基于 ramfs 进行 OTA

kill_remaining() { # [ <signal> [ <loop> ] ]
	local loop_limit=10

	local sig="${1:-TERM}"
	local loop="${2:-0}"
	local run=true
	local stat
	local proc_ppid=$(cut -d' ' -f4  /proc/$$/stat)

	echo -n "Sending $sig to remaining processes ... "

	while $run; do
		run=false
		for stat in /proc/[0-9]*/stat; do
			[ -f "$stat" ] || continue

			local pid name state ppid rest
			read pid name state ppid rest < $stat
			name="${name#(}"; name="${name%)}"

			# Skip PID1, our parent, ourself and our children
			[ $pid -ne 1 -a $pid -ne $proc_ppid -a $pid -ne $$ -a $ppid -ne $$ ] || continue

			local cmdline
			read cmdline < /proc/$pid/cmdline

			# Skip kernel threads
			[ -n "$cmdline" ] || continue

			echo -n "$name "
			kill -$sig $pid 2>/dev/null

			[ $loop -eq 1 ] && run=true
		done

		let loop_limit--
		[ $loop_limit -eq 0 ] && {
			echo
			echo "Failed to kill all processes."
			exit 1
		}
	done
	echo
}

switch_to_ramfs() {
         # 将一些基础文件拷贝到ram中，构造ramfs
	for binary in \
		/bin/busybox /bin/ash /bin/sh /bin/mount /bin/umount	\
		pivot_root mount_root reboot sync kill sleep		\
		md5sum hexdump cat zcat bzcat dd tar			\
		ls basename find cp mv rm mkdir rmdir mknod touch chmod \
		'[' printf wc grep awk sed cut				\
		mtd partx losetup mkfs.ext4 nandwrite flash_erase	\
		ubiupdatevol ubiattach ubiblock ubiformat		\
		ubidetach ubirsvol ubirmvol ubimkvol			\
		snapshot snapshot_tool					\
                # 除了上面列出来的，还可以将自定义的一些文件赋值到 $RAMFS_COPY_BIN 中，这样就无需改动官方的这份文件
		$RAMFS_COPY_BIN
	do
		local file="$(which "$binary" 2>/dev/null)"
		[ -n "$file" ] && install_bin "$file"
	done
	install_file /etc/resolv.conf /lib/*.sh /lib/functions/*.sh /lib/upgrade/*.sh /lib/upgrade/do_stage2 /usr/share/libubox/jshn.sh $RAMFS_COPY_DATA

	[ -L "/lib64" ] && ln -s /lib $RAM_ROOT/lib64

	supivot $RAM_ROOT /mnt || {
		echo "Failed to switch over to ramfs. Please reboot."
		exit 1
	}

        #原本的根文件系统，变成挂载在 /mnt 下，现在可以卸载掉
	/bin/mount -o remount,ro /mnt
	/bin/umount -l /mnt

	grep /overlay /proc/mounts > /dev/null && {
		/bin/mount -o noatime,remount,ro /overlay
		/bin/umount -l /overlay
	}
}

# Exec new shell from ramfs
exec /bin/busybox ash -c "$COMMAND"