path: root/DEBUG_NOTES.md

# 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).

## ✅ ROOT CAUSE + FIX (this session) — ExitBootServices retry wipes the free list

**Root cause (confirmed by QEMU simulation + OptiPlex screen markers):**

1. `capture_memory_map` (main.rs, pre-EBS) → `init_heap` builds the free
   list from the raw map; `map_key` stashed.
2. `gop::init` → `SetMode` — on real hardware SetMode may allocate/reallocate
   memory, making `map_key` stale — then allocates the live display shadow
   (first heap alloc, `mem::forget`'d) and `vt::init` builds the cells grids.
3. `ExitBootServices(image_handle, map_key)` **fails** (stale key) → retry
   `refresh_and_exit` → `capture_memory_map` → **`init_heap` REBUILT the
   free list from the raw map, wiping every pre-EBS live allocation's
   state** — the shadow's 1.8 MB span and the vt grids became FREE while
   still live.
4. Post-EBS allocations carved **inside the live shadow** (the Opt's vt
   marker String at exactly `heap+0x443e0`, the family-A crash ptr); the
   display renders into the shadow → clobbers those objects → **zeroed
   back-pointers** (family A `ptr=0x404443e0 block=0x0`) and **gray text
   pixels** (family B `0xaaaaaa00aaaaaa` = 2× `0x00AAAAAA` = PALETTE[7]
   fg-7, NOT a firmware scrub).

**Fix (committed):** `refresh_and_exit` now re-captures the map and returns
a FRESH EBS key WITHOUT rebuilding the free list (`refresh_map_key`, the
free list is the authoritative live-allocation state).  Verified in QEMU
with a forced EBS failure: `verify: ok`, `probe free=false`, `flhead
in_shadow=false` — identical to the no-failure boot.  Test hook:
`memory::SIMULATE_EBS_FAILURE` (one-shot, defaults off).

**OptiPlex markers that exposed it:** `probe@+0x443e0 free=YES`,
`w0=203a7476 w1=78302063` (= "vt: c 0x" — the freed marker String),
`blk=0x404443c0 sz=0x770`, `flhead=0x404443c0 in-shadow=YES`, `shadow
=0x40400000 freed=no`, `fb=0xe0000000 in-heap=no`.

## 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.
- **CORRECTION (this session): `0x00AAAAAA` is NOT a UEFI firmware scrub.
  It is the VGA GRAY text colour: `rgb(170,170,170)` packs to
  `(170<<16)|(170<<8)|170 = 0x00AAAAAA`, which is `PALETTE[7]` = fg 7 — the
  vt's default foreground.** Every rendered glyph pixel is a `0x00AAAAAA`
  word in the shadow; the shadow's background is `0x00000000` (bg 0).
  - Family B `0xaaaaaa00aaaaaa` = **two adjacent gray TEXT pixels** — the
    memory read as heap held rendered text ⇒ it was **inside the live
    shadow** (see root cause above).
  - Family A zeros = the shadow's **black background** at the same span.
  - ⇒ BOTH families = the allocator handed out memory inside the live
    shadow after the EBS-retry free-list rebuild.
- 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.  QEMU proves both grids are 100%
  BLANK at shell entry.  (Note: 0x70020 ≠ 0x00AAAAAA — the two "fills" are
  cleanly distinguishable; the family-A/B dump content is TEXT/bg, not
  BLANK cells.)

## 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 two
   `0x00AAAAAA` words are **gray text pixels** (PALETTE[7]) — the memory at
   that address was rendered shadow content. Something handed the allocator a
   pointer that was never a live allocation (or a live-shadow block that
   leaked into the free list).

