src: add linux getauxval(AT_SECURE) in SafeGetenv

This commit attempts to fix the following TODO:
// TODO(bnoordhuis) Should perhaps also check whether
getauxval(AT_SECURE) is non-zero on Linux.

This can be manually tested at the moment using the following steps:

$ setcap cap_net_raw+ep out/Release/node
$ NODE_PENDING_DEPRECATION="1" out/Release/node -p
"process.binding('config').pendingDeprecation"
true
$ useradd test
$ su test
$ NODE_PENDING_DEPRECATION="1" out/Release/node -p
"process.binding('config').pendingDeprecation"
undefined

PR-URL: https://github.com/nodejs/node/pull/12548
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

src/node.cc

@@ -230,6 +230,8 @@ bool config_expose_internals = false;
 
 bool v8_initialized = false;
 
+bool linux_at_secure = false;
+
 // process-relative uptime base, initialized at start-up
 static double prog_start_time;
 
@@ -965,13 +967,15 @@ Local<Value> UVException(Isolate* isolate,
 // Look up environment variable unless running as setuid root.
 bool SafeGetenv(const char* key, std::string* text) {
 #ifndef _WIN32
-  // TODO(bnoordhuis) Should perhaps also check whether getauxval(AT_SECURE)
-  // is non-zero on Linux.
   if (getuid() != geteuid() || getgid() != getegid()) {
     text->clear();
     return false;
   }
 #endif
+  if (linux_at_secure) {
+    text->clear();
+    return false;
+  }
   if (const char* value = getenv(key)) {
     *text = value;
     return true;

src/node_main.cc

@@ -71,7 +71,32 @@ int wmain(int argc, wchar_t *wargv[]) {
 }
 #else
 // UNIX
+#ifdef __linux__
+#include <elf.h>
+#ifdef __LP64__
+#define Elf_auxv_t Elf64_auxv_t
+#else
+#define Elf_auxv_t Elf32_auxv_t
+#endif  // __LP64__
+extern char** environ;
+#endif  // __linux__
+
+namespace node {
+  extern bool linux_at_secure;
+}  // namespace node
+
 int main(int argc, char *argv[]) {
+#if defined(__linux__)
+  char** envp = environ;
+  while (*envp++ != nullptr) {}
+  Elf_auxv_t* auxv = reinterpret_cast<Elf_auxv_t*>(envp);
+  for (; auxv->a_type != AT_NULL; auxv++) {
+    if (auxv->a_type == AT_SECURE) {
+      node::linux_at_secure = auxv->a_un.a_val;
+      break;
+    }
+  }
+#endif
   // Disable stdio buffering, it interacts poorly with printf()
   // calls elsewhere in the program (e.g., any logging from V8.)
   setvbuf(stdout, nullptr, _IONBF, 0);