os: make program ctors and dtors visible to LDSO

This is needed later when eliminating the need for a startup lib in
dynamic programs to enable LDSO to call ctors and dtors of the program.

Issue #1042

base/mk/prg.mk

@@ -115,6 +115,12 @@ ifeq ($(SHARED_LIBS),)
 LD_SCRIPTS  := $(LD_SCRIPT_STATIC)
 FILTER_DEPS := $(DEPS:.lib=)
 else
+
+#
+# Add a list of symbols that shall always be added to the dynsym section
+#
+LD_OPT += --dynamic-list=$(call select_from_repositories,src/platform/genode_dyn.dl)
+
 LD_SCRIPTS  := $(LD_SCRIPT_DYN)
 LD_CMD      += -Wl,--dynamic-linker=$(DYNAMIC_LINKER).lib.so \
                -Wl,--eh-frame-hdr

os/src/platform/genode_dyn.dl

@@ -0,0 +1,16 @@
+/*
+ * Symbols that shall always be added to the section '.dynsym'
+ */
+{
+	/*
+	 * The ctors and dtors symbols are not used by a dynamic program as such
+	 * programs have no '_main' function. Hence the linker doesn't add them to
+	 * the dynsym section by itself. However, we need to lookup the ctors and
+	 * dtors arrays of the dynamic program in LDSO, which is why we must
+	 * explicitely add them to the dynsym section.
+	 */
+	_ctors_start;
+	_ctors_end;
+	_dtors_start;
+	_dtors_end;
+};