# ChromeOS Flex on hp-chell's internal eMMC

hp-chell (HP Chromebook 13 G1, MrChromebox UEFI firmware) dual-boots:

| OS | Disk | Notes |
|---|---|---|
| KDE neon | `sda`, external USB/SD card (117GB) | primary OS, owns GRUB |
| ChromeOS Flex | `mmcblk0`, internal eMMC (29GB) | installed from neon, see below |

Because neon lives entirely on external media, the eMMC can be wiped and
reinstalled from the running neon system with no risk to neon.

## Installing / reinstalling

```
sudo ./install-flex-emmc.sh ~/Downloads/chromeos_<ver>_reven_recovery_stable-channel_<keys>.bin.zip
```

- Accepts the `.bin.zip` straight from Google's Flex download or an
  already-extracted `.bin` (zips are extracted to `/var/tmp/flex-install`).
- Destination defaults to `/dev/mmcblk0`; pass a second argument to
  override. **The destination is wiped entirely.**
- Refuses to run if the destination holds the running root filesystem,
  shows the target's layout, and asks you to type `yes`.
- Takes about 2.5 minutes. Full installer log: `/var/log/flex-install.log`.
  UEFI boot entries from before the run are saved to
  `/var/log/flex-install-efiboot-before.txt` and the current ones are
  printed at the end.
- Updates the GRUB chainload entry (below) with the new EFI partition's
  UUID and runs `update-grub`.

Last installed: 2026-09-27, Flex `16640.57.0` (stable, `reven`).

## Why not just `dd` the image

The Flex recovery `.bin` is a full GPT disk image, but it's the
**installer** layout, not an installed system:

| Partition | Installer image (raw `dd`) | Proper install |
|---|---|---|
| ROOT-A | 2.4GB | 4GB |
| ROOT-B | 2MB placeholder | 4GB (A/B updates) |
| STATE | fixed 6GB | rest of the disk (~20.6GB) |
| unallocated | ~20.7GB | none |

Flex decides whether it's running from installer media with
`/usr/sbin/is_running_from_installer`, which compares the ROOT-A and
ROOT-B sizes. A raw `dd` therefore boots as live media ("Install
ChromeOS Flex / Try it first"), can't auto-update, and never grows
STATE. **There's no flag to set**, and resizing in place isn't
practical (ROOT-B sits in a 2MB gap between the ESP and ROOT-A). This
was the state from 2026-09-04 (raw `dd`) until the 2026-09-27 reinstall.

The fix is to run the image's own `chromeos-install`, which is what the
"Install ChromeOS Flex" button runs, against the eMMC.

## How the script works

1. Adds a temporary udev rule (`/run/udev/rules.d/99-flex-install-ignore.rules`,
   `UDISKS_IGNORE=1`) so the Plasma desktop doesn't auto-mount eMMC
   partitions mid-install, then unmounts anything already mounted there.
2. Attaches the `.bin` read-only as a partitioned loop device.
3. In a private mount namespace (`unshare -m`; every mount is discarded on
   exit), mounts the image's ROOT-A read-only and builds a chroot:
   tmpfs `/tmp` `/run` `/var` `/mnt/stateful_partition`, bind `/dev`
   `/sys`, fresh `/proc`.
4. **Masks every disk except the destination**: each `/dev/sd*`,
   `/dev/nvme*`, other `/dev/mmcblk*`, and `/dev/vd*` node gets
   `/dev/null` bind-mounted over it, and `/dev/disk` and `/dev/block`
   are hidden. Inside the chroot, reading `/dev/sda` returns 0 bytes, so
   neon is unreachable.
5. Runs:
   ```
   chromeos-install --dst /dev/mmcblk0 --payload_image /tmp/payload.bin \
       --skip_dst_removable --yes --target_bios efi --debug
   ```
6. Checks the log for success, re-reads the partition table, and updates
   GRUB. Cleanup (loop device, udev rule) runs on exit, including on failure.

