latency-test, latency-histogram: warn when rtapi_app lacks RT privileges

Without 'sudo make setuid' (or 'sudo make setcap') rtapi_app runs
unprivileged: no SCHED_FIFO, no locked memory, so latency readings are
wildly inflated and easy to mistake for a code regression. Warn, for a
non-root user, when rtapi_app is neither setuid root nor carries the
cap_sys_nice capability.

Closes #4044

Author: grandixximo

scripts/latency-histogram

@@ -111,6 +111,26 @@ proc which_exe {name} {
   return -code error "$name: executable not found"
 } ;# which_exe
 
+proc check_rt_privileges {} {
+  # Without 'sudo make setuid' (or 'sudo make setcap') rtapi_app runs
+  # unprivileged: realtime threads get no SCHED_FIFO and no locked memory, so
+  # the latency readings are wildly inflated.  Warn rather than mislead.
+  if {![catch {exec id -u} uid] && $uid == 0} return ;# root has every privilege
+  if {[catch {which_exe rtapi_app} rtapi]} return ;# cannot locate, stay quiet
+  if {![catch {file attributes $rtapi -permissions} mode] && ($mode & 04000)} return ;# setuid root
+  if {![catch {exec getcap $rtapi} caps] && [string match *cap_sys_nice* $caps]} return ;# file caps
+  set msg "Warning: rtapi_app is not setuid root and has no realtime capabilities."
+  append msg "\nRealtime threads get no SCHED_FIFO priority or locked memory,"
+  append msg "\nso the latency shown will be far worse than reality."
+  append msg "\nRun 'sudo make setuid' (or 'sudo make setcap') in src/."
+  puts stderr $msg
+  if {$::LH(use_x)} {
+    package require Tk
+    tk_messageBox -parent . -icon warning -type ok \
+        -title "Latency Warning" -message $msg
+  }
+} ;# check_rt_privileges
+
 proc program_check {plist} {
   foreach prog $plist {
     if [catch {
@@ -944,6 +964,7 @@ proc windowToFile { win } {
 if ![info exists ::LH(start)] {
   set_defaults
   config
+  check_rt_privileges
   progress "Loading packages"
   load_packages
   signal trap SIGINT finish

scripts/latency-test

@@ -16,6 +16,25 @@ kversion=$(uname -v)
 calc() { awk "BEGIN { print ($1); }" < /dev/null; }
 icalc() { awk "BEGIN { printf \"%.0f\n\", ($1); }" < /dev/null; }
 
+# Without 'sudo make setuid' (or 'sudo make setcap') rtapi_app runs
+# unprivileged: realtime threads get no SCHED_FIFO and no locked memory, so the
+# latency readings are wildly inflated.  Warn rather than mislead.
+check_rt_privileges () {
+    [ "$(id -u)" = 0 ] && return 0          # root has every privilege
+    rtapi_app=$(command -v rtapi_app) || return 0
+    [ -n "$rtapi_app" ] || return 0
+    [ -u "$rtapi_app" ] && return 0         # setuid root
+    if command -v getcap >/dev/null 2>&1; then
+        case $(getcap "$rtapi_app" 2>/dev/null) in
+            *cap_sys_nice*) return 0 ;;     # file capabilities
+        esac
+    fi
+    echo "Warning: rtapi_app is not setuid root and has no realtime capabilities." >&2
+    echo "         Realtime threads get no SCHED_FIFO priority or locked memory," >&2
+    echo "         so the latency shown will be far worse than reality." >&2
+    echo "         Run 'sudo make setuid' (or 'sudo make setcap') in src/." >&2
+}
+
 parse_time () {
     case $1 in
     -)   echo "0" ;;
@@ -170,4 +189,6 @@ L=\$(($SIGNALS
 echo \$L > "$HOME"/.latency
 EOF
 
+check_rt_privileges
+
 halrun lat.hal