libdl: Add object file dependencies to track references

Tracking references lets us manage when an object file can be
unloaded. If an object file has references to it, it cannot be
unloaded.

Modules that depend on each other cannot be unloaded.

Updates #3605

10 files changed, 619 insertions, 140 deletions

diff --git a/cpukit/include/rtems/rtl/rtl-obj-fwd.h b/cpukit/include/rtems/rtl/rtl-obj-fwd.h
index 39e28c58e1..6d41483aa2 100644
--- a/cpukit/include/rtems/rtl/rtl-obj-fwd.h
+++ b/cpukit/include/rtems/rtl/rtl-obj-fwd.h
@@ -27,6 +27,12 @@ struct rtems_rtl_obj_sect;
 typedef struct rtems_rtl_obj_sect rtems_rtl_obj_sect;
 
 /**
+ * The forward declaration of the obj depends structure.
+ */
+struct rtems_rtl_obj_depends;
+typedef struct rtems_rtl_obj_depends rtems_rtl_obj_depends;
+
+/**
  * The forward declaration of the obj structure.
  */
 struct rtems_rtl_obj;
diff --git a/cpukit/include/rtems/rtl/rtl-obj.h b/cpukit/include/rtems/rtl/rtl-obj.h
index be77a81c9d..976d6c293f 100644
--- a/cpukit/include/rtems/rtl/rtl-obj.h
+++ b/cpukit/include/rtems/rtl/rtl-obj.h
@@ -139,12 +139,35 @@ struct rtems_rtl_obj_sect
 };
 
 /**
+ * Object file dependents. This is a list of tables of pointers to the object
+ * modules the object file depends on. The table is a list of tables because
+ * unresolved externals can exist when an object file is loaded and resolved
+ * later when the dependent object file is loaded.
+ */
+struct rtems_rtl_obj_depends
+{
+  rtems_chain_node node;        /**< The node's link in the chain. */
+  size_t           dependents;  /**< The number of dependent object pointers. */
+  rtems_rtl_obj*   depends[];   /**< Dependtent objects. More follow. */
+};
+
+/**
+ * Dependency iterator.
+ */
+typedef bool (*rtems_rtl_obj_depends_iterator) (rtems_rtl_obj* obj,
+                                                rtems_rtl_obj* dependent,
+                                                void*          data);
+
+/**
  * Object file descriptor flags.
  */
-#define RTEMS_RTL_OBJ_LOCKED     (1 << 0) /**< Lock the object file so it cannot
-                                           *   be unloaded. */
-#define RTEMS_RTL_OBJ_UNRESOLVED (1 << 1) /**< The object file has unresolved
-                                           *   external symbols. */
+#define RTEMS_RTL_OBJ_LOCKED       (1 << 0) /**< Lock the object file so it cannot
+                                             *   be unloaded. */
+#define RTEMS_RTL_OBJ_UNRESOLVED   (1 << 1) /**< The object file has unresolved
+                                             *   external symbols. */
+#define RTEMS_RTL_OBJ_BASE         (1 << 2) /**< The base image. */
+#define RTEMS_RTL_OBJ_RELOC_TAG    (1 << 3) /**< Tag the object as visited when reloc
+                                             *   parsing. */
 
