feat: add external process runtime (#1799)

app/src/main/cpp/prop_override.cpp

@@ -0,0 +1,62 @@
+// Library for overriding Android system properties via environment variables.
+//
+// Usage: LD_PRELOAD=prop_override.so PROP_dalvik.vm.heapsize=123M getprop dalvik.vm.heapsize
+// Output: 123M
+#include <string>
+#include <cstring>
+#include <cstdlib>
+#include <dlfcn.h>
+
+// Source: https://android.googlesource.com/platform/system/core/+/100b08a848d018eeb1caa5d5e7c7c2aaac65da15/libcutils/include/cutils/properties.h
+#define PROP_VALUE_MAX 92
+// This is the mangled name of "android::base::GetProperty".
+#define GET_PROPERTY_MANGLED_NAME "_ZN7android4base11GetPropertyERKNSt3__112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEES9_"
+
+extern "C" typedef int (*property_get_ptr)(const char *, char *, const char *);
+typedef std::string (*GetProperty_ptr)(const std::string &, const std::string &);
+
+char *GetPropOverride(const std::string &key) {
+    auto envKey = "PROP_" + key;
+
+    return getenv(envKey.c_str());
+}
+
+// See: https://android.googlesource.com/platform/system/core/+/100b08a848d018eeb1caa5d5e7c7c2aaac65da15/libcutils/properties.cpp
+extern "C" int property_get(const char *key, char *value, const char *default_value) {
+    auto replacement = GetPropOverride(std::string(key));
+    if (replacement) {
+        int len = strnlen(replacement, PROP_VALUE_MAX);
+
+        strncpy(value, replacement, len);
+        return len;
+    }
+
+    static property_get_ptr original = NULL;
+    if (!original) {
+        // Get the address of the original function.
+        original = reinterpret_cast<property_get_ptr>(dlsym(RTLD_NEXT, "property_get"));
+    }
+
+    return original(key, value, default_value);
+}
+
+// Defining android::base::GetProperty ourselves won't work because std::string has a slightly different "path" in the NDK version of the C++ standard library.
+// We can get around this by forcing the function to adopt a specific name using the asm keyword.
+std::string GetProperty(const std::string &, const std::string &) asm(GET_PROPERTY_MANGLED_NAME);
+
+
+// See: https://android.googlesource.com/platform/system/libbase/+/1a34bb67c4f3ba0a1ea6f4f20ac9fe117ba4fe64/properties.cpp
+// This isn't used for the properties we want to override, but property_get is deprecated so that could change in the future.
+std::string GetProperty(const std::string &key, const std::string &default_value) {
+    auto replacement = GetPropOverride(key);
+    if (replacement) {
+        return std::string(replacement);
+    }
+
+    static GetProperty_ptr original = NULL;
+    if (!original) {
+        original = reinterpret_cast<GetProperty_ptr>(dlsym(RTLD_NEXT, GET_PROPERTY_MANGLED_NAME));
+    }
+
+    return original(key, default_value);
+}