (wrong expectation — BLANK is not
  zero), and its probe loop `break`-ed on the first non-zero cell, so probes
  1–3 were never written and stayed at their zero-initialized slots.  The
  serial line `c=[0x70020,0x0,0x0,0x0]` was therefore bogus on **every healthy
  boot**.  A full-grid dump on QEMU proves it: `nz=3700 blank=3700` — both
  grids are 100% BLANK at shell entry.  The OptiPlex's `0x00AAAAAA` firmware
  scrub is still cleanly distinguishable (scrub ≠ 0x70020).

## The two confirmed bugs

1. **(A) scroll-crash family** — heap allocator returns an in-heap block whose
   header is zeroed (`ptr=0x404443e0`). Dropping it panics in the guard.
2. **(B) 0xAA family** — freed garbage pointer `0xaaaaaa00aaaaaa`; the memory at
   that address is firmware scrub fill. Something handed the allocator a pointer
   that was never a live allocation (or the VecData fields themselves got
   clobbered with scrub values — the cap was `0xAA` garbage in the boot crash).

## What we've fixed / proven already

- **IST=1 stack collision → ROOT CAUSE of ACTIVE_VOL corruption**: vec 14 ran on
  `PF_STACK`, a 4 KiB `.bss` static overlapping FAT `ACTIVE_VOL`. Fixed in
  `gdt.rs`: heap-allocated 32 KiB IST stacks. (Done, earlier.)
- **AHCI/DMA EXONERATED**: user removed all disks; crashes persisted.
- **All QEMU repro attempts were clean** (mem/klog/tail-crash) until the content
  probe upgrade — which tripped the *content check only* on a healthy boot for
  the reason above (expectation bug + break bug).  Fixed: the check now expects
  `blank_raw` (0x70020), records all four probe values, and adds a full-grid
  nz/blank summary line.  QEMU now reports `verify: ok` on every boot.
- **Clean heap geometry (OptiPlex)**: heap = `0x40400000..0xc7e34000` (~2.2 GB),
  boot stack ~1 MiB above heap end.
- **Shadow geometry (QEMU)**: shadow = first alloc at `heap+0x20` (`0x1980020`),
  480000 words → span `[heap+0x20, heap+0x1D4C20)`; cells data at
  `heap+0x1D4C40`; VecDatas at ~`heap+0x1D8630`.
- **Shadow-rejection hypothesis (unproven)**: on the OptiPlex the shadow alloc
  may have been rejected (or the cells alloc landed inside the would-be shadow
  span), leaving that block free + scrub-filled. `0x40444970 = heap+0x44970` is
  *inside* the would-be shadow span. The last crash's `r9=0x405d8630 =
  heap+0x1D8630` is the cells VecData region.
- **Original #GP resolved**: crashed image was RELEASE; drop of `Parsed` struct;
  cause = real memory corruption (laytest excluded compiler bug).
- **Stale PDB trap**: fresh PDB re-resolved shell-entry #GP →
  `rsnix::vt::put_byte+0x16a` (`mov %ax,(%rdx,%rcx,4)`) — u16 char at 4-byte
  stride into cells buffer via garbage ptr from `r9=0x405d8630`. bt chain:
  `shell::run → GopWriter::write_str → fmt::Formatter::pad_integral →
  klog::LineBuf::write_str → core::fmt::write → Display(u32)`.
- **Crash decoded**: the dropped object in the last boot crash was a 126-byte
  String (`cap 0x7e`) = the vt verify klog line (19-char prefix + 106-char text +
  `\n`); `ptr=0x40444970` is STALE (inside the would-be shadow span, back-pointer
  never written).

## Allocator layout (verified, `memory.rs`)

- `HEADER_SIZE=16`, `MINI_SIZE=8`, `MIN_BLOCK=32`, `MIN_ALIGN=16`
- payload = `align_up(block+24, align)`
- split: rewrites rem header + `free_list_insert` + back-pointer at `payload-8`
- `back_pointer_ok(ptr)` validates `[ptr-8]`; guard v0.5.7 (`0187b80`) reports +
  halts on failure; `corrupt` QEMU hook verified.