 /**
  * RTL Object. There is one for each object module loaded plus one for the base
@@ -154,7 +177,8 @@ struct rtems_rtl_obj
 {
   rtems_chain_node    link;         /**< The node's link in the chain. */
   uint32_t            flags;        /**< The status of the object file. */
-  uint32_t            users;        /**< References to the object file. */
+  size_t              users;        /**< Users of this object file, number of loads. */
+  size_t              refs;         /**< References to the object file. */
   int                 format;       /**< The format of the object file. */
   const char*         fname;        /**< The file name for the object. */
   const char*         oname;        /**< The object file name. Can be
@@ -164,27 +188,28 @@ struct rtems_rtl_obj
                                      *   in a lib */
   off_t               ooffset;      /**< The object offset in the archive. */
   size_t              fsize;        /**< Size of the object file. */
-  rtems_chain_control sections;     /**< The sections of interest in the
-                                     *   object file. */
+  rtems_chain_control sections;     /**< The sections of interest in the object
+                                     *   file. */
+  rtems_chain_control dependents;   /**< The dependent object files. */
   rtems_rtl_obj_sym*  local_table;  /**< Local symbol table. */
   size_t              local_syms;   /**< Local symbol count. */
   size_t              local_size;   /**< Local symbol memory usage. */
   rtems_rtl_obj_sym*  global_table; /**< Global symbol table. */
   size_t              global_syms;  /**< Global symbol count. */
   size_t              global_size;  /**< Global symbol memory usage. */
-  uint32_t            unresolved;   /**< The number of unresolved relocations. */
+  size_t              unresolved;   /**< The number of unresolved relocations. */
   void*               text_base;    /**< The base address of the text section
                                      *   in memory. */
   size_t              text_size;    /**< The size of the text section. */
   void*               const_base;   /**< The base address of the const section
                                      *   in memory. */
-  void*               eh_base;      /**< The base address of the eh section
-                                     *   in memory. */
+  void*               eh_base;      /**< The base address of the eh section in
+                                     *   memory. */
   size_t              eh_size;      /**< The size of the eh section. */
   void*               data_base;    /**< The base address of the data section
                                      *   in memory. */
-  void*               bss_base;     /**< The base address of the bss section
-                                     *   in memory. */
+  void*               bss_base;     /**< The base address of the bss section in
+                                     *   memory. */
   size_t              bss_size;     /**< The size of the bss section. */
   size_t              exec_size;    /**< The amount of executable memory
                                      *   allocated */
@@ -192,9 +217,11 @@ struct rtems_rtl_obj
   uint32_t            checksum;     /**< The checksum of the text sections. A
                                      *   zero means do not checksum. */
   uint32_t*           sec_num;      /**< The sec nums of each obj. */
-  uint32_t            obj_num;      /**< The count of elf files in an rtl obj. */
+  uint32_t            obj_num;      /**< The count of elf files in an rtl
+                                     *   obj. */
   struct link_map*    linkmap;      /**< For GDB. */
-  void*               loader;       /**< The file details specific to a loader. */
+  void*               loader;       /**< The file details specific to a
+                                     *   loader. */
 };
 
 /**
@@ -443,6 +470,46 @@ rtems_rtl_obj_sect* rtems_rtl_obj_find_section_by_mask (const rtems_rtl_obj* obj
                                                         uint32_t             mask);
 
 /**
+ * Allocate a table for dependent objects.
+ *
+ * @param obj The object file's descriptor.
+ * @param dependents The size of the table.
+ * @retval true The table was allocated.
+ * @retval false The alloction failed.
+ */
+bool rtems_rtl_obj_alloc_dependents (rtems_rtl_obj* obj, size_t dependents);
+
+/**
+ * Erase the object file descriptor's dependents.
+ *
+ * @param obj The object file's descriptor.
+ */
+void rtems_rtl_obj_erase_dependents (rtems_rtl_obj* obj);
+
+/**
+ * Add an object file to the dependents table.
+ *
+ * @param obj The object file's descriptor.
+ * @param dependent The dependent object file to add.
+ * @retval true The dependent has been added to the table.
+ * @retval false There is no space in the table.
+ */
+bool rtems_rtl_obj_add_dependent (rtems_rtl_obj* obj, rtems_rtl_obj* dependent);
+
+/**
+ * Iterate over the module dependenices.
+ *
+ * @param obj The object file's descriptor.
+ * @param handler The iterator handler. Returns true to end.
+ * @param data User data passed to the iterator.
+ * @retval true The iterator handler returned true.
+ * @retval false The iterator handler returned false.
+ */
+bool rtems_rtl_obj_iterate_dependents (rtems_rtl_obj*                 obj,
+                                       rtems_rtl_obj_depends_iterator iterator,
+                                       void*                          data);
+
+/**
  * The text section size. Only use once all the sections has been added. It
  * includes alignments between sections that are part of the object's text
  * area. The consts sections are included in this section.
@@ -643,6 +710,20 @@ void rtems_rtl_obj_run_ctors (rtems_rtl_obj* obj);
 void rtems_rtl_obj_run_dtors (rtems_rtl_obj* obj);
 
