Obtain FTL's PID from the PID file (#4103)

* Try to obtain FTL's PID from the PID file. If this fails, try to identify the main process using pgrep --oldest (instead of relying on pkill finding the right one by itself). This allows the script to work in even when FTL is running inside the memory checker valgrind.

* Rename FTL_PID -> FTL_PID_FILE

* Remove the pgrep fallback after discussions about that it should be more obvious to users if something strange happened to their PID file. Also, simplify the routine using a bashism in the end.

* Shorten if [[ regex ]]

* Use unset instead of emptying the PID variable

Signed-off-by: DL6ER <dl6er@dl6er.de>

Co-authored-by: Dan Schaper <dan.schaper@pi-hole.net>

pihole

@@ -16,6 +16,7 @@ readonly PI_HOLE_SCRIPT_DIR="/opt/pihole"
 # error due to modifying a readonly variable.
 setupVars="/etc/pihole/setupVars.conf"
 PI_HOLE_BIN_DIR="/usr/local/bin"
+readonly FTL_PID_FILE="/run/pihole-FTL.pid"
 
 readonly colfile="${PI_HOLE_SCRIPT_DIR}/COL_TABLE"
 source "${colfile}"
@@ -98,8 +99,25 @@ versionFunc() {
   exit 0
 }
 
+# Get PID of main pihole-FTL process
+getFTLPID() {
+  local pid
+
+  if [ -s "${FTL_PID_FILE}" ]; then
+    # -s: FILE exists and has a size greater than zero
+    pid="$(<"$FTL_PID_FILE")"
+    # Exploit prevention: unset the variable if there is malicious content
+    # Verify that the value read from the file is numeric
+    [[ "$pid" =~ [^[:digit:]] ]] && unset pid
+  fi
+
+  # If FTL is not running, or the PID file contains malicious stuff, substitute
+  # negative PID to signal this to the caller
+  echo "${pid:=-1}"
+}
+
 restartDNS() {
-  local svcOption svc str output status
+  local svcOption svc str output status pid icon
   svcOption="${1:-restart}"
 
   # Determine if we should reload or restart
@@ -108,17 +126,34 @@ restartDNS() {
     # Note 1: This will NOT re-read any *.conf files
     # Note 2: We cannot use killall here as it does
     #         not know about real-time signals
-    svc="pkill -RTMIN pihole-FTL"
-    str="Reloading DNS lists"
+    pid="$(getFTLPID)"
+    if [[ "$pid" -eq "-1" ]]; then
+      svc="true"
+      str="FTL is not running"
+      icon="${INFO}"
+    else
+      svc="kill -RTMIN ${pid}"
+      str="Reloading DNS lists"
+      icon="${TICK}"
+    fi
   elif [[ "${svcOption}" =~ "reload" ]]; then
     # Reloading of the DNS cache has been requested
     # Note: This will NOT re-read any *.conf files
-    svc="pkill -HUP pihole-FTL"
-    str="Flushing DNS cache"
+    pid="$(getFTLPID)"
+    if [[ "$pid" -eq "-1" ]]; then
+      svc="true"
+      str="FTL is not running"
+      icon="${INFO}"
+    else
+      svc="kill -HUP ${pid}"
+      str="Flushing DNS cache"
+      icon="${TICK}"
+    fi
   else
     # A full restart has been requested
     svc="service pihole-FTL restart"
     str="Restarting DNS server"
+    icon="${TICK}"
   fi
 
   # Print output to Terminal, but not to Web Admin
@@ -128,7 +163,7 @@ restartDNS() {
   status="$?"
 
   if [[ "${status}" -eq 0 ]]; then
-    [[ -t 1 ]] && echo -e "${OVER}  ${TICK} ${str}"
+    [[ -t 1 ]] && echo -e "${OVER}  ${icon} ${str}"
     return 0
   else
     [[ ! -t 1 ]] && local OVER=""