src: allow CAP_NET_BIND_SERVICE in SafeGetenv

This commit updates SafeGetenv to check if the current process has the
effective capability cap_net_bind_service set, and if so allows
environment variables to be read.

The motivation for this change is a use-case where Node is run in a
container, and the is a requirement to be able to listen to ports
below 1024. This is done by setting the capability of
cap_net_bind_service. In addition there is a need to set the
environment variable `NODE_EXTRA_CA_CERTS`. But currently this
environment variable will not be read when the capability has been set
on the executable.

src/node_credentials.cc

@@ -10,6 +10,7 @@
 
 #if !defined(_MSC_VER)
 #include <unistd.h>  // setuid, getuid
+#include <sys/capability.h>
 #endif
 
 namespace node {
@@ -33,11 +34,32 @@ bool linux_at_secure = false;
 
 namespace credentials {
 
-// Look up environment variable unless running as setuid root.
+#if !defined(__CloudABI__) && !defined(_WIN32)
+// Returns true if the current process has effective capabilities and the
+// passed-in capability is in that set.
+bool HasCapability(int capability) {
+  DCHECK(cap_valid(capability));
+  struct __user_cap_header_struct cap_header_data = {
+    _LINUX_CAPABILITY_VERSION_3, getpid()
+  };
+  struct __user_cap_data_struct cap_data;
+
+  if (capget(&cap_header_data, &cap_data) == -1) {
+    return false;
+  }
+
+  return cap_data.effective & CAP_TO_MASK(capability);
+}
+#endif
+
+// Look up the environment variable and allow the lookup if the current
+// process has the capability CAP_NET_BIND_SERVICE set. If the current process
+// does not have any capabilities set and the process is running as setuid root
+// then lookup will not be allowed.
 bool SafeGetenv(const char* key, std::string* text, Environment* env) {
 #if !defined(__CloudABI__) && !defined(_WIN32)
-  if (per_process::linux_at_secure || getuid() != geteuid() ||
-      getgid() != getegid())
+  if (!HasCapability(CAP_NET_BIND_SERVICE) && per_process::linux_at_secure ||
+      getuid() != geteuid() || getgid() != getegid())
     goto fail;
 #endif