v0.5.5: crash reports go to a dedicated CRASHLOG USB stick

The fault handler now writes crash.log to the volume labeled CRASHLOG
(label-selected via fat::crash_vol), falling back to slot 0.  On real
hardware the boot USB may be unwritable, so a second stick labeled
CRASHLOG survives the write; the shell auto-prints the previous crash
report from it at next boot (read_crash_log resolves /disk/<uid>/crash.log
through the FAT path, since Disk nodes hold no cached children).

- fat.rs: volume_label() + crash_vol() (no allocation)
- fs.rs: write_crash_log targets crash_vol(); read_crash_log prefers the
  CRASHLOG disk node; drop now-unused first_disk()
- mkdisk.sh: --label option
- test-fault.sh: boots with a dedicated CRASHLOG stick (mkfs -F 32 -n
  CRASHLOG), verifies crash.log lands there and NOT on the ESP

2 files changed, 31 insertions, 7 deletions

diff --git a/scripts/mkdisk.sh b/scripts/mkdisk.sh
index 6bdd50c..2c62f76 100755
--- a/scripts/mkdisk.sh
+++ b/scripts/mkdisk.sh
@@ -9,6 +9,7 @@
 #   ./scripts/mkdisk.sh --size 128M     # different size (default 64M)
 #   ./scripts/mkdisk.sh /path/img       # custom image path
 #   ./scripts/mkdisk.sh --no-seed       # skip the seed/ directory
+#   ./scripts/mkdisk.sh --label CRASHLOG # volume label (default RSDOS)
 #
 # The image is created with `mkfs.fat -C`, which is the option that
 # actually creates the file — plain `mkfs.fat` only writes into an
@@ -23,6 +24,7 @@ IMG="/tmp/disk.img"
 SIZE="64M"
 FRESH=0
 SEED_DIR="$PROJECT_DIR/seed"
+LABEL="RSDOS"
 
 while [ $# -gt 0 ]; do
         case "$1" in
@@ -32,6 +34,10 @@ while [ $# -gt 0 ]; do
                 SIZE="$1"
                 ;;
         --no-seed) SEED_DIR="" ;;
+        --label)
+                shift
+                LABEL="$1"
+                ;;
         --help | -h)
                 head -20 "$0" | grep "^#" | sed 's/^# \?//'
                 exit 0
@@ -65,9 +71,9 @@ if [ -z "$SIZE_BLOCKS" ] || [ "$SIZE_BLOCKS" -le 0 ] 2>/dev/null; then
         exit 1
 fi
 
-echo "mkdisk: creating $IMG ($SIZE FAT32, label RSDOS)"
+echo "mkdisk: creating $IMG ($SIZE FAT32, label $LABEL)"
 rm -f "$IMG"
-mkfs.fat -C -F 32 -n RSDOS "$IMG" "$SIZE_BLOCKS"
+mkfs.fat -C -F 32 -n "$LABEL" "$IMG" "$SIZE_BLOCKS"
 
 if [ -n "$SEED_DIR" ] && [ -d "$SEED_DIR" ]; then
         if ! command -v mcopy >/dev/null 2>&1; then
diff --git a/scripts/test-fault.sh b/scripts/test-fault.sh
index 49ba791..59f741f 100755
--- a/scripts/test-fault.sh
+++ b/scripts/test-fault.sh
@@ -52,12 +52,18 @@ echo "==> 2/5 deploy to ESP"
 mkdir -p "$ESP_DIR/EFI/BOOT"
 cp target/x86_64-unknown-uefi/debug/rsdos.efi "$ESP_DIR/EFI/BOOT/BOOTX64.EFI"
 
-echo "==> 3/5 boot QEMU (headless)"
+echo "==> 3/6 boot QEMU (headless, + dedicated CRASHLOG stick)"
 kill "$(cat "$QPID" 2>/dev/null)" 2>/dev/null || true
 sleep 1
 rm -f "$SOCK" "$LOG"
+# A second FAT32 stick labeled CRASHLOG: crash reports must land here
+# (label-selected by the kernel), not on the boot device.
+CRASH_IMG=/tmp/rsdos-crash.img
+"$SCRIPT_DIR/mkdisk.sh" --fresh --label CRASHLOG --no-seed "$CRASH_IMG" >/dev/null
 "$QEMU" -bios "$OVMF" \
         -drive file=fat:rw:"$ESP_DIR",format=raw \
+        -drive file="$CRASH_IMG",format=raw,if=none,id=crash \
+        -device ide-hd,drive=crash,bus=ide.2 \
         -serial file:"$LOG" \
         -monitor unix:"$SOCK",server,nowait \
         -m 256M -machine q35 -cpu qemu64 -no-reboot -display none \
@@ -66,11 +72,12 @@ echo $! >"$QPID"
 
 wait_log "$LOG" "boot: entering shell" 90 || fail "kernel never reached the shell"
 
-echo "==> 4/5 boot diagnostics present"
+echo "==> 4/6 boot diagnostics present"
 grep -aq "gop: fb base=" "$LOG" || fail "fb geometry diagnostic missing"
 grep -aq "gop: mode .* selected" "$LOG" || fail "mode-selection diagnostic missing"
+grep -aq "CRASHLOG" "$LOG" || fail "CRASHLOG stick not mounted"
 
-echo "==> 5/5 inject a page fault (fault command)"
+echo "==> 5/6 inject a page fault (fault command)"
 python3 "$SCRIPT_DIR/sendkeys.py" --sock "$SOCK" \
         "clear" \
         "fault"
@@ -83,8 +90,19 @@ grep -aq "cr2=0x400000000000" "$LOG" || fail "CR2 not reported (faulting address
 grep -aq "task=0 \"shell\"" "$LOG" || fail "faulting task not identified"
 grep -aq "!!! halting" "$LOG" || fail "no halt message"
 
+echo "==> 6/6 crash.log persisted on the CRASHLOG stick"
+grep -aq "crashlog: vol .* (CRASHLOG)" "$LOG" || fail "crashlog did not target the CRASHLOG stick"
+grep -aq "crashlog: wrote" "$LOG" || fail "crashlog write did not complete"
+mdir -i "$CRASH_IMG" "::" 2>/dev/null | grep -aqi "crash" \
+        || fail "crash.log missing on the CRASHLOG stick"
+# The boot ESP must NOT receive the crash log.
+if [ -f "$ESP_DIR/crash.log" ]; then
+        fail "crash.log landed on the boot ESP instead of the CRASHLOG stick"
+fi
+
 # The screen dump itself is exercised in-kernel (dump_lines); the serial
 # path sharing the same format line is verified above.
 echo ""
-echo "PASS — fault dump works (vec, err, rip, cr2, task → screen + serial)"
-echo "  every CPU exception now halts with a diagnosable dump, not a freeze"
+echo "PASS — fault dump + crash.log on the CRASHLOG stick"
+echo "  every CPU exception halts with a diagnosable dump; the report"
+echo "  survives reboot on the dedicated CRASHLOG volume"