实现在树莓派的GPIO接口上用跳线(短路帽)轻松选择启用或者禁用overlayfs影子系统

前面我发过一篇blog说明如何使用overlayfs打造一个只读的不怕意外关机的树莓派Raspberry Pi。

https://blog.csdn.net/zhufu86/article/details/78906046

下面我来实现在树莓派的GPIO接口上用跳线(短路帽)选择启用或者禁用overlayfs影子系统。

【接线】

我是用了用了如下材料

--- 跳线帽(短路帽) * 1

--- 连接线 * 2 (可选)

--- LED * 1 (可选)

--- 电阻 1k欧姆 * 1 (可选)

如上图所示，在BCM GPIO 4 以及和右边的一位针脚（GND）上使用跳线帽，在BCM GPIO 18和其右边的一位针脚（GND）上连接LED，并串联1K电阻，LED的阴极接GND，阳极接电阻，电阻的另一端接GPIO 18。

接LED是为了在启动时指示脚本的运行状态，并不是必须要接的，如果不接也可以。

【原理】

需要在原overlayRoot.sh脚本的基础上，代码开头增加检测GPIO端口的代码。

首先，要mount sysfs，以便于后面访问GPIO

mount -t sysfs sysfs /sys
if [ $? -ne 0 ]; thenfail "ERROR: could not mount sysfs"
fi

设置GPIO引脚变量，可以按照你自己的实际接线更改设置。

# The jumper is on BCM GPIO 4
GPIO_PIN_JP=4
# The LED is connected on BCM GPIO 18
GPIO_PIN_LED=18

定义闪LED的函数，功能是点亮LED 0.4秒钟并熄灭。

# Function - Flash/light-up the LED one time
FlashLED()
{# Export LED pinecho $GPIO_PIN_LED > /sys/class/gpio/export# Set LED pin as output portecho out > /sys/class/gpio/gpio$GPIO_PIN_LED/direction# Set LED pin output HIGHecho 1 > /sys/class/gpio/gpio$GPIO_PIN_LED/valuesleep 0.4# Set LED pin output LOWecho 0 > /sys/class/gpio/gpio$GPIO_PIN_LED/value# Set LED pin as input portecho in > /sys/class/gpio/gpio$GPIO_PIN_LED/direction# Unexport LED pinecho $GPIO_PIN_LED > /sys/class/gpio/unexportsleep 0.4
}

如果不打算连接LED，而且GPIO 18也可能别有他用，可以把这段代码改为无用代码：

# Function - Flash/light-up the LED one time
FlashLED()
{echo I am a LED.
}

后面的代码开始干正事儿。

首先点亮LED 0.4秒后熄灭。

# Flash the LED one time
FlashLED

然后每隔50ms检测一次有没有跳线短接GPIO 4到GND，连续检测10次

# Export Jumper pin
echo "$GPIO_PIN_JP" > /sys/class/gpio/export
# Set Jumper pin as input port
echo in > /sys/class/gpio/gpio$GPIO_PIN_JP/directioni=1
count=0while [ "$i" -le 10 ]
do# Detect the jumper pin every 0.05secsleep 0.05   # If the Jumper is shorted to GNDif [ $(cat "/sys/class/gpio/gpio${GPIO_PIN_JP}/value") = 0 ]; thencount=$((count+1))fii=$((i+1))
done
# Uncomment to print the count value for test purpose
#echo $count# Unexport the Jumper pin
echo $GPIO_PIN_JP > /sys/class/gpio/unexport

如果发现10次检测有9次以上为低电平（即GPIO短路到GND），则点亮LED 0.4秒后熄灭，然后继续常规的init过程（/sbin/init）而不开启overlayfs模式，否则继续后面的开启overlayfs模式的代码。继续之前不忘记unmount sysfs。

if [ $count -ge 9 ]; then# if the Jumper is shorted to GND, exit to the normal init.# Flash the LED one timeFlashLEDumount /sysecho [Info.] Overlayfs jumper setting DISABLE.# Continue with regular initexec /sbin/init#exit 0elseumount /sysecho [Info.] Overlayfs jumper setting ENABLE.# Continue with overlayfs activation process.
fi