## 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 (strengthened by QEMU forced-reject test)**:
  on the OptiPlex the shadow alloc may have been rejected (or its block
  freed), leaving that span free + text/zero-filled. `0x40444970 =
  heap+0x44970` is *inside* the would-be shadow span.  Forcing rejection in
  QEMU reproduces family A exactly: `probe heap+0x443e0 IN free blk` with
  all-zero content.
- **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)

- **0x00AAAAAA scan verdict (this session)**: `shadow scan: 11333w/480000`
  hits are all at **glyph positions of on-screen text** (fg=7 gray pixels).
  The "scrub" reading at the crash offset (`probe mem: 00000000 00aaaaaa
  00aaaaaa 00000000 …`) is the heapdbg output line **rendered at that exact
  shadow location** — the probe reads its own text.  The shadow far rows
  (r300, r500) read 0 ⇒ init zeroing is correct and nothing over-writes
  untouched shadow rows in QEMU.  FB pixel at (171,1) = (170,170,170) GRAY
  = 0x00AAAAAA — shadow and FB agree; there is NO shadow/FB desync.
- The shadow scan is still a useful health signal: untouched far rows must
  stay 0; text-row hits are expected (that's what the shadow holds).

- 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)

**FIRST OPTIPLEX REPORT (this session, previous image):**

```
heapdbg: probe@+0x443e0 free=YES w0=203a7476 w1=78302063
heapdbg: shadow=0x40400000 freed=no
```

- `w0=0x203a7476 w1=0x78302063` = LE ASCII **"vt: c 0x"** — the leftover
  bytes of the *vt verify marker String* (`debug_verify`'s
  `alloc::format!("vt: c {:#x} …")`), which was **allocated at exactly
  `heap+0x443e0`** (the historical family-A crash ptr) and freed before
  the marker ran.  ⇒ on the Opt the heap's first-fit hands out
  **`heap+0x443e0` — INSIDE the live shadow span
  [`heap+0x20`, `heap+0x1D4C20`)** — while `shadow=0x40400000 freed=no`
  (the shadow Vec's block header claims allocated, forget'd).
- QEMU control: same marker prints `probe free=false … flhead=0x1b706d0
  in_shadow=false` — the probe address is NOT in the free list.
- ⇒ The free list contains the live shadow span on the Opt.  The two new
  boot lines (`fb … in-heap=YES/no`, `flhead … in-shadow=YES/no`) plus
  the probe's containing `blk=…` tell us whether the overlap is the whole
  heap block (shadow carve never happened / init_heap after alloc), the
  fb inside the heap (display clobbers), or a mid-span block.

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))**: `heapdbg: probe@+0x443e0 free=YES|no w0=… w1=… blk={base} sz={size}`
   — crash offset free?  content (zeros=shadow bg, `00aaaaaa`=gray text,
   ASCII=leftover string bytes)?  and the **base/size of the free block
   containing it** (0x40400000 = whole-heap block → the shadow carve
   never happened; inside the shadow span → overlap; a normal small
   block → ordinary freed allocation).
2. **Second-to-last row (bottom_row(1))**: `heapdbg: shadow={ptr| -} freed=YES|no`
   — the shadow block base + header free flag.
3. **Third-to-last (bottom_row(2))**: `heapdbg: fb={base} size={size} in-heap=YES|no`
   — is the framebuffer inside the heap range (display clobbers heap)?
4. **Fourth-to-last (bottom_row(3))**: `heapdbg: flhead={base} size={size} in-shadow=YES|no`
   — does the free-list head overlap the shadow span?
5. Also: `vt: c {ptr} cap=… ok|GARBAGE | r {ptr} ok` marker (was
   bottom_row(1), now overwritten by the shadow line) — the cells ptr
   value is the linchpin for a stale-VecData theory: equals `0x40444970`?
6. 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 boot heapdbg values are mirrored to serial
(`heapdbg: probe free=… shadow=… freed=… fb_in_heap=… flhead=… in_shadow=…`)
and captured in the klog ring — 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 the fixed image to the OptiPlex** (BOOTX64.EFI in `esp/`).
   Expected bottom rows now: `probe@+0x443e0 free=no` (was YES),
   `flhead=… in-shadow=no` (was YES), `vt: … ok`.  Run the old repro
   (`klog -t 2`, `ls`, `help`) — should no longer crash.
2. If the Opt still misbehaves: relay the four bottom lines again — the
   fix only holds if the free list no longer overlaps the shadow.
3. Reattach the window-manager milestone once confirmed.

(QEMU control arm is green with and without a forced EBS failure:
`verify: ok`, probe NOT free, shadow base NOT in free list, full shell
sequence clean.  `0x00AAAAAA` = gray text colour; shadow far rows stay 0.)