Dec 07, 2015

Resizing first FAT32 partition to microSD card size on boot from Raspberry Pi

One other thing I've needed to do recently is to have Raspberry Pi OS resize its /boot FAT32 partition to full card size (i.e. "make it as large as possible") from right underneath itself.

RPis usually have first FAT (fat16 / fat32 / vfat) partition needed by firmware to load config.txt and uboot stuff off, and that is the only partition one can see in Windows OS when plugging microSD card into card-reader (which is a kinda arbitrary OS limitation).

Map of the usual /dev/mmcblk0 on RPi (as seen in parted):

Number  Start   End     Size    Type     File system  Flags
        32.3kB  1049kB  1016kB           Free Space
 1      1049kB  106MB   105MB   primary  fat16        lba
 2      106MB   1887MB  1782MB  primary  ext4

Resizing that first partition is naturally difficult, as it is followed by ext4 one with RPi's OS, but when you want to have small (e.g. <2G) and easy-to-write "rpi.img" file for any microSD card, there doesn't seem to be a way around that - img have to have as small initial partitions as possible to fit on any card.

Things get even more complicated by the fact that there don't seem to be any tools around for resizing FAT fs'es, so it has to be re-created from scratch.

There is quite an easy way around all these issues however, which can be summed-up as a sequence of the following steps:

  • Start while rootfs is mounted read-only or when it can be remounted as such, i.e. on early boot.

    Before=systemd-remount-fs.service in systemd terms.

  • Grab sfdisk/parted map of the microSD and check if there's "Free Space" chunk left after last (ext4/os) partition.

    If there is, there's likely a lot of it, as SD cards increase in 2x size factors, so 4G image written on larger card will have 4+ gigs there, in fact a lot more for 16G or 32G cards.

    Or there can be only a few megs there, in case of matching card size, where it's usually a good idea to make slightly smaller images, as actual cards do vary in size a bit.

  • "dd" whole rootfs to the end of the microSD card.

    This is safe with read-only rootfs, and dumb "dd" approach to copying it (as opposed to dmsetup + mkfs + cp) seem to be simpliest and least error-prone.

  • Update partition table to have rootfs in the new location (at the very end of the card) and boot partition covering rest of the space.

  • Initiate reboot, so that OS will load from the new rootfs location.

  • Starting on early-boot again, remount rootfs rw if necessary, temporary copy all contents of boot partition (which should still be small) to rootfs.

  • Run mkfs.vfat on the new large boot partition and copy stuff back to it from rootfs.

  • Reboot once again, in case whatever boot timeouts got triggered.

  • Avoid running same thing on all subsequent boots.

    E.g. touch /etc/boot-resize-done and have ConditionPathExists=!/etc/boot-resize-done in the systemd unit file.

That should do it \o/

resize-rpi-fat32-for-card (in fgtk repo) is a script I wrote to do all of this stuff, exactly as described.

systemd unit file for the thing (can also be printed by running script with "--print-systemd-unit" option):

Before=systemd-remount-fs.service -.mount



It does use lsblk -Jnb JSON output to get rootfs device and partition, and get whether it's mounted read-only, then parted -ms /dev/... unit B print free to grab machine-readable map of the device.

sfdisk -J (also JSON output) could've been better option than parted (extra dep, which is only used to get that one map), except it doesn't conveniently list "free space" blocks and device size, pity.

If partition table doesn't have extra free space at the end, "fsstat" tool from sleuthkit is used to check whether FAT filesystem covers whole partition and needs to be resized.

After that, and only if needed, either "dd + sfdisk" or "cp + mkfs.vfat + cp back" sequence gets executed, followed by a reboot command.

Extra options for the thing:

  • "--skip-ro-check" - don't bother checkin/forcing read-only rootfs before "dd" step, which should be fine, if there's no activity there (e.g. early boot).

  • "--done-touch-file" - allows to specify location of file to create (if missing) when "no resize needed" state gets reached.

    Script doesn't check whether this file exists and always does proper checks of partition table and "fsstat" when deciding whether something has to be done, only creates the file at the end (if it doesn't exist already).

  • "--overlay-image" uses splash.go tool that I've mentioned earlier (be sure to compile it first, ofc) to set some "don't panic, fs resize in progress" image (resized/centered and/or with text and background) during the whole process, using RPi's OpenVG GPU API, covering whatever console output.

  • Misc other stuff for setup/debug - "--print-systemd-unit", "--debug", "--reboot-delay".

    Easy way to debug the thing with these might be to add StandardOutput=tty to systemd unit's Service section and ... --debug --reboot-delay 60 options there, or possibly adding extra ExecStart=/bin/sleep 60 after the script (and changing its ExecStart= to ExecStart=-, so delay will still happen on errors).

    This should provide all the info on what's happening in the script (has plenty of debug output) to the console (one on display or UART).

One more link to the script: resize-rpi-fat32-for-card

Nov 28, 2015

Raspberry Pi early boot splash / logo screen

Imagine you have RPi with some target app (e.g. kiosk-mode browser) starting in X session, and want to have a full-screen splash for the whole time that device will be booting, and no console output or getty's of any kind, and no other splash screens in there - only black screen to logo to target app.