## Gotchas when running `chromeos-install` from another Linux

Each of these broke an attempt during the 2026-09-27 install:

- **`No recognized cros_XXX bios option on kernel command line`**:
  `chromeos_postinst` reads `/proc/cmdline` for `cros_efi` (or
  `cros_legacy` etc.), even though it's passed `--bios efi`. The script
  bind-mounts a copy of the command line with `cros_efi` appended over
  `/proc/cmdline`. `chromeos-postinst` then re-chroots into the freshly
  written root with a *non-recursive* `mount --bind /proc`, which drops
  that overmount. So the image's `chroot` (a coreutils shim at
  `/bin/chroot`) is also overmounted with a wrapper that re-applies the
  fake cmdline inside the target root before exec'ing the real
  `coreutils --coreutils-prog=chroot`.
- **`sfdisk: This disk is currently in use`**: udisks auto-mounted the
  new STATE partition (`/media/matt/H-STATE`) from a previous attempt.
  The udev rule in step 1 above handles this.
- **Exit status can't be trusted**: `chromeos-install` exited 0 while
  its inner `chromeos-install.sh` had failed. It also logs harmless
  `ERROR cros_installer` lines on success (metrics
  `/var/lib/metrics/uma-events`, "Couldn't check the current
  permission"). The script treats the run as successful only if the log
  has `Installation to '/dev/mmcblk0' complete.` and no final `Error:` line.
- **`Touch(/mnt/stateful_partition/.install_completed) failed`**:
  postinst marks the *installer's* STATE as done. In the chroot that's
  the read-only image, so the script gives it a throwaway tmpfs.
- Harmless noise: `initctl` (no Upstart), `Failed to find physical
  volume`, `BLKRRPART: Device or resource busy`, and ChromeOS ACPI
  `GetAcpiSysfsPath` errors (not stock ChromeOS firmware).
- `nohup … &` over SSH got killed when the session closed, so long runs
  go through `systemd-run --unit=... -p StandardOutput=file:/var/log/...`.
  The log must not be in `/tmp` if another user owns the file (root is
  denied by `fs.protected_regular`).

## Resulting layout (2026-09-27)

```
mmcblk0      29.1G
├─mmcblk0p1  20.6G ext4   STATE
├─mmcblk0p2    64M        KERN-A    (priority=1 successful=1)
├─mmcblk0p3     4G ext4   ROOT-A
├─mmcblk0p4    64M        KERN-B
├─mmcblk0p5     4G ext4   ROOT-B
├─mmcblk0p6-7 512B        KERN-C / ROOT-C
├─mmcblk0p8    16M ext4   OEM
├─mmcblk0p9-11 512B       reserved / reserved / RWFW
└─mmcblk0p12  256M vfat   EFI-SYSTEM (UUID CBE4-703E)
```

The ESP is copied from the image, so its FAT UUID (`CBE4-703E`) survived
the reinstall unchanged. The UEFI boot entries and `BootOrder` were
untouched.

## Booting Flex

Two ways:

1. **From GRUB** (neon's menu, shown for 5s): the entry "ChromeOS Flex
   (internal eMMC)" in `/etc/grub.d/40_custom` chainloads Flex's own EFI
   loader (`crdyboot`) from the eMMC's ESP:
   ```
   menuentry "ChromeOS Flex (internal eMMC)" {
       insmod part_gpt
       insmod fat
       insmod chain
       search --no-floppy --fs-uuid --set=root CBE4-703E
       chainloader /EFI/BOOT/BOOTX64.EFI
   }
   ```
   Run `sudo update-grub` after editing `40_custom`. The install script
   rewrites the `--fs-uuid` value automatically.
2. **From the firmware boot picker** (MrChromebox UEFI, `Esc` during
   POST): `efibootmgr -v` shows `neon` (Boot0005, on the USB card) and
   `Internal eMMC` (Boot0001), with `BootOrder` `0005,0001,0000,0003,0002`.
   With the USB card unplugged, the firmware falls through to `Internal
   eMMC` and boots Flex with no menu.

## History

- **2026-09-04**: first install by streaming the image with
  `unzip -p … | dd of=/dev/mmcblk0 bs=4M conv=fsync` (~18 min; eMMC
  writes dropped from ~130MB/s to ~9MB/s as its SLC cache filled, which
  is normal for this class of eMMC). Booted as live/installer media, as
  explained above. The note at the time that Flex expands STATE on
  first boot was wrong.
- **2026-09-27**: reinstalled properly with `install-flex-emmc.sh`.