base-files: functions.sh: fix config_get() on invalid identifiers

When passing a section or option value to config_get() which contains
characters that happen to be valid variable interpolation expressions,
the function returns a nonsensical expression result instead of the
expected empty string.

When the passed section or option name contains other characters which
are not valid within a shell variable name, a substitution error is
occuring instead.

The issue can be easily reproduced by one of the following examples:

    root@OpenWrt:~# . /lib/functions.sh
    root@OpenWrt:~# config load system
    root@OpenWrt:~# config_get variable invalid-section option
    root@OpenWrt:~# echo "$variable"
    section_option:-

    root@OpenWrt:~# . /lib/functions.sh
    root@OpenWrt:~# config load system
    root@OpenWrt:~# config_get variable section invalid-option
    root@OpenWrt:~# echo "$variable"
    option:-

    root@OpenWrt:~# . /lib/functions.sh
    root@OpenWrt:~# config load system
    root@OpenWrt:~# config_get variable section invalid@option
    -ash: eval: syntax error: bad substitution

Fix this issue by only performing interpolations when the given section
and option arguments are free of illegal characters.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

package/base-files/Makefile

@@ -12,7 +12,7 @@ include $(INCLUDE_DIR)/version.mk
 include $(INCLUDE_DIR)/feeds.mk
 
 PKG_NAME:=base-files
-PKG_RELEASE:=225
+PKG_RELEASE:=226
 PKG_FLAGS:=nonshared
 
 PKG_FILE_DEPENDS:=$(PLATFORM_DIR)/ $(GENERIC_PLATFORM_DIR)/base-files/

package/base-files/files/lib/functions.sh

@@ -107,9 +107,14 @@ config_unset() {
 # config_get <variable> <section> <option> [<default>]
 # config_get <section> <option>
 config_get() {
-	case "$3" in
-		"") eval echo "\"\${CONFIG_${1}_${2}:-\${4}}\"";;
-		*)  eval export ${NO_EXPORT:+-n} -- "${1}=\${CONFIG_${2}_${3}:-\${4}}";;
+	case "$2${3:-$1}" in
+		*[^A-Za-z0-9_]*) : ;;
+		*)
+			case "$3" in
+				"") eval echo "\"\${CONFIG_${1}_${2}:-\${4}}\"";;
+				*)  eval export ${NO_EXPORT:+-n} -- "${1}=\${CONFIG_${2}_${3}:-\${4}}";;
+			esac
+		;;
 	esac
 }