In case of average Raspberry Pi boot, there is:

  • A firmware "color test" splash when device gets powered-on.

    Removed with disable_splash=1 in /boot/config.txt.

  • "Rainbow-colored square" over-current indicator popping up on the right, regardless of PSU or cables, it seems.

    avoid_warnings=1 in the same config.txt

  • As kernel boots - Raspberry Pi logo embedded in it.

    logo.nologo to /boot/cmdline.txt.

    Replacing that logo with proper splash screen is not really an option, as logos that work there have to be tiny - like 80x80 pixels tiny.

    Anything larger than that gives fbcon_init: disable boot-logo (boot-logo bigger than screen), so in-kernel logo isn't that useful, and it's a pain to embed it there anyway (kernel rebuild!).

  • Lots of console output - from kernel and init both.

    cmdline.txt: console=null quiet

  • Getty showing its login prompt.

    systemctl disable getty@tty1

  • More printk stuff, as various kernel modules get initialized and hardware detected.

    console=null in cmdline.txt should've removed all that.

    consoleblank=0 loglevel=1 rootfstype=ext4 helps if console=null is not an option, e.g. because "fbi" should set logo there (see below).

    Need for "rootfstype" is kinda funny, because messages from kernel trying to mount rootfs as ext2/ext3 seem to be emergency-level or something.

  • Removing all the stuff above should (finally!) get a peaceful black screen, but what about the actual splash image?

    fbi -d /dev/fb0 --once --noverbose\
      --autozoom /path/to/image.png </dev/tty1 >/dev/tty1

    Or, in early-boot-systemd terms:

    ExecStart=/usr/bin/fbi -d /dev/fb0\
      --once --noverbose --autozoom /path/to/image.png

    "fbi" is a tool from fbida project.

    console=null should NOT be in cmdline for this tool to work (see above).

    First time you run it, you'll probably get:

    ioctl VT_GETSTATE: Inappropriate ioctl for device (not a linux console?)

    A lot of people on the internets seem to suggest something like "just run it from Alt + F1 console", which definitely isn't an option for this case, but I/O redirection to /dev/tty (as shown above) seem to work.

  • Blank black screen and whatever flickering on X startup.

    Running X on a different VT from "fbi" seem to have nice effect that if X will have to be restarted for some reason (e.g. whole user session gets restarted due to target app's watchdog + StartLimitAction=), VT will switch back to a nice logo, not some text console.

    To fix blackness in X before-and-after WM, there're tools like feh:

    feh --bg-scale /path/to/image.png

    That's not instant though, as X usually takes its time starting up, so see more on it below.

  • Target app startup cruft - e.g. browser window without anything loaded yet, or worse - something like window elements being drawn.

    • There can be some WM tricks to avoid showing unprepared window, including "start minimized, then maximize", switching "virtual desktops", overlay windows, transparency with compositors, etc.

      Depends heavily on WM, obviously, and needs one that can be controlled from the script (which is rather common among modern standalone WMs).

    • Another trick is to start whole X without switching VT - i.e. X -novtswitch vt2 - and switch to that VT later when both X and app signal that they're ready, or just been given enough time.

      Until switch happens, splash logo is displayed, courtesy of "fbi" tool.

    • On Raspberry Pi in particular, there're some direct-to-display VideoCore APIs, which allow to overlay anything on top of whatever Linux or X draw in their VTs while starting-up.

      This is actually a cool thing - e.g. starting omxplayer --no-osd --no-keys /path/to/image.png.mp4 (mp4 produced from still image) early on boot (it doesn't need X or anything!) will remove the need for most previous steps, as it will eclipse all the other video output.

      "omxplayer" maybe isn't the best tool for the job, as it's not really meant to display still images, but it's fast and liteweight enough.

      Better alternative I've found is to use OpenVG API via openvg lib, which has nice Go (golang) version, and wrote an overlay-image.go tool to utilize it for this simple "display image and hang forever" (to be stopped when boot finishes) purpose.

      Aforementioned Go tool has "-resize" flag to scale the image to current display size with "convert" and cache it with ".cache-WxH" suffix, and "-bg-color" option to set margins' color otherwise (for e.g. logo centered with solid color around it). Can be built (be sure to set $GOPATH first) with: go get && go build .

  • Finally some destination state with target app showing what it's supposed to.

    Yay, we got here!

Not a very comprehensive or coherent guide, but might be useful to sweep all the RPi nasties under an exquisite and colorful rug ;)

Update 2015-11-30: Added link to overlay-image.go tool.

Update 2015-11-30: A bit different version (cleaned-up, with build-dep on "" instead of optional call to "convert") of this tool has been added to openvg lib repo under "go-client/splash".

Nov 25, 2015

Replacing built-in RTC with i2c battery-backed one on BeagleBone Black from boot

BeagleBone Black (BBB) boards have - and use - RTC (Real-Time Clock - device that tracks wall-clock time, including calendar date and time of day) in the SoC, which isn't battery-backed, so looses track of time each time device gets power-cycled.