【完整的overlayRoot.sh代码】

如下：

#!/bin/sh
#  Read-only Root-FS for Raspian using overlayfs
#  Version 1.0
#
#  Created 2017 by Pascal Suter @ DALCO AG, Switzerland
#  to work on Raspian as custom init script
#  (raspbian does not use an initramfs on boot)
#
#  Modified 2017-Apr-21 by Tony McBeardsley
#
#  * Modified 2018-Jul-18 by Yang Chen
#
#  This program is free software: you can redistribute it and/or modify
#  it under the terms of the GNU General Public License as published by
#  the Free Software Foundation, either version 3 of the License, or
#  (at your option) any later version.
#
#  This program is distributed in the hope that it will be useful,
#  but WITHOUT ANY WARRANTY; without even the implied warranty of
#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
#  GNU General Public License for more details.
#
#    You should have received a copy of the GNU General Public License
#    along with this program.  If not, see
#    <http://www.gnu.org/licenses/>.
#
#
#  Tested with Raspbian mini, 2017-01-11
#
#  This script will mount the root filesystem read-only and overlay it with a temporary tempfs
#  which is read-write mounted. This is done using the overlayFS which is part of the linux kernel
#  since version 3.18.
#  when this script is in use, all changes made to anywhere in the root filesystem mount will be lost
#  upon reboot of the system. The SD card will only be accessed as read-only drive, which significantly
#  helps to prolong its life and prevent filesystem coruption in environments where the system is usually
#  not shut down properly
#
#  Install:
#  copy this script to /sbin/overlayRoot.sh and add "init=/sbin/overlayRoot.sh" to the cmdline.txt
#  file in the raspbian image's boot partition.
#  I strongly recommend to disable swapping before using this. it will work with swap but that just does
#  not make sens as the swap file will be stored in the tempfs which again resides in the ram.
#  run these commands on the booted raspberry pi BEFORE you set the init=/sbin/overlayRoot.sh boot option:
#  sudo dphys-swapfile swapoff
#  sudo dphys-swapfile uninstall
#  sudo update-rc.d dphys-swapfile remove
#
#  To install software, run upgrades and do other changes to the raspberry setup, simply remove the init=
#  entry from the cmdline.txt file and reboot, make the changes, add the init= entry and reboot once more.fail(){echo -e "$1"/bin/bash
}# Add the function +++
# Use a jumper on the Head connector to activate(OPEN) or
# deactivate(SHORT) the overlayfs mode
# Added by C.Yang 20180718
# jumper function code start @@@@@mount -t sysfs sysfs /sys
if [ $? -ne 0 ]; thenfail "ERROR: could not mount sysfs"
fi# The jumper is on BCM GPIO 4
GPIO_PIN_JP=4
# The LED is connected on BCM GPIO 18
GPIO_PIN_LED=18# Function - Flash/light-up the LED one time
FlashLED()
{# Export LED pinecho $GPIO_PIN_LED > /sys/class/gpio/export# Set LED pin as output portecho out > /sys/class/gpio/gpio$GPIO_PIN_LED/direction# Set LED pin output HIGHecho 1 > /sys/class/gpio/gpio$GPIO_PIN_LED/valuesleep 0.4# Set LED pin output LOWecho 0 > /sys/class/gpio/gpio$GPIO_PIN_LED/value# Set LED pin as input portecho in > /sys/class/gpio/gpio$GPIO_PIN_LED/direction# Unexport LED pinecho $GPIO_PIN_LED > /sys/class/gpio/unexportsleep 0.4
}# Flash the LED one time
FlashLED# Export Jumper pin
echo "$GPIO_PIN_JP" > /sys/class/gpio/export
# Set Jumper pin as input port
echo in > /sys/class/gpio/gpio$GPIO_PIN_JP/directioni=1
count=0while [ "$i" -le 10 ]
