fix: promote loader symbols to global scope before loading embedded .so

src/lib/loader.c

@@ -280,6 +280,15 @@ static void __attribute__((constructor)) loader() {
   ensure_libpthread_is_loaded();
   ensure_librt_is_loaded();
 
+  // When the loader is dlopen'd with RTLD_GLOBAL, glibc does not promote its
+  // symbols to global scope until dlopen returns. The embedded .so references
+  // ddprof_lib_state (defined here in the loader) as an undefined symbol, so
+  // loading it with RTLD_NOW during our constructor fails with
+  // "undefined symbol: ddprof_lib_state".
+  // Fix: re-open ourselves with RTLD_NOLOAD | RTLD_GLOBAL to promote our
+  // symbols before loading the embedded .so.
+  my_dlopen("libdd_profiling.so", RTLD_GLOBAL | RTLD_NOLOAD | RTLD_NOW);
+
   s_profiling_lib_handle = my_dlopen(lib_profiling_path, RTLD_LOCAL | RTLD_NOW);
   free(lib_profiling_path);
   if (s_profiling_lib_handle) {

test/CMakeLists.txt

@@ -534,6 +534,19 @@ if(NOT CMAKE_BUILD_TYPE STREQUAL "SanitizedDebug")
              COMMAND ${CMAKE_SOURCE_DIR}/tools/check_for_unsafe_libc_functions.py
                      $<TARGET_FILE:dd_profiling-embedded>)
   endif()
+
+  # Test that the loader works when dlopen'd with RTLD_GLOBAL (the pattern
+  # used by applications that load libdd_profiling.so at runtime).
+  add_executable(loader_rtld_global_test loader_rtld_global_test.c)
+  target_link_libraries(loader_rtld_global_test PRIVATE dl)
+  add_dependencies(loader_rtld_global_test dd_profiling-shared)
+  add_test(
+    NAME loader_rtld_global
+    COMMAND loader_rtld_global_test
+    WORKING_DIRECTORY ${CMAKE_BINARY_DIR})
+  set_tests_properties(
+    loader_rtld_global
+    PROPERTIES ENVIRONMENT "TEST_DD_PROFILING_LIB=$<TARGET_FILE:dd_profiling-shared>")
 endif()
 
 if(NOT CMAKE_BUILD_TYPE STREQUAL "SanitizedDebug")

test/loader_rtld_global_test.c

@@ -0,0 +1,77 @@
+// Unless explicitly stated otherwise all files in this repository are licensed
+// under the Apache License Version 2.0. This product includes software
+// developed at Datadog (https://www.datadoghq.com/). Copyright 2021-Present
+// Datadog, Inc.
+
+// Test that libdd_profiling.so (the loader) works when loaded at runtime
+// with RTLD_GLOBAL.
+//
+// This reproduces the pattern used by applications that dlopen the profiling
+// library at startup (e.g., after reading a config flag).
+// The loader's constructor extracts and dlopen's the embedded .so, which
+// references ddprof_lib_state as an extern symbol defined in the loader.
+// On glibc, RTLD_GLOBAL only takes effect after dlopen returns, so without
+// the self-promotion fix the embedded library fails with:
+//   "undefined symbol: ddprof_lib_state"
+
+#include <dlfcn.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+typedef int (*start_fn_t)(void);
+typedef void (*stop_fn_t)(int);
+
+int main(void) {
+  const char *lib_path = getenv("TEST_DD_PROFILING_LIB");
+  if (!lib_path) {
+    lib_path = "./libdd_profiling.so";
+  }
+
+  fprintf(stderr, "loading %s with RTLD_LAZY | RTLD_GLOBAL...\n", lib_path);
+
+  void *handle = dlopen(lib_path, RTLD_LAZY | RTLD_GLOBAL);
+  if (!handle) {
+    fprintf(stderr, "FAIL: dlopen: %s\n", dlerror());
+    return 1;
+  }
+
+  // Verify ddprof_lib_state is resolvable in global scope
+  void *sym = dlsym(RTLD_DEFAULT, "ddprof_lib_state");
+  if (!sym) {
+    fprintf(stderr, "FAIL: ddprof_lib_state not in global scope\n");
+    dlclose(handle);
+    return 1;
+  }
+
+  // Call ddprof_start_profiling. If the embedded library failed to load
+  // (the bug we're testing for), this returns -1.
+  start_fn_t start = (start_fn_t)dlsym(handle, "ddprof_start_profiling");
+  if (!start) {
+    fprintf(stderr, "FAIL: ddprof_start_profiling not found\n");
+    dlclose(handle);
+    return 1;
+  }
+
+  int rc = start();
+  // The profiler may fail to start for environment reasons (no perf events,
+  // etc.), but a return of -1 specifically means the embedded library was
+  // never loaded (the loader's start function returns -1 when its function
+  // pointer is NULL).
+  if (rc == -1) {
+    fprintf(stderr,
+            "FAIL: ddprof_start_profiling returned -1 "
+            "(embedded library not loaded)\n");
+    dlclose(handle);
+    return 1;
+  }
+
+  // Clean up: stop profiling
+  stop_fn_t stop = (stop_fn_t)dlsym(handle, "ddprof_stop_profiling");
+  if (stop) {
+    stop(1000);
+  }
+
+  fprintf(stderr, "PASS: loader constructor succeeded with RTLD_GLOBAL\n");
+  dlclose(handle);
+  return 0;
+}