base-files: don't evaluate block-device uevent

Current code and also before commit da52dd0c83 was vulnerable to shell
injection using volume lables in the GPT partition table of block
devices. Given that partition names can be freely defined in GPT tables
we really shouldn't evaluate a string which is potentially crafted with
evil intentions. Hence rather use `export -n` to absorb the uevent's
variables into the environment.

Fixes commit da52dd0c83 (base-files: quote values when evaluating uevent)
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[mschiffer@universe-factory.net: suggested export -n usage]

package/base-files/files/lib/upgrade/common.sh

@@ -101,7 +101,7 @@ get_magic_long() {
 }
 
 export_bootdevice() {
-	local cmdline uuid disk uevent
+	local cmdline uuid disk uevent line
 	local MAJOR MINOR DEVNAME DEVTYPE
 
 	if read cmdline < /proc/cmdline; then
@@ -134,7 +134,9 @@ export_bootdevice() {
 		esac
 
 		if [ -e "$uevent" ]; then
-			eval "$(sed "s/=\(.*\)/=\'\1\'/" < "$uevent")"
+			while read line; do
+				export -n "$line"
+			done < "$uevent"
 			export BOOTDEV_MAJOR=$MAJOR
 			export BOOTDEV_MINOR=$MINOR
 			return 0
@@ -146,10 +148,12 @@ export_bootdevice() {
 
 export_partdevice() {
 	local var="$1" offset="$2"
-	local uevent MAJOR MINOR DEVNAME DEVTYPE
+	local uevent line MAJOR MINOR DEVNAME DEVTYPE
 
 	for uevent in /sys/class/block/*/uevent; do
-		eval "$(sed "s/=\(.*\)/=\'\1\'/" < "$uevent")"
+		while read line; do
+			export -n "$line"
+		done < "$uevent"
 		if [ $BOOTDEV_MAJOR = $MAJOR -a $(($BOOTDEV_MINOR + $offset)) = $MINOR -a -b "/dev/$DEVNAME" ]; then
 			export "$var=$DEVNAME"
 			return 0