LibELF: Split the DynamicLoader's loading mechanism into two steps

load_from_image() becomes map() and link(). This allows us to map
an object before mapping its dependencies.

This solves an issue where fixed-position executables (like GCC)
would clash with the ASLR placement of their own shared libraries.

Userland/Libraries/LibC/dlfcn.cpp

@@ -88,9 +88,8 @@ void* dlopen(const char* filename, int flags)
         return nullptr;
     }
 
-    if (!loader->load_from_image(flags,
+    auto object = loader->map();
-            0 // total_tls_size = 0, FIXME: Support TLS when using dlopen()
+    if (!object || !loader->link(flags, /* total_tls_size (FIXME) */ 0)) {
-            )) {
         g_dlerror_msg = String::formatted("Failed to load ELF object {}", filename);
         return nullptr;
     }

Userland/Libraries/LibELF/DynamicLinker.cpp

@@ -181,6 +181,10 @@ static void load_elf(const String& name)
 {
     dbgln<DYNAMIC_LOAD_DEBUG>("load_elf: {}", name);
     auto loader = g_loaders.get(name).value();
+
+    auto dynamic_object = loader->map();
+    ASSERT(dynamic_object);
+
     for (const auto& needed_name : get_dependencies(name)) {
         dbgln<DYNAMIC_LOAD_DEBUG>("needed library: {}", needed_name);
         String library_name = get_library_name(needed_name);
@@ -189,8 +193,8 @@ static void load_elf(const String& name)
         }
     }
 
-    auto dynamic_object = loader->load_from_image(RTLD_GLOBAL | RTLD_LAZY, g_total_tls_size);
+    bool success = loader->link(RTLD_GLOBAL | RTLD_LAZY, g_total_tls_size);
-    ASSERT(dynamic_object);
+    ASSERT(success);
 
     g_loaded_objects.set(name, *dynamic_object);
     g_global_objects.append(*dynamic_object);

Userland/Libraries/LibELF/DynamicLoader.cpp

@@ -143,12 +143,12 @@ void* DynamicLoader::symbol_for_name(const char* name)
     return m_dynamic_object->base_address().offset(symbol.value()).as_ptr();
 }
 
-RefPtr<DynamicObject> DynamicLoader::load_from_image(unsigned flags, size_t total_tls_size)
+RefPtr<DynamicObject> DynamicLoader::map()
 {
     ASSERT(!m_dynamic_object);
 
     if (!m_valid) {
-        dbgln("DynamicLoader::load_from_image failed: image is invalid");
+        dbgln("DynamicLoader::map failed: image is invalid");
         return nullptr;
     }
 
@@ -158,14 +158,14 @@ RefPtr<DynamicObject> DynamicLoader::load_from_image(unsigned flags, size_t tota
     m_dynamic_object->set_tls_offset(m_tls_offset);
     m_dynamic_object->set_tls_size(m_tls_size);
 
-    auto rc = load_stage_2(flags, total_tls_size);
-    if (!rc) {
-        dbgln("DynamicLoader::load_from_image failed at load_stage_2");
-        return nullptr;
-    }
     return m_dynamic_object;
 }
 
+bool DynamicLoader::link(unsigned flags, size_t total_tls_size)
+{
+    return load_stage_2(flags, total_tls_size);
+}
+
 bool DynamicLoader::load_stage_2(unsigned flags, size_t total_tls_size)
 {
     ASSERT(flags & RTLD_GLOBAL);

Userland/Libraries/LibELF/DynamicLoader.h

@@ -49,7 +49,10 @@ public:
 
     // Load a full ELF image from file into the current process and create an DynamicObject
     // from the SHT_DYNAMIC in the file.
-    RefPtr<DynamicObject> load_from_image(unsigned flags, size_t total_tls_size);
+    // Note that the DynamicObject will not be linked yet. Callers are responsible for calling link() to finish it.
+    RefPtr<DynamicObject> map();
+
+    bool link(unsigned flags, size_t total_tls_size);
 
     // Stage 2 of loading: dynamic object loading and primary relocations
     bool load_stage_2(unsigned flags, size_t total_tls_size);