This represents a problem if keeping track of time is necessary and there's no network access (or a patchy one) to sync this internal RTC when board boots up.

Easy solution to that, of course, is plugging external RTC device, with plenty of cheap chips with various precision available, most common being Dallas/Maxim ICs like DS1307 or DS3231 (a better one of the line) with I2C interface, which are all supported by Linux "ds1307" module.

Enabling connected chip at runtime can be easily done with a command like this:

echo ds1307 0x68 >/sys/bus/i2c/devices/i2c-2/new_device

(see this post on Fortune Datko blog and/or this one on minix-i2c blog for ways to tell reliably which i2c device in /dev corresponds to which bus and pin numbers on BBB headers, and how to check/detect/enumerate connected devices there)

This obviously doesn't enable device straight from the boot though, which is usually accomplished by adding the thing to Device Tree, and earlier with e.g. 3.18.x kernels it had to be done by patching and re-compiling platform dtb file used on boot.

But since 3.19.x kernels (and before 3.9.x), easier way seem to be to use Device Tree Overlays (usually "/lib/firmware/*.dtbo" files, compiled by "dtc" from dts files), which is kinda like patching Device Tree, only done at runtime.

Code for such patch in my case ("i2c2-rtc-ds3231.dts"), with 0x68 address on i2c2 bus and "ds3231" kernel module (alias for "ds1307", but more appropriate for my chip):


/* dtc -O dtb -o /lib/firmware/BB-RTC-02-00A0.dtbo -b0 i2c2-rtc-ds3231.dts */
/* bone_capemgr.enable_partno=BB-RTC-02 */
/* */

/ {
  compatible = "ti,beaglebone", "ti,beaglebone-black", "ti,beaglebone-green";
  part-number = "BB-RTC-02";
  version = "00A0";

  fragment@0 {
    target = <&i2c2>;

    __overlay__ {
      pinctrl-names = "default";
      pinctrl-0 = <&i2c2_pins>;
      status = "okay";
      clock-frequency = <100000>;
      #address-cells = <0x1>;
      #size-cells = <0x0>;

      rtc: rtc@68 {
        compatible = "dallas,ds3231";
        reg = <0x68>;

As per comment in the overlay file, can be compiled ("dtc" comes from "dtc-overlay" package on ArchLinuxARM) to the destination with:

dtc -O dtb -o /lib/firmware/BB-RTC-02-00A0.dtbo -b0 i2c2-rtc-ds3231.dts

And then loaded on early boot (as soon as rootfs with "/lib/firmware" gets mounted) with "bone_capemgr.enable_partno=" cmdline addition, and should be put to something like "/boot/uEnv.txt", for example (with dtb path from command above):


Docs in repository have more details and examples on how to write and manage these.

That should ensure that this second RTC appears as "/dev/rtc1" (rtc0 is an internal one) on system startup, but unfortunately it still won't be the first one and kernel will already pick up time from internal rtc0 by the time this one gets detected.

Furthermore, systemd-enabled userspace (as in e.g. ArchLinuxARM) interacts with RTC via systemd-timedated and systemd-timesyncd, which both use "/dev/rtc" symlink (and can't be configured to use other devs), which by default udev points to rtc0 as well, and rtc1 - no matter how early it appears - gets completely ignored there as well.

So two issues are with "system clock" that kernel keeps and userspace daemons using wrong RTC, which is default in both cases.

"/dev/rtc" symlink for userspace gets created by udev, according to "/usr/lib/udev/rules.d/50-udev-default.rules", and can be overidden by e.g. "/etc/udev/rules.d/55-i2c-rtc.rules":

SUBSYSTEM=="rtc", KERNEL=="rtc1", SYMLINK+="rtc", OPTIONS+="link_priority=10", TAG+="systemd"

This sets "link_priority" to 10 to override SYMLINK directive for same "rtc" dev node name from "50-udev-default.rules", which has link_priority=-100.

Also, TAG+="systemd" makes systemd track device with its "dev-rtc.device" unit (auto-generated, see systemd.device(5) for more info), which is useful to order userspace daemons depending on that symlink to start strictly after it's there.

"userspace daemons" in question on a basic Arch are systemd-timesyncd and systemd-timedated, of which only systemd-timesyncd starts early on boot, before all other services, including systemd-timedated, and (for early-boot clock-dependant services).

So basically if proper "/dev/rtc" and system clock gets initialized before systemd-timesyncd (or whatever replacement, like ntpd or chrony), correct time and rtc device will be used for all system daemons (which start later) from here on.

Adding that extra step can be done as a separate systemd unit (to avoid messing with shipped systemd-timesyncd.service), e.g. "i2c-rtc.service":

Before=systemd-timesyncd.service ntpd.service chrony.service

DeviceAllow=/dev/rtc rw
ExecStart=/usr/bin/hwclock --hctosys


Note that Before= above should include whatever time-sync daemon is used on the machine, and there's no harm in listing non-existant or unused units there jic.

Most security-related stuff and conditions are picked from systemd-timesyncd unit file, which needs roughly same access permissions as "hwclock" here.

