Yocto构建只读文件系统

嵌入式调试过程中，mount命令发现系统分区是只读的，不利于开发阶段的调试，例如想单独安装调试软件都不行。

/dev/mmcblk0p5 on / type ext4 (ro,relatime)　

于是想让系统变为rw支持读写的，条条大路通罗马，Linux世界里要实现一个功能有N个方法，系统是否只读可以由多个因素决定，如：

1. 文件系统的类型，例如squashfs文件系统就是一个压缩只读的文件系统，如果系统采用squashfs文件系统的话，那么系统起来后是不能写的
2. /etc/fstab中指定分区的读写类型

例如下例中系统根目录是只读的，不能写

cat /etc/fstab
# stock fstab - you probably want to override this with a machine specific one

/dev/root            /                    auto       ro              0  0　

那么在Yocto系统如何修改fstab控制系统各个分区的读写属性呢?

以我的Yocto版本为例，其中有一个base-files_3.0.14.bb，会设置系统的fstab，这里默认是读写属性，如果想设置自己的系统为只读，可以创建对应的base-files_3.0.14.bbappend在其中对fstab修改，

但是我的例子中分析完base-files相关的recipe后发现他们没有修改为只读属性。于是如下命令查找所有layers下的对fstab相关的bb文件

find  layers/ -type f -name "*bb*" -exec grep -H fstab {} \;　　

发现layers/oe-core/meta/classes/rootfs-postcommands.bbclass中的read_only_rootfs_hook函数有对读写属性操作。

进一步跟踪定位发现，所以IMAGE_FEATURES如果设置为read-only-rootfs那么fstab中将设置为只读。

# Tweak the mount options for rootfs in /etc/fstab if read-only-rootfs is enabled
ROOTFS_POSTPROCESS_COMMAND += '${@bb.utils.contains("IMAGE_FEATURES", "read-only-rootfs", "read_only_rootfs_hook; ", "",d)}'　

接下来就好说了，查看recipe中设置IMAGE_FEATURES的地方就好了。

BTW: 由read_only_rootfs_hook实现来看，不同的系统只读设置是有很多可能的，详情参照read_only_rootfs_hook。

 

 

 

FYI：read_only_rootfs_hook函数

#
# A hook function to support read-only-rootfs IMAGE_FEATURES
#
read_only_rootfs_hook () {
        # Tweak the mount option and fs_passno for rootfs in fstab
        if [ -f ${IMAGE_ROOTFS}/etc/fstab ]; then
                sed -i -e '/^[#[:space:]]*\/dev\/root/{s/defaults/ro/;s/\([[:space:]]*[[:digit:]]\)\([[:space:]]*\)[[:digit:]]$/\1\20/}' ${IMAGE_ROOTFS}/etc/fstab
        fi

        # Tweak the "mount -o remount,rw /" command in busybox-inittab inittab
        if [ -f ${IMAGE_ROOTFS}/etc/inittab ]; then
                sed -i 's|/bin/mount -o remount,rw /|/bin/mount -o remount,ro /|' ${IMAGE_ROOTFS}/etc/inittab
        fi

        # If we're using openssh and the /etc/ssh directory has no pre-generated keys,
        # we should configure openssh to use the configuration file /etc/ssh/sshd_config_readonly
        # and the keys under /var/run/ssh.
        if [ -d ${IMAGE_ROOTFS}/etc/ssh ]; then
                if [ -e ${IMAGE_ROOTFS}/etc/ssh/ssh_host_rsa_key ]; then
                        echo "SYSCONFDIR=\${SYSCONFDIR:-/etc/ssh}" >> ${IMAGE_ROOTFS}/etc/default/ssh
                        echo "SSHD_OPTS=" >> ${IMAGE_ROOTFS}/etc/default/ssh
                else
                        echo "SYSCONFDIR=\${SYSCONFDIR:-/var/run/ssh}" >> ${IMAGE_ROOTFS}/etc/default/ssh
                        echo "SSHD_OPTS='-f /etc/ssh/sshd_config_readonly'" >> ${IMAGE_ROOTFS}/etc/default/ssh
                fi
        fi

        # Also tweak the key location for dropbear in the same way.
        if [ -d ${IMAGE_ROOTFS}/etc/dropbear ]; then
                if [ ! -e ${IMAGE_ROOTFS}/etc/dropbear/dropbear_rsa_host_key ]; then
                        echo "DROPBEAR_RSAKEY_DIR=/var/lib/dropbear" >> ${IMAGE_ROOTFS}/etc/default/dropbear
                fi
        fi

        if ${@bb.utils.contains("DISTRO_FEATURES", "sysvinit", "true", "false", d)}; then
                # Change the value of ROOTFS_READ_ONLY in /etc/default/rcS to yes
                if [ -e ${IMAGE_ROOTFS}/etc/default/rcS ]; then
                        sed -i 's/ROOTFS_READ_ONLY=no/ROOTFS_READ_ONLY=yes/' ${IMAGE_ROOTFS}/etc/default/rcS
                fi
                # Run populate-volatile.sh at rootfs time to set up basic files