do# Detect the jumper pin every 0.05secsleep 0.05   # If the Jumper is shorted to GNDif [ $(cat "/sys/class/gpio/gpio${GPIO_PIN_JP}/value") = 0 ]; thencount=$((count+1))fii=$((i+1))
done
# Uncomment to print the count value for test purpose
#echo $count# Unexport the Jumper pin
echo $GPIO_PIN_JP > /sys/class/gpio/unexportif [ $count -ge 9 ]; then# if the Jumper is shorted to GND, exit to the normal init.# Flash the LED one timeFlashLEDumount /sysecho [Info.] Overlayfs jumper setting DISABLE.# Continue with regular initexec /sbin/init#exit 0elseumount /sysecho [Info.] Overlayfs jumper setting ENABLE.# Continue with overlayfs activation process.
fi
# jumper function code END   @@@@@# Load overlay module
modprobe overlay
if [ $? -ne 0 ]; thenfail "ERROR: missing overlay kernel module"
fi# Mount /proc
mount -t proc proc /proc
if [ $? -ne 0 ]; thenfail "ERROR: could not mount proc"
fi# Create a writable fs on /mnt to then create our mountpoints
mount -t tmpfs inittemp /mnt
if [ $? -ne 0 ]; thenfail "ERROR: could not create a temporary filesystem to mount the base filesystems for overlayfs"
fi# Mount a tmpfs under /mnt/rw
mkdir /mnt/rw
mount -t tmpfs root-rw /mnt/rw
if [ $? -ne 0 ]; thenfail "ERROR: could not create tempfs for upper filesystem"
fi# Identify root fs device, PARTUUID, mount options and fs type#rootDev=`blkid -o list | awk '$3 == "/" {print $1}'`
# Changed here(point to / ) in case the cmd above doesn't work # By ChenYang 20171122
rootDev=/dev/mmcblk0p2
rootPARTUUID=`awk '$2 == "/" {print $1}' /etc/fstab`
rootMountOpt=`awk '$2 == "/" {print $4}' /etc/fstab`
rootFsType=`awk '$2 == "/" {print $3}' /etc/fstab`# Mount original root filesystem readonly under /mnt/lower
mkdir /mnt/lower
mount -t ${rootFsType} -o ${rootMountOpt},ro ${rootDev} /mnt/lower
if [ $? -ne 0 ]; thenfail "ERROR: could not ro-mount original root partition"
fi# Mount the overlay filesystem
mkdir /mnt/rw/upper
mkdir /mnt/rw/work
mkdir /mnt/newroot
mount -t overlay -o lowerdir=/mnt/lower,upperdir=/mnt/rw/upper,workdir=/mnt/rw/work overlayfs-root /mnt/newroot
if [ $? -ne 0 ]; thenfail "ERROR: could not mount overlayFS"
fi# Create mountpoints inside the new root filesystem-overlay
mkdir /mnt/newroot/ro
mkdir /mnt/newroot/rw# Remove root mount from fstab (this is already a non-permanent modification)
grep -v "$rootPARTUUID" /mnt/lower/etc/fstab > /mnt/newroot/etc/fstab
echo "#the original root mount has been removed by overlayRoot.sh" >> /mnt/newroot/etc/fstab
echo "#this is only a temporary modification, the original fstab" >> /mnt/newroot/etc/fstab
echo "#stored on the disk can be found in /ro/etc/fstab" >> /mnt/newroot/etc/fstab# Change to the new overlay root
cd /mnt/newroot
pivot_root . mnt
exec chroot . sh -c "$(cat <<END# Move ro and rw mounts to the new rootmount --move /mnt/mnt/lower/ /roif [ $? -ne 0 ]; thenecho "ERROR: could not move ro-root into newroot"/bin/bashfimount --move /mnt/mnt/rw /rwif [ $? -ne 0 ]; thenecho "ERROR: could not move tempfs rw mount into newroot"/bin/bashfi# Unmount unneeded mounts so we can unmout the old readonly rootumount /mnt/mntumount /mnt/procumount /mnt/devumount /mnt# Continue with regular initexec /sbin/init
END
)"# End of file

【备注】

启动之前使用跳线帽短路GPIO 4和GND，则启动过程会看到LED闪烁两次，启动后可以看到overlayfs并未启用。

启动之前如果空开GPIO 4和GND，即不短接跳线帽，则启动过程只会看到LED闪烁一次，启动后可以看到overlayfs已启用。

不连接LED，脚本也是可以正常工作的。GPIO 18如果别有他用，则需要按文中所述方法修改闪烁LED的那段函数。

理论上来说应该在内核启动时设置跳线帽连接的那个GPIO（我的例子里为GPIO 4）为上拉状态，不过我没有做任何处理，似乎也没问题。

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