procd: make mDNS TXT record parsing more solid

mDNS broadcast can't accept empty TXT record and would fail
registration.

Current procd_add_mdns_service checks only if the first passed arg is
empty but don't make any verification on the other args permittins
insertion of empty values in TXT record.

Example:

	procd_add_mdns "blah" \
				"tcp" "50" \
				"1" \
				"" \
				"3"

Produce:

{ "blah_50": { "service": "_blah._tcp.local", "port": 50, "txt": [ "1", "", "3" ] } }

The middle empty TXT record should never be included as it's empty.

This can happen with scripts that make fragile parsing and include
variables even if they are empty.

Prevent this and make the TXT record more solid by checking every
provided TXT record and include only the non-empty ones.

The fixed JSON is the following:

{ "blah_50": { "service": "_blah._tcp.local", "port": 50, "txt": [ "1", "3" ] } }

Fixes: b0d9dcf84dd0 ("procd: update to latest git HEAD")
Reported-by: Paul Donald <newtwen@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15331
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

diff --git a/package/system/procd/files/procd.sh b/package/system/procd/files/procd.sh
index 8ee25f4f08b4715dd214623e3edba6d1db1379f9..5dc8ec42b7a4ef7bdffba23acd3b8fbe28915cd9 100644 (file)
--- a/package/system/procd/files/procd.sh
+++ b/package/system/procd/files/procd.sh
@@ -592,18 +592,21 @@ _procd_set_config_changed() {
 }
 
 procd_add_mdns_service() {
-       local service proto port
+       local service proto port txt_count=0
        service=$1; shift
        proto=$1; shift
        port=$1; shift
        json_add_object "${service}_$port"
        json_add_string "service" "_$service._$proto.local"
        json_add_int port "$port"
-       [ -n "$1" ] && {
-               json_add_array txt
-               for txt in "$@"; do json_add_string "" "$txt"; done
-               json_select ..
-       }
+       for txt in "$@"; do
+               [ -z "$txt" ] && continue
+               txt_count=$((txt_count+1))
+               [ $txt_count -eq 1 ] && json_add_array txt
+               json_add_string "" "$txt"
+       done
+       [ $txt_count -gt 0 ] && json_select ..
+
        json_select ..
 }