[Backtracing][Runtime] Disable backtraces for setuid binaries.

We really, really shouldn't be running the external backtracer for setuid
binaries.  It's just too dangerous.

So don't do that.  And if someone tries to force us, emit a warning.

rdar://105391747

Author: al45tair

stdlib/public/runtime/Backtrace.cpp

@@ -217,15 +217,20 @@ const char *presetToString(Preset preset) {
 BacktraceInitializer::BacktraceInitializer() {
   const char *backtracing = swift::runtime::environment::SWIFT_BACKTRACE();
 
+#if !_WIN32
+  // Force off for setuid processes.
+  if (issetugid()) {
+    _swift_backtraceSettings.enabled = OnOffTty::Off;
+  }
+#endif
+
   if (backtracing)
     _swift_parseBacktracingSettings(backtracing);
 
 #if TARGET_OS_OSX
-  // Make sure that we don't pass on setuid privileges, and that all fds
-  // are closed except for stdin/stdout/stderr.
+  // Make sure that all fds are closed except for stdin/stdout/stderr.
   posix_spawnattr_init(&backtraceSpawnAttrs);
-  posix_spawnattr_setflags(&backtraceSpawnAttrs,
-                           POSIX_SPAWN_RESETIDS | POSIX_SPAWN_CLOEXEC_DEFAULT);
+  posix_spawnattr_setflags(&backtraceSpawnAttrs, POSIX_SPAWN_CLOEXEC_DEFAULT);
 
   posix_spawn_file_actions_init(&backtraceFileActions);
   posix_spawn_file_actions_addinherit_np(&backtraceFileActions, STDIN_FILENO);
@@ -241,6 +246,26 @@ BacktraceInitializer::BacktraceInitializer() {
     _swift_backtraceSettings.enabled = OnOffTty::Off;
   }
 #else
+  #if !_WIN32
+  if (issetugid()) {
+    if (_swift_backtraceSettings.enabled != OnOffTty::Off) {
+      // You'll only see this warning if you do e.g.
+      //
+      //    SWIFT_BACKTRACE=enable=on /path/to/some/setuid/binary
+      //
+      // as opposed to
+      //
+      //    /path/to/some/setuid/binary
+      //
+      // i.e. when you're trying to force matters.
+      swift::warning(0,
+                     "swift runtime: backtrace-on-crash is not supported for "
+                     "setuid executables.\n");
+      _swift_backtraceSettings.enabled = OnOffTty::Off;
+    }
+  }
+  #endif // !_WIN32
+
   if (_swift_backtraceSettings.enabled == OnOffTty::TTY)
     _swift_backtraceSettings.enabled =
       isStdoutATty() ? OnOffTty::On : OnOffTty::Off;