 /**
+ * Increment the object file reference count.
+ *
+ * @param obj The object file's descriptor.
+ */
+void rtems_rtl_obj_inc_reference (rtems_rtl_obj* obj);
+
+/**
+ * Decrement the object file reference count.
+ *
+ * @param obj The object file's descriptor.
+ */
+void rtems_rtl_obj_dec_reference (rtems_rtl_obj* obj);
+
+/**
  * Load the object file, reading all sections into memory, symbols and
  * performing any relocation fixups.
  *
diff --git a/cpukit/include/rtems/rtl/rtl-sym.h b/cpukit/include/rtems/rtl/rtl-sym.h
index be246a908d..aff9339ea1 100644
--- a/cpukit/include/rtems/rtl/rtl-sym.h
+++ b/cpukit/include/rtems/rtl/rtl-sym.h
@@ -92,7 +92,7 @@ bool rtems_rtl_symbol_global_add (rtems_rtl_obj*       obj,
  *
  * @param name The name as an ASCIIZ string.
  * @retval NULL No symbol found.
- * @return rtems_rtl_obj_sym_t* Reference to the symbol.
+ * @return rtems_rtl_obj_sym* Reference to the symbol.
  */
 rtems_rtl_obj_sym* rtems_rtl_symbol_global_find (const char* name);
 
@@ -102,7 +102,7 @@ rtems_rtl_obj_sym* rtems_rtl_symbol_global_find (const char* name);
  * @param obj The object file to search.
  * @param name The name as an ASCIIZ string.
  * @retval NULL No symbol found.
- * @return rtems_rtl_obj_sym_t* Reference to the symbol.
+ * @return rtems_rtl_obj_sym* Reference to the symbol.
  */
 rtems_rtl_obj_sym* rtems_rtl_symbol_obj_find (rtems_rtl_obj* obj,
                                               const char*    name);
diff --git a/cpukit/include/rtems/rtl/rtl.h b/cpukit/include/rtems/rtl/rtl.h
index e30dba90ef..398ac2cd3f 100644
--- a/cpukit/include/rtems/rtl/rtl.h
+++ b/cpukit/include/rtems/rtl/rtl.h
@@ -28,6 +28,7 @@
 #include <rtems/rtl/rtl-obj.h>
 #include <rtems/rtl/rtl-obj-cache.h>
 #include <rtems/rtl/rtl-obj-comp.h>
+#include <rtems/rtl/rtl-sym.h>
 #include <rtems/rtl/rtl-unresolved.h>
 
 #ifdef __cplusplus
@@ -71,6 +72,11 @@ extern "C" {
 #define RTEMS_RTL_UNRESOLVED_BLOCK_SIZE (64)
 
 /**
+ * The number of dependency record per block in the dependency table.
+ */
+#define RTEMS_RTL_DEPENDENCY_BLOCK_SIZE (16)
+
+/**
  * The global debugger interface variable.
  */
 extern struct r_debug _rtld_debug;
@@ -173,6 +179,16 @@ void rtems_rtl_obj_decompress (rtems_rtl_obj_comp** decomp,
                                off_t                offset);
 
 /**
+ * Update the mask in the object files. You can clear flags and then set
+ * flags. A zero (0) does not clear or set the flags. This is global to all
+ * object files that are laoded.
+ *
+ * @param clear The flag's clear mask, a 0 does not clear any flags.
+ * @param set The flag's set mask, a 0 does not set any flags.
+ */
+void rtems_rtl_obj_update_flags (uint32_t clear, uint32_t set);
+
+/**
  * Lock the Run-time Linker.
  *
  * @return rtems_rtl_data* The RTL data after being locked.
@@ -207,6 +223,15 @@ rtems_rtl_obj* rtems_rtl_check_handle (void* handle);
 rtems_rtl_obj* rtems_rtl_find_obj (const char* name);
 
 /**
+ * Find the object file a symbol is exported from.
+ *
+ * @param sym The symbol to search with.
+ * @retval NULL No object file found.
+ * @return rtems_rtl_obj* Reference to the symbol.
+ */
+rtems_rtl_obj* rtems_rtl_find_obj_with_symbol (const rtems_rtl_obj_sym* sym);
+
+/**
  * Load an object file into memory relocating it. It will not be resolved
  * against other symbols in other object files or the base image.
  *
diff --git a/cpukit/libdl/rtl-elf.c b/cpukit/libdl/rtl-elf.c
index c1821bcfda..762130b9e7 100644
--- a/cpukit/libdl/rtl-elf.c
+++ b/cpukit/libdl/rtl-elf.c
@@ -64,31 +64,40 @@ rtems_rtl_elf_machine_check (Elf_Ehdr* ehdr)
   return true;
 }
 
-bool
-rtems_rtl_elf_find_symbol (rtems_rtl_obj* obj,
-                           const Elf_Sym* sym,
-                           const char*    symname,
-                           Elf_Word*      value)
+static bool
+rtems_rtl_elf_find_symbol (rtems_rtl_obj*      obj,
+                           const Elf_Sym*      sym,
+                           const char*         symname,
+                           rtems_rtl_obj_sym** symbol,
+                           Elf_Word*           value)
 {
   rtems_rtl_obj_sect* sect;
 