With udev rule and that systemd service (don't forget to "systemctl enable" it), boot sequence goes like this:

  • Kernel inits internal rtc0 and sets system clock to 1970-01-01.
  • Kernel starts systemd.
  • systemd mounts local filesystems and starts i2c-rtc asap.
  • i2c-rtc, due to Wants/After=dev-rtc.device, starts waiting for /dev/rtc to appear.
  • Kernel detects/initializes ds1307 i2c device.
  • udev creates /dev/rtc symlink and tags it for systemd.
  • systemd detects tagging event and activates dev-rtc.device.
  • i2c-rtc starts, adjusting system clock to realistic value from battery-backed rtc.
  • systemd-timesyncd starts, using proper /dev/rtc and correct system clock value.
  • activates, as it is scheduled to, after systemd-timesyncd and i2c-rtc.
  • From there, boot goes on to, and starts all the daemons.

udev rule is what facilitates symlink and tagging, i2c-rtc.service unit is what makes boot sequence wait for that /dev/rtc to appear and adjusts system clock right after that.

Haven't found an up-to-date and end-to-end description with examples anywhere, so here it is. Cheers!

Oct 22, 2015

Arch Linux chroot and sshd from boot on rooted Android with SuperSU

Got myself Android device only recently, and first thing I wanted to do, of course, was to ssh into it.

But quick look at the current F-Droid and Play apps shows that they either based on a quite limited dropbear (though perfectly fine for one-off shell access) or "based on openssh code", where infamous Debian OpenSSL code patch comes to mind, not to mention that most look like ad-ridden proprietary piece of crap.

Plus I'd want a proper package manager, shell (zsh), tools and other stuff in there, not just some baseline bash and busybox, and chroot with regular linux distro is a way to get all that plus standard OpenSSH daemon.

Under the hood, modern Android (5.11 in my case, with CM 12) phone is not much more than Java VM running on top of SELinux-enabled (which I opted to keep for Android stuff) Linux kernel, on top of some multi-core ARMv7 CPU (quadcore in my case, rather identical to one in RPi2).

Steps I took to have sshd running on boot with such device:

  • Flush stock firmware (usually loaded with adware and crapware) and install some pre-rooted firmware image from or

    My phone is a second-hand Samsung Galaxy S3 Neo Duos (GT-I9300I), and I picked resurrection remix CM-based ROM for it, built for this phone from t3123799, with Open GApps arm 5.1 Pico (minimal set of google stuff).

    That ROM (like most of them, it appears) comes with bash, busybox and - most importantly - SuperSU, which runs its unrestricted init, and is crucial to getting chroot-script to start on boot. All three of these are needed.

    Under Windows, there's Odin suite for flashing zip with all the goodies to USB-connected phone booted into "download mode".

    On Linux, there's Heimdall (don't forget to install adb with that, e.g. via pacman -S android-tools android-udev on Arch), which can dd img files for diff phone partitions, but doesn't seem to support "one zip with everything" format that most firmwares come in.

    Instead of figuring out which stuff from zip to upload where with Heimdall, I'd suggest grabbing a must-have TWRP recovery.img (small os that boots in "recovery mode", grabbed one for my phone from t2906840), flashing it with e.g. heimdall flash --RECOVERY recovery.twrp- and then booting into it to install whatever main OS and apps from zip's on microSD card.

    TWRP (or similar CWM one) is really useful for a lot of OS-management stuff like app-packs installation (e.g. Open GApps) or removal, updates, backups, etc, so I'd definitely suggest installing one of these as recovery.img as the first thing on any Android device.

  • Get or make ARM chroot tarball.

    I did this by bootstrapping a stripped-down ARMv7 Arch Linux ARM chroot on a Raspberry Pi 2 that I have around:

    # pacman -Sg base
     ### Look through the list of packages,
     ###  drop all the stuff that won't ever be useful in chroot on the phone,
     ###  e.g. pciutils, usbutils, mdadm, lvm2, reiserfsprogs, xfsprogs, jfsutils, etc
     ### pacstrap chroot with whatever is left
    # mkdir droid-chroot
    # pacstrap -i -d droid-chroot bash bzip2 coreutils diffutils file filesystem \
        findutils gawk gcc-libs gettext glibc grep gzip iproute2 iputils less \
        licenses logrotate man-db man-pages nano pacman perl procps-ng psmisc \
        sed shadow sysfsutils tar texinfo util-linux which
     ### Install whatever was forgotten in pacstrap
    # pacman -r droid-chroot -S --needed atop busybox colordiff dash fping git \
        ipset iptables lz4 openssh patch pv rsync screen xz zsh fcron \
        python2 python2-pip python2-setuptools python2-virtualenv
     ### Make a tar.gz out of it all
    # rm -f droid-chroot/var/cache/pacman/pkg/*
    # tar -czf droid-chroot.tar.gz droid-chroot

    Same can be obviously done with debootstrap or whatever other distro-of-choice bootstrapping tool, but likely has to be done on a compatible architecture - something that can run ARMv7 binaries, like RPi2 in my case (though VM should do too) - to run whatever package hooks upon installs.