## vt grid facts

- `put_cell(r,c,ch)` sets `ch` + fg/bg from statics `SGR_FG=7`/`SGR_BG=0`.
- Cell raw encoding: `ch | fg<<16 | bg<<24`; BLANK = 0; cursor-inverse blank =
  `0x07000000` (bg and fg swapped). **`0x70020` is NOT a cursor-inverse cell** —
  it's a literal space with fg=7 (ch=0x20, fg=7, bg=0).
- `flush()` diffs cells vs rendered; cursor pass writes inverse colors into both
  FB and `rendered`.
- `term_println(s)` = `gop_print!` + blank line + explicit `serial::write_line`.
  Display mirror to serial is NOT automatic for `gop_println!`.
- Boot order after vt init: shadow → boot markers → memory self-test → page
  allocator + self-tests → paging adopt + self-test → sched init →
  rtc/pci/ahci/fat/fs → **cursor spawn** → **`debug_verify()`** → `shell::run()`
  (first banner `rs-dos v0.5.7 …`, then `=====`, then crash-log read).

## Current committed work (post-`ab633ce`)

- `gop::boot_marker_at(msg, row)` + `gop::text_rows()` + `gop::bottom_row(off)`
  — shadow marker now on the LAST text row, vt verify marker one above it
  (both survive the banner, prompt, and crash.log replay).
- `debug_verify()` fixed: compares probes against `blank_raw` (0x70020), not
  zero; records all four probe values (no early break); probes BOTH cells and
  rendered; adds a full-grid `grids c nz=… blank=… r nz=… blank=…` summary
  (serial).  Marker format:
  `vt: c {ptr:#x} cap={cap:#x} ok|GARBAGE | r {ptr:#x} ok|GARBAGE` at
  `bottom_row(1)`.
- QEMU verified: `verify: ok` every boot; screendump shows the banner (gray),
  prompt, and both bottom markers.  test-msg + test-fault PASS; debug and
  release builds 0 warnings.

## QEMU reproduction of the content-check failure (RESOLVED)

- Serial showed `probes c=[0x70020,0x0,0x0,0x0] r=[0x70020,0x0,0x0,0x0]` — this
  was **misread** as "a single stray cell".  The full-grid dump (3700 reads)
  showed `nz=3700 blank=3700` in both grids: **every cell is BLANK (0x70020),
  the grids are healthy**.  The probe array's 1–3 slots were zero-initialized
  leftovers from the early-`break` bug.
- Memory dump proves grids are fully healthy on QEMU: cells block base
  `0x1b54c20` `{size=0x39f0, free=0}`, back-ptr written; rendered block base
  `0x1b58650`; both grids contain the shell banner post-draw. **No heap
  corruption on QEMU.**
- QEMU verify geometry: cells ptr `0x1b54c40` cap `0xe74` len `0xe74`; rendered
  ptr `0x1b58670`.
- ⇒ `CONTENT-GARBAGE` was never evidence of OptiPlex corruption — the check was
  comparing against the wrong expectation.  Now: probes compare against
  `blank_raw` (0x70020); every probe value is logged; a `grids c nz=… blank=…`
  summary is the definitive health signal.

## The pre-verify `0x70020` at grid(0,0) — MYSTERY SOLVED

**There was no writer.** `0x70020` is the grid's own init fill:
`BLANK = Cell{ch:32, fg:7, bg:0}` encodes as `0x70020`, and `vt::init` builds
both grids with `vec![BLANK; rows*cols]`.  The old content check expected
all-zero cells (wrong), and the probe loop's early `break` made probes 1–3
look like zeros.  Nothing writes a pre-banner space; the shell banner (gray
VGA-7 text, rows 0–1) and the `>` prompt (row 2) are the only vt writes
before the markers.