-  if (ELF_ST_TYPE(sym->st_info) == STT_NOTYPE ||
-      sym->st_shndx == SHN_COMMON)
+  /*
+   * If the symbol type is STT_NOTYPE the symbol references a global
+   * symbol. The gobal symbol table is searched to find it and that value
+   * returned. If the symbol is local to the object module the section for the
+   * symbol is located and it's base added to the symbol's value giving an
+   * absolute location.
+   */
+  if (ELF_ST_TYPE(sym->st_info) == STT_NOTYPE || sym->st_shndx == SHN_COMMON)
   {
     /*
      * Search the object file then the global table for the symbol.
      */
-    rtems_rtl_obj_sym* symbol = rtems_rtl_symbol_obj_find (obj, symname);
-    if (!symbol)
+    *symbol = rtems_rtl_symbol_obj_find (obj, symname);
+    if (!*symbol)
     {
       rtems_rtl_set_error (EINVAL, "global symbol not found: %s", symname);
       return false;
     }
 
-    *value = (Elf_Addr) symbol->value;
+    *value = (Elf_Addr) (*symbol)->value;
     return true;
   }
 
+  *symbol = NULL;
+
   sect = rtems_rtl_obj_find_section_by_index (obj, sym->st_shndx);
   if (!sect)
   {
@@ -100,11 +109,161 @@ rtems_rtl_elf_find_symbol (rtems_rtl_obj* obj,
   return true;
 }
 