    Easier way (that won't require having spare ARM box or vm) would be to take pre-made image for any ARMv7 platform, from list or such, but unless there's something very generic (which e.g. "ArchLinuxARM-armv7-latest.tar.gz" seem to be), there's likely be some platform-specific cruft like kernel, modules, firmware blobs, SoC-specific tools and such... nothing that can't be removed at any later point with package manager or simple "rm", of course.

    While architecture in my case is ARMv7, which is quite common nowadays (2015), other devices can have Intel SoCs with x86 or newer 64-bit ARMv8 CPUs. For x86, bootstrapping can obviously be done on pretty much any desktop/laptop machine, if needed.

  • Get root shell on device and unpack chroot tarball there.

    adb shell (pacman -S android-tools android-udev or other distro equivalent, if missing) should get "system" shell on a USB-connected phone.

    (btw, "adb" access have to be enabled on the phone via some common "tap 7 times on OS version in Settings-About then go to Developer-options" dance, if not already)

    With SuperSU (or similar "su" package) installed, next step would be running "su" there to get unrestricted root, which should work.

    /system and /data should be on ext4 (check mount | grep ext4 for proper list), f2fs or such "proper" filesystem, which is important to have for all the unix permission bits and uid/gid info which e.g. FAT can't handle (loopback img with ext4 can be created in that case, but shouldn't be necessary in case of internal flash storage, which should have proper fs'es already).

    /system is a bad place for anything custom, as it will be completely flushed on most main-OS changes (when updating ROM from zip with TWRP, for instance), and is mounted with "ro" on boot anyway.

    Any subdir in /data seem to work fine, though one obvious pre-existing place - /data/local - is probably a bad idea, as it is used by some Android dev tools already.

    With busybox and proper bash on the phone, unpacking tarball from e.g. microSD card should be easy:

    # mkdir -m700 /data/chroots
    # cd /data/chroots
    # tar -xpf /mnt/sdcard/droid-chroot.tar.gz

    It should already work, too, so...

    # cd droid-chroot
    # mount -o bind /dev dev \
        && mount -o bind /dev/pts dev/pts \
        && mount -t proc proc proc \
        && mount -t sysfs sysfs sys
    # env -i TERM=$TERM SHELL=/bin/zsh HOME=/root $(which chroot) . /bin/zsh

    ...should produce a proper shell in a proper OS, yay! \o/

    Furthermore, to be able to connect there directly, without adb or USB cable, env -i $(which chroot) . /bin/sshd should work too.

    For sshd in particular, one useful thing to do here is:

    # $(which chroot) . /bin/ssh-keygen -A
 populate /etc/ssh with keys, which are required to start sshd.

  • Setup init script to run sshd or whatever init-stuff from that chroot on boot.

    Main trick here is to run it with unrestricted SELinux context (unless SELinux is disabled entirely, I guess).

    This makes /system/etc/init.d using "sysinit_exec" and /data/local/ with "userinit_exec" unsuitable for the task, only something like "init" ("u:r:init:s0") will work.

    SELinux on Android is documented in Android docs, and everything about SELinux in general applies there, of course, but some su-related roles like above "userinit_exec" actually come with CyanogenMod or whatever similar hacks on top of the base Android OS.

    Most relevant info on this stuff comes with SuperSU though (or rather libsuperuser) -

    That doc has info on how to patch policies, to e.g. transition to unrestricted role for chroot init, setup sub-roles for stuff in there (to also use SELinux in a chroot), which contexts are used where, and - most useful in this case - which custom "init" dirs are used at which stages of the boot process.

    Among other useful stuff, it specifies/describes /system/su.d init-dir, from which SuperSU runs scripts/binaries with unrestricted "init" context, and very early in the process too, hence it is most suitable for starting chroot from.

    So, again, from root (after "su") shell:

    # mount -o remount,rw /system
    # mkdir -m700 /system/su.d
    # cat >/system/su.d/ <<EOF
    exec /data/local/chroots.bash
    # chmod 700 /system/su.d/
    # cat >/data/local/chroots.bash <<EOF
    export PATH=/sbin:/vendor/bin:/system/sbin:/system/bin:/system/xbin
    [[ $(du -m "$log" | awk '{print $1}') -gt 20 ]] && mv "$log"{,.old}
    exec >>"$log" 2>&1
    echo " --- Started $(TZ=UTC date) --- "
    log -p i -t chroots "Starting chroot: droid-chroot"
    /data/chroots/ &
    log -p i -t chroots "Finished chroots init"
    echo " --- Finished $(TZ=UTC date) --- "
    # chmod 700 /data/local/chroots.bash
    # cd /data/chroots
    # mkdir -p droid-chroot/mnt/storage
    # ln -s droid-chroot/
    # cat >droid-chroot/ <<EOF
    set -e -o pipefail
    usage() {
      bin=$(basename $0)
      echo >&2 "Usage: $bin [ stop | chroot ]"
      exit ${1:-0}
    [[ "$#" -gt 1 || "$1" = -h || "$1" = --help ]] && usage
    cd /data/chroots/droid-chroot
    sshd_pid=$(cat run/ 2>/dev/null ||:)
    mountpoint -q dev || mount -o bind /dev dev
    mountpoint -q dev/pts || mount -o bind /dev/pts dev/pts
    mountpoint -q proc || mount -t proc proc proc
    mountpoint -q sys || mount -t sysfs sysfs sys
    mountpoint -q tmp || mount -o nosuid,nodev,size=20%,mode=1777 -t tmpfs tmpfs tmp
    mountpoint -q run || mount -o nosuid,nodev,size=20% -t tmpfs tmpfs run
    mountpoint -q mnt/storage || mount -o bind /data/media/0 mnt/storage
    case "$1" in
        [[ -z "$sshd_pid" ]] || kill "$sshd_pid"
        exit 0 ;;
        exec env -i\
          TERM="$TERM" SHELL=/bin/zsh HOME=/root\
          /system/xbin/chroot . /bin/zsh ;;
      *) [[ -z "$1" ]] || usage 1 ;;
    [[ -n "$sshd_pid" ]]\
      && kill -0 "$sshd_pid" 2>/dev/null\
      || exec env -i /system/xbin/chroot . /bin/sshd
    # chmod 700 droid-chroot/

