Fix recursive ELF constructor check

The flag to avoid calling ELF constructors recursively (in the case
of recursive .so dependencies) was being set after the dangerous
recursive constructor call had already been made.

This fixes the libc's debug malloc implementation.

Change-Id: I5e601f0ea19ab1df81b8b1ad4df25c3eab0ccda4

linker/linker.c

@@ -1622,6 +1622,19 @@ void call_constructors_recursive(soinfo *si)
     if (si->constructors_called)
         return;
 
+    // Set this before actually calling the constructors, otherwise it doesn't
+    // protect against recursive constructor calls. One simple example of
+    // constructor recursion is the libc debug malloc, which is implemented in
+    // libc_malloc_debug_leak.so:
+    // 1. The program depends on libc, so libc's constructor is called here.
+    // 2. The libc constructor calls dlopen() to load libc_malloc_debug_leak.so.
+    // 3. dlopen() calls call_constructors_recursive() with the newly created
+    //    soinfo for libc_malloc_debug_leak.so.
+    // 4. The debug so depends on libc, so call_constructors_recursive() is
+    //    called again with the libc soinfo. If it doesn't trigger the early-
+    //    out above, the libc constructor will be called again (recursively!).
+    si->constructors_called = 1;
+
     if (si->flags & FLAG_EXE) {
         TRACE("[ %5d Calling preinit_array @ 0x%08x [%d] for '%s' ]\n",
               pid, (unsigned)si->preinit_array, si->preinit_array_count,
@@ -1665,7 +1678,6 @@ void call_constructors_recursive(soinfo *si)
         TRACE("[ %5d Done calling init_array for '%s' ]\n", pid, si->name);
     }
 
-    si->constructors_called = 1;
 }
 
 static void call_destructors(soinfo *si)