Also learned while verifying with a screendump:

- The shell banner occupies grid rows **0 and 1** (motd + `=====` line), and
  the prompt sits at row 2 — so a marker at FB row 1 (the old shadow marker)
  is **clobbered** by the banner, and row 2's first glyphs get eaten by the
  prompt + cursor.  Markers whose values matter now live at the **bottom**
  (`gop::bottom_row(0)` = last row, `bottom_row(1)` = second-to-last), which
  also survive a stale crash.log replay (~20 lines at grid rows 2+).
- Banner text renders VGA gray `0xAAAAAA` (fg 7), boot markers white `0xFFFFFF`
  — a white-only pixel matcher misses the banner.  QEMU screendump decode of
  the bottom rows: `vt: c 0x1b54c40 cap=0xe74 ok | r 0x1b58670 ok` and
  `gop: shadow 0x1980020 1.83 MiB`.

**Excluded candidates (all still true, now moot):** cursor task (FB/shadow
only), flush cursor-inverse pass (0x07000000 ≠ 0x70020), boot markers (direct
FB), memory self-test, shell banner (runs after verify), backspace (BLANK).

## OptiPlex flash — what to look for (the linchpin)

New image puts on screen (markers are **bottom-anchored** now — they survive
the banner rows 0–1, the prompt row 2, AND a stale crash.log replay at rows
2+; use `gop::bottom_row(off)` to place any future ones):

1. **Last row (bottom_row(0))**: `gop: shadow {ptr:#x} {MiB}` vs
   `gop: NO-SHADOW (fallback)`.
   - If NO-SHADOW ⇒ shadow-rejection hypothesis gains weight (first block left
     free + scrub-filled).
2. **Second-to-last row (bottom_row(1))**: `vt: c {ptr:#x} cap={cap:#x} ok|GARBAGE | r {ptr:#x} ok|GARBAGE`.
   - **The cells ptr value is the linchpin**: if it equals `0x40444970`, the
     cells VecData itself is stale at shell entry (matches boot-crash decode).
   - If cap shows `0xAA…` garbage ⇒ VecData fields scrubbed (bug family B).
   - `ok` = all probed cells read BLANK (0x70020); `GARBAGE` = probe ≠ BLANK
     (the OptiPlex firmware scrub reads back `0x00AAAAAA`).
3. If it crashes: the full guard band (`hdr:` + all four `mem:` lines, 16 stack
   words) plus the bottom markers if still visible.

Serial-side (QEMU only): the verify lines are captured in the klog ring
(`serial_println!` → ring), so a healthy OptiPlex boot can also dump them with
the `klog` shell command.

## Build/flash/test loop (traps)

- Build 0 warnings, edition 2024, `no_std`, `x86_64-unknown-uefi`, workspace
  v0.5.7. Zero new deps.
- Real-hardware boot chain: rsboot at `EFI/BOOT/BOOTX64.EFI` → kernel at
  `bzImage`. QEMU dev loop boots kernel as BOOTX64.EFI.
- CRASHLOG stick must be FAT32; `mdir` grep gotcha: match `crash`
  case-insensitively.
- **Stale qemu holds flock** → `pkill -9 -f 'qemu[-]system'` in a SEPARATE
  command; `rm -f esp/NvVars` between boots.
- Use repo `scripts/` infra: test-msg/test-fault/test-mouse; monitor socket; HMP
  `mouse_move`.
- Fault-context serial rule: never `serial_println!` in ISR/fault code; use
  `serial::write_raw`; guard report must not allocate (stack `FmtBuf`).
- Marker on row 0 gets overwritten by the shell MOTD — use `boot_marker_at(msg, row>0)`.
- Stale PDBs misname symbols — re-resolve with a fresh PDB per build (PDB math
  is decimal); `r9` = struct base with Vec layout `{ptr@0, cap@8, len@16}`.

## Next steps (priority order)

1. **Flash to OptiPlex** and relay the two bottom markers: shadow state (last
   row) + vt marker (`bottom_row(1)` — the cells ptr value is the linchpin:
   equals `0x40444970`? cap scrubbed `0xAA…`? `ok` vs `GARBAGE`?).  If crash:
   full `hdr:` + all four `mem:` lines.
2. Reattach the window-manager milestone once the crash family is resolved.

(QEMU control arm is green: content check now passes on every healthy boot;
no further QEMU-side investigation is needed.)