    To unpack all that wall-of-shell a bit:

    • Very simple /system/su.d/ is created, so that it can easily be replaced if/when /system gets flushed by some update, and also so that it won't need to be edited (needing rw remount) ever.

    • /data/local/chroots.bash is an actual init script for whatever chroots, with Android logging stuff (accessible via e.g. adb logcat, useful to check if script was ever started) and simplier more reliable (and rotated) log in /data/local/chroots.log.

    • /data/chroots/ is a symlink to init script in /data/chroots/droid-chroot/, so that this script can be easily edited from inside of the chroot itself.

    • /data/chroots/droid-chroot/ is the script that mounts all the stuff needed for the chroot and starts sshd there.

      Can also be run from adb root shell to do the same thing, with "stop" arg to kill that sshd, or with "chroot" arg to do all the mounts and chroot into the thing from whatever current sh.

      Basically everything to with that chroot from now on can/should be done through that script.

    "cat" commands can obviously be replaced with "nano" and copy-paste there, or copying same (or similar) scripts from card or whatever other paths (to avoid pasting them into shell, which might be less convenient than Ctrl+S in $EDITOR).

  • Reboot, test sshd, should work.

Anything other than sshd can also be added to that init script, to make some full-featured dns + web + mail + torrents server setup start in chroot.

With more than a few daemons, it'd probably be a good idea to start just one "daemon babysitter" app from there, such as runit, daemontools or whatever. Maybe even systemd will work, though unlikely, given how it needs udev, lots of kernel features and apis initialized in its own way, and such.

Obvious caveat for running a full-fledged linux separately from main OS is that it should probably be managed through local webui's or from some local terminal app, and won't care much about power management and playing nice with Android stuff.

Android shouldn't play nice with such parasite OS either, cutting network or suspending device when it feels convenient, without any regard for conventional apps running there, though can be easily configured not to.

As I'm unlikely to want this device as a phone ever (who needs these, anyway?), turning it into something more like wireless RPi2 with a connected management terminal (represented by Android userspace) sounds like the only good use for it so far.

Update 2016-05-16: Added note on ssh-keygen and rm for pacman package cache after pacstrap.

Jan 30, 2015

Enabling i2c1 on BeagleBone Black without Device Tree overlays

Important: This way was pretty much made obsolete by Device Tree overlays, which have returned (as expected) in 3.19 kernels - it'd probably be easier to use these in most cases.

BeagleBone Black board has three i2c buses, two of which are available by default on Linux kernel with patches from RCN (Robert Nelson).

There are plenty of links on how to enable i2c1 on old-ish (by now) 3.8-series kernels, which had "Device Tree overlays" patches, but these do not apply to 3.9-3.18, though it looks like they might make a comeback in the future (LWN).
Probably it's just a bit too specific task to ask an easy answer for.

Overlays in pre-3.9 allowed to write a "patch" for a Device Tree, compile it and then load it at runtime, which is not possible without these, but perfectly possible and kinda-easy to do by compiling a dtb and loading it on boot.

It'd be great if there was a prebuilt dtb file in Linux with just i2c1 enabled (not just for some cape, bundled with other settings), but unfortunately, as of patched 3.18.4, there doesn't seem to be one, hence the following patching process.

For that, getting the kernel sources (whichever were used to build the kernel, ideally) is necessary.

In Arch Linux ARM (which I tend to use with such boards), this can be done by grabbing the PKGBUILD dir, editing the "PKGBUILD" file, uncommenting the "return 1" under "stop here - this is useful to configure the kernel" comment and running makepkg -sf (from "base-devel" package set on arch) there.
That will just unpack the kernel sources, put the appropriate .config file there and run make prepare on them.