+/**
+ * Relocation worker routine.
+ */
+typedef bool (*rtems_rtl_elf_reloc_handler)(rtems_rtl_obj*      obj,
+                                            bool                is_rela,
+                                            void*               relbuf,
+                                            rtems_rtl_obj_sect* targetsect,
+                                            rtems_rtl_obj_sym*  symbol,
+                                            Elf_Sym*            sym,
+                                            const char*         symname,
+                                            Elf_Word            symvalue,
+                                            bool                resolved,
+                                            void*               data);
+
+/**
+ * Relocation parser data.
+ */
+typedef struct
+{
+  size_t dependents; /**< The number of dependent object files. */
+  size_t unresolved; /**< The number of unresolved symbols. */
+} rtems_rtl_elf_reloc_data;
+
+static bool
+rtems_rtl_elf_reloc_parser (rtems_rtl_obj*      obj,
+                            bool                is_rela,
+                            void*               relbuf,
+                            rtems_rtl_obj_sect* targetsect,
+                            rtems_rtl_obj_sym*  symbol,
+                            Elf_Sym*            sym,
+                            const char*         symname,
+                            Elf_Word            symvalue,
+                            bool                resolved,
+                            void*               data)
+{
+  rtems_rtl_elf_reloc_data* rd = (rtems_rtl_elf_reloc_data*) data;
+  /*
+   * If the symbol has been resolved and there is a symbol name it is a global
+   * symbol and from another object file so add it as a dependency.
+   */
+  if (!resolved)
+  {
+    ++rd->unresolved;
+  }
+  else if (resolved && symname != NULL)
+  {
+    /*
+     * Find the symbol's object file. It cannot be NULL so ignore that result
+     * if returned, it means something is corrupted. We are in an iterator.
+     */
+    rtems_rtl_obj*  sobj = rtems_rtl_find_obj_with_symbol (symbol);
+    if (sobj != NULL)
+    {
+      /*
+       * A dependency is not the base kernel image or itself. Tag the object as
+       * having been visited so we count it only once.
+       */
+      if (sobj != rtems_rtl_baseimage () && obj != sobj &&
+          (sobj->flags & RTEMS_RTL_OBJ_RELOC_TAG) == 0)
+      {
+        sobj->flags |= RTEMS_RTL_OBJ_RELOC_TAG;
+        ++rd->dependents;
+      }
+    }
+  }
+  return true;
+}
+
+static bool
+rtems_rtl_elf_reloc_relocator (rtems_rtl_obj*      obj,
+                               bool                is_rela,
+                               void*               relbuf,
+                               rtems_rtl_obj_sect* targetsect,
+                               rtems_rtl_obj_sym*  symbol,
+                               Elf_Sym*            sym,
+                               const char*         symname,
+                               Elf_Word            symvalue,
+                               bool                resolved,
+                               void*               data)
+{
+  const Elf_Rela* rela = (const Elf_Rela*) relbuf;
+  const Elf_Rel*  rel = (const Elf_Rel*) relbuf;
+
+  if (!resolved)
+  {
+    uint16_t       flags = 0;
+    rtems_rtl_word rel_words[3];
+
+    if (is_rela)
+    {
+      flags = 1;
+      rel_words[REL_R_OFFSET] = rela->r_offset;
+      rel_words[REL_R_INFO] = rela->r_info;
+      rel_words[REL_R_ADDEND] = rela->r_addend;
+    }
+    else
+    {
+      rel_words[REL_R_OFFSET] = rel->r_offset;
+      rel_words[REL_R_INFO] = rel->r_info;
+      rel_words[REL_R_ADDEND] = 0;
+    }
+
+    if (!rtems_rtl_unresolved_add (obj,
+                                   flags,
+                                   symname,
+                                   targetsect->section,
+                                   rel_words))
+      return false;
+
+    ++obj->unresolved;
+  }
+  else
+  {
+    rtems_rtl_obj* sobj;
+
+    if (is_rela)
+    {
+      if (rtems_rtl_trace (RTEMS_RTL_TRACE_RELOC))
+        printf ("rtl: rela: sym:%s(%d)=%08jx type:%d off:%08jx addend:%d\n",
+                symname, (int) ELF_R_SYM (rela->r_info),
+                (uintmax_t) symvalue, (int) ELF_R_TYPE (rela->r_info),
+                (uintmax_t) rela->r_offset, (int) rela->r_addend);
+      if (!rtems_rtl_elf_relocate_rela (obj, rela, targetsect,
+                                        symname, sym->st_info, symvalue))
+        return false;
+    }
+    else
+    {
+      if (rtems_rtl_trace (RTEMS_RTL_TRACE_RELOC))
+        printf ("rtl: rel: sym:%s(%d)=%08jx type:%d off:%08jx\n",
+                symname, (int) ELF_R_SYM (rel->r_info),
+                (uintmax_t) symvalue, (int) ELF_R_TYPE (rel->r_info),
+                (uintmax_t) rel->r_offset);
+      if (!rtems_rtl_elf_relocate_rel (obj, rel, targetsect,
+                                       symname, sym->st_info, symvalue))
+        return false;
+    }
+
+    sobj = rtems_rtl_find_obj_with_symbol (symbol);
+    if (sobj != NULL)
+    {
+      if (rtems_rtl_obj_add_dependent (obj, sobj))
+        rtems_rtl_obj_inc_reference (sobj);
+    }
+  }
+
+  return true;
+}
+
 static bool
-rtems_rtl_elf_relocator (rtems_rtl_obj*      obj,
-                         int                 fd,
-                         rtems_rtl_obj_sect* sect,
-                         void*               data)
+rtems_rtl_elf_relocate_worker (rtems_rtl_obj*              obj,
+                               int                         fd,
+                               rtems_rtl_obj_sect*         sect,
+                               rtems_rtl_elf_reloc_handler handler,
+                               void*                       data)
 {
   rtems_rtl_obj_cache* symbols;
   rtems_rtl_obj_cache* strings;
@@ -155,15 +314,16 @@ rtems_rtl_elf_relocator (rtems_rtl_obj*      obj,
 
   for (reloc = 0; reloc < (sect->size / reloc_size); ++reloc)
   {
-    uint8_t         relbuf[reloc_size];
-    const Elf_Rela* rela = (const Elf_Rela*) relbuf;
-    const Elf_Rel*  rel = (const Elf_Rel*) relbuf;
-    Elf_Sym         sym;
-    const char*     symname = NULL;
-    off_t           off;
-    Elf_Word        type;
-    Elf_Word        symvalue = 0;
-    bool            relocate;
+    uint8_t            relbuf[reloc_size];
+    const Elf_Rela*    rela = (const Elf_Rela*) relbuf;
+    const Elf_Rel*     rel = (const Elf_Rel*) relbuf;
+    rtems_rtl_obj_sym* symbol = NULL;
+    Elf_Sym            sym;
+    const char*        symname = NULL;
+    off_t              off;
+    Elf_Word           rel_type;
+    Elf_Word           symvalue = 0;
+    bool               resolved;
 
     off = obj->ooffset + sect->offset + (reloc * reloc_size);
 
