libvndksupport: Add android_is_in_vendor_process()

In preparation for cleaning up conditionally compiled VNDK libraries,
we need a function that determines if the current process is running in
the system process or the vendor process.

Bug: 119423884
Test: Add a dummy VNDK library that prints different string in system
      vs vendor process.  Use the library in different processs and
      check the output.

Change-Id: Ic00927d8eadd2a21fcba9cb64cc93eaadd5bdd2d

libvndksupport/include/vndksupport/linker.h

@@ -20,6 +20,16 @@
 extern "C" {
 #endif
 
+/*
+ * Returns whether the current process is a vendor process.
+ *
+ * Note that this is only checking what process is running and has nothing to
+ * do with what namespace the caller is loaded at.  For example, a VNDK-SP
+ * library loaded by SP-HAL calling this function may still get a 'false',
+ * because it is running in a system process.
+ */
+int android_is_in_vendor_process();
+
 void* android_load_sphal_library(const char* name, int flag);
 
 int android_unload_sphal_library(void* handle);

libvndksupport/libvndksupport.map.txt

@@ -1,5 +1,6 @@
 LIBVNDKSUPPORT {
   global:
+    android_is_in_vendor_process; # vndk apex
     android_load_sphal_library; # vndk apex
     android_unload_sphal_library; # vndk apex
   local:

libvndksupport/linker.c

@@ -45,6 +45,17 @@ static struct android_namespace_t* get_vendor_namespace() {
     return vendor_namespace;
 }
 
+int android_is_in_vendor_process() {
+    if (android_get_exported_namespace == NULL) {
+        ALOGD("android_get_exported_namespace() not available. Assuming system process.");
+        return 0;
+    }
+
+    // In vendor process, 'vndk' namespace is not visible, whereas in system
+    // process, it is.
+    return android_get_exported_namespace("vndk") == NULL;
+}
+
 void* android_load_sphal_library(const char* name, int flag) {
     struct android_namespace_t* vendor_namespace = get_vendor_namespace();
     if (vendor_namespace != NULL) {