With kernel sources unpacked, the file that you'd want to patch is "arch/arm/boot/dts/am335x-boneblack.dts" (or whichever other dtb you're loading via uboot):

--- am335x-boneblack.dts.bak    2015-01-29 18:20:29.547909768 +0500
+++ am335x-boneblack.dts        2015-01-30 20:56:43.129213998 +0500
@@ -23,6 +23,14 @@

 &ocp {
+       /* i2c */
+       P9_17_pinmux {
+               status = "disabled";
+       };
+       P9_18_pinmux {
+               status = "disabled";
+       };
        /* clkout2 */
        P9_41_pinmux {
                status = "disabled";
@@ -33,6 +41,13 @@

+&i2c1 {
+       status = "okay";
+       pinctrl-names = "default";
+       pinctrl-0 = <&i2c1_pins>;
+       clock-frequency = <100000>;
 &mmc1 {
        vmmc-supply = <&vmmcsd_fixed>;

Then "make dtbs" can be used to build dtb files only, and not the whole kernel (which would take a while on BBB).

Resulting *.dtb (e.g. "am335x-boneblack.dtb" for "am335x-boneblack.dts", in the same dir) can be put into "dtbs" on boot partition and loaded from uEnv.txt (or whatever uboot configs are included from there).

Reboot, and i2cdetect -l should show i2c-1:

# i2cdetect -l
i2c-0   i2c             OMAP I2C adapter                    I2C adapter
i2c-1   i2c             OMAP I2C adapter                    I2C adapter
i2c-2   i2c             OMAP I2C adapter                    I2C adapter

As I've already mentioned before, this might not be the optimal way to enable the thing in kernels 3.19 and beyond, if "device tree overlays" patches will land there - it should be possible to just load some patch on-the-fly there, without all the extra hassle described above.

Update 2015-03-19: Device Tree overlays landed in 3.19 indeed, but if migrating to use these is too much hassle for now, here's a patch for 3.19.1-bone4 am335x-bone-common.dtsi to enable i2c1 and i2c2 on boot (applies in the same way, make dtbs, copy am335x-boneblack.dtb to /boot/dtbs).

Jan 28, 2015

Sample code for using ST7032I I2C/SMBus driver in Midas LCD with python

There seem to be a surprising lack of python code on the net for this particular device, except for this nice pi-ras blog post, in japanese.
So, to give google some more food and a bit of commentary in english to that post - here goes.

I'm using Midas MCCOG21605C6W-SPTLYI 2x16 chars LCD panel, connected to 5V VDD and 3.3V BeagleBone Black I2C bus:

simple digital clock on lcd

Code for the above LCD clock "app" (python 2.7):

import smbus, time

class ST7032I(object):

  def __init__(self, addr, i2c_chan, **init_kws):
    self.addr, self.bus = addr, smbus.SMBus(i2c_chan)

  def _write(self, data, cmd=0, delay=None):
    self.bus.write_i2c_block_data(self.addr, cmd, list(data))
    if delay: time.sleep(delay)

  def init(self, contrast=0x10, icon=False, booster=False):
    assert contrast < 0x40 # 6 bits only, probably not used on most lcds
    pic_high = 0b0111 << 4 | (contrast & 0x0f) # c3 c2 c1 c0
    pic_low = ( 0b0101 << 4 |
      icon << 3 | booster << 2 | ((contrast >> 4) & 0x03) ) # c5 c4
    self._write([0x38, 0x39, 0x14, pic_high, pic_low, 0x6c], delay=0.01)
    self._write([0x0c, 0x01, 0x06], delay=0.01)

  def move(self, row=0, col=0):
    assert 0 <= row <= 1 and 0 <= col <= 15, [row, col]
    self._write([0b1000 << 4 | (0x40 * row + col)])

  def addstr(self, chars, pos=None):
    if pos is not None:
      row, col = (pos, 0) if isinstance(pos, int) else pos
      self.move(row, col)
    self._write(map(ord, chars), cmd=0x40)

  def clear(self):

if __name__ == '__main__':
  lcd = ST7032I(0x3e, 2)
  while True:
    ts_tuple = time.localtime()
    lcd.addstr(time.strftime('date: %y-%m-%d', ts_tuple), 0)
    lcd.addstr(time.strftime('time: %H:%M:%S', ts_tuple), 1)
Note the constants in the "init" function - these are all from "INITIALIZE(5V)" sequence on page-8 of the Midas LCD datasheet , setting up things like voltage follower circuit, OSC frequency, contrast (not used on my panel), modes and such.
Actual reference on what all these instructions do and how they're decoded can be found on page-20 there.

Even with the same exact display, but connected to 3.3V, these numbers should probably be a bit different - check the datasheet (e.g. page-7 there).

Also note the "addr" and "i2c_chan" values (0x3E and 2) - these should be taken from the board itself.

"i2c_chan" is the number of the device (X) in /dev/i2c-X, of which there seem to be usually more than one on ARM boards like RPi or BBB.
For instance, Beaglebone Black has three I2C buses, two of which are available on the expansion headers (with proper dtbs loaded).
See this post on Fortune Datko blog and/or this one on minix-i2c blog for one way to tell reliably which device in /dev corresponds to which hardware bus and pin numbers.