@@ -203,75 +363,26 @@ rtems_rtl_elf_relocator (rtems_rtl_obj*      obj,
     /*
      * See if the record references an external symbol. If it does find the
      * symbol value. If the symbol cannot be found flag the object file as
-     * having unresolved externals and store the externals. The load of an
+     * having unresolved externals and store the external. The load of an
      * object after this one may provide the unresolved externals.
      */
     if (is_rela)
-      type = ELF_R_TYPE(rela->r_info);
+      rel_type = ELF_R_TYPE(rela->r_info);
     else
-      type = ELF_R_TYPE(rel->r_info);
-
-    relocate = true;
-
-    if (rtems_rtl_elf_rel_resolve_sym (type))
-    {
-      if (!rtems_rtl_elf_find_symbol (obj, &sym, symname, &symvalue))
-      {
-        uint16_t       flags = 0;
-        rtems_rtl_word rel_words[3];
-
-        relocate = false;
-
-        if (is_rela)
-        {
-          flags = 1;
-          rel_words[REL_R_OFFSET] = rela->r_offset;
-          rel_words[REL_R_INFO] = rela->r_info;
-          rel_words[REL_R_ADDEND] = rela->r_addend;
-        }
-        else
-        {
-          rel_words[REL_R_OFFSET] = rel->r_offset;
-          rel_words[REL_R_INFO] = rel->r_info;
-          rel_words[REL_R_ADDEND] = 0;
-        }
+      rel_type = ELF_R_TYPE(rel->r_info);
 
-        if (!rtems_rtl_unresolved_add (obj,
-                                       flags,
-                                       symname,
-                                       targetsect->section,
-                                       rel_words))
-          return false;
+    resolved = true;
 
-        ++obj->unresolved;
-      }
-    }
+    if (rtems_rtl_elf_rel_resolve_sym (rel_type))
+      resolved = rtems_rtl_elf_find_symbol (obj,
+                                            &sym, symname,
+                                            &symbol, &symvalue);
 
-    if (relocate)
-    {
-      if (is_rela)
-      {
-        if (rtems_rtl_trace (RTEMS_RTL_TRACE_RELOC))
-          printf ("rtl: rela: sym:%s(%d)=%08jx type:%d off:%08jx addend:%d\n",
-                  symname, (int) ELF_R_SYM (rela->r_info),
-                  (uintmax_t) symvalue, (int) ELF_R_TYPE (rela->r_info),
-                  (uintmax_t) rela->r_offset, (int) rela->r_addend);
-        if (!rtems_rtl_elf_relocate_rela (obj, rela, targetsect,
-                                          symname, sym.st_info, symvalue))
-          return false;
-      }
-      else
-      {
-        if (rtems_rtl_trace (RTEMS_RTL_TRACE_RELOC))
-          printf ("rtl: rel: sym:%s(%d)=%08jx type:%d off:%08jx\n",
-                  symname, (int) ELF_R_SYM (rel->r_info),
-                  (uintmax_t) symvalue, (int) ELF_R_TYPE (rel->r_info),
-                  (uintmax_t) rel->r_offset);
-        if (!rtems_rtl_elf_relocate_rel (obj, rel, targetsect,
-                                         symname, sym.st_info, symvalue))
-          return false;
-      }
-    }
+    if (!handler (obj,
+                  is_rela, relbuf, targetsect,
+                  symbol, &sym, symname, symvalue, resolved,
+                  data))
+      return false;
   }
 
   /*
@@ -283,6 +394,28 @@ rtems_rtl_elf_relocator (rtems_rtl_obj*      obj,
   return true;
 }
 
+static bool
+rtems_rtl_elf_relocs_parser (rtems_rtl_obj*      obj,
+                             int                 fd,
+                             rtems_rtl_obj_sect* sect,
+                             void*               data)
+{
+  bool r = rtems_rtl_elf_relocate_worker (obj, fd, sect,
+                                          rtems_rtl_elf_reloc_parser, data);
+  rtems_rtl_obj_update_flags (RTEMS_RTL_OBJ_RELOC_TAG, 0);
+  return r;
+}
+
+static bool
+rtems_rtl_elf_relocs_locator (rtems_rtl_obj*      obj,
+                              int                 fd,
+                              rtems_rtl_obj_sect* sect,
+                              void*               data)
+{
+  return rtems_rtl_elf_relocate_worker (obj, fd, sect,
+                                        rtems_rtl_elf_reloc_relocator, data);
+}
+
 bool
 rtems_rtl_obj_relocate_unresolved (rtems_rtl_unresolv_reloc* reloc,
                                    rtems_rtl_obj_sym*        sym)
@@ -290,8 +423,9 @@ rtems_rtl_obj_relocate_unresolved (rtems_rtl_unresolv_