And the address is easy to get from the datasheet (lcd I have uses only one static slave address), or detect via i2cdetect -r -y <i2c_chan>, e.g.:

# i2cdetect -r -y 2
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- --
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- 3e --
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
50: -- -- -- -- UU UU UU UU -- -- -- -- -- -- -- --
60: -- -- -- -- -- -- -- -- 68 -- -- -- -- -- -- --
70: -- -- -- -- -- -- -- --

Here I have DS1307 RTC on 0x68 and an LCD panel on 0x3E address (again, also specified in the datasheet).

Both "i2cdetect" command-line tool and python "smbus" module are part of i2c-tools project, which is developed under lm-sensors umbrella.
On Arch or source-based distros these all come with "i2c-tools" package, but on e.g. debian, python module seem to be split into "python-smbus".

Plugging these bus number and the address for your particular hardware into the script above and maybe adjusting the values there for your lcd panel modes should make the clock show up and tick every second.

In general, upon seeing tutorial on some random blog (like this one), please take it with a grain of salt, because it's highly likely that it was written by a fairly incompetent person (like me), since engineers who deal with these things every day don't see above steps as any kind of accomplishment - it's a boring no-brainer routine for them, and they aren't likely to even think about it, much less write tutorials on it (all trivial and obvious, after all).

Nevertheless, hope this post might be useful to someone as a pointer on where to look to get such device started, if nothing else.

Oct 05, 2014

Simple aufs setup for Arch Linux ARM and boards like RPi, BBB or Cubie

Experimenting with all kinds of arm boards lately (nyms above stand for Raspberry Pi, Beaglebone Black and Cubieboard), I can't help but feel a bit sorry of microsd cards in each one of them.

These are even worse for non-bulk writes than SSD, having less erase cycles plus larger blocks, and yet when used for all fs needs of the board, even typing "ls" into shell will usually emit a write (unless shell doesn't keep history, which sucks).

Great explaination of how they work can be found on LWN (as usual).

Easy and relatively hassle-free way to fix the issue is to use aufs, but as doing it for the whole rootfs requires initramfs (which is not needed here otherwise), it's a lot easier to only use it for commonly-writable parts - i.e. /var and /home in most cases.

Home for "root" user is usually /root, so to make it aufs material as well, it's better to move that to /home (which probably shouldn't be a separate fs on these devices), leaving /root as a symlink to that.

It seem to be impossible to do when logged-in as /root (mv will error with EBUSY), but trivial from any other machine:

# mount /dev/sdb2 /mnt # mount microsd
# cd /mnt
# mv root home/
# ln -s home/root
# cd
# umount /mnt

As aufs2 is already built into Arch Linux ARM kernel, only thing that's left is to add early-boot systemd unit for mounting it, e.g. /etc/systemd/system/aufs.service:




# Remount /home and /var as aufs
ExecStart=/bin/mount -t tmpfs tmpfs /aufs/rw
ExecStart=/bin/mkdir -p -m0755 /aufs/rw/var /aufs/rw/home
ExecStart=/bin/mount -t aufs -o br:/aufs/rw/var=rw:/var=ro none /var
ExecStart=/bin/mount -t aufs -o br:/aufs/rw/home=rw:/home=ro none /home

# Mount "pure" root to /aufs/ro for syncing changes
ExecStart=/bin/mount --bind / /aufs/ro
ExecStart=/bin/mount --make-private /aufs/ro

And then create the dirs used there and enable unit:

# mkdir -p /aufs/{rw,ro}
# systemctl enable aufs

Now, upon rebooting the board, you'll get aufs mounts for /home and /var, making all the writes there go to respective /aufs/rw dirs on tmpfs while allowing to read all the contents from underlying rootfs.

To make sure systemd doesn't waste extra tmpfs space thinking it can sync logs to /var/log/journal, I'd also suggest to do this (before rebooting with aufs mounts):

# rm -rf /var/log/journal
# ln -s /dev/null /var/log/journal

Can also be done via journald.conf with Storage=volatile.

One obvious caveat with aufs is, of course, how to deal with things that do expect to have permanent storage in /var - examples can be a pacman (Arch package manager) on system updates, postfix or any db.
For stock Arch Linux ARM though, it's only pacman on manual updates.

And depending on the app and how "ok" can loss of this data might be, app dir in /var (e.g. /var/lib/pacman) can be either moved + symlinked to /srv or synced before shutdown or after it's done with writing (for manual oneshot apps like pacman).

For moving stuff back to permanent fs, aubrsync from aufs2-util.git can be used like this:

# aubrsync move /var/ /aufs/rw/var/ /aufs/ro/var/

As even pulling that from shell history can be a bit tedious, I've made a simplier ad-hoc wrapper - aufs_sync - that can be used (with mountpoints similar to presented above) like this:

# aufs_sync
Usage: aufs_sync { copy | move | check } [module]
Example (flushes /var): aufs_sync move var

# aufs_sync check
--- ... just does "find /aufs/rw"

# aufs_sync move
--- does "aubrsync move" for all dirs in /aufs/rw

Just be sure to check if any new apps might write something important there (right after installing these) and do symlinks (to something like /srv) for their dirs, as even having "aufs_sync copy" on shutdown definitely won't prevent data loss for these on e.g. sudden power blackout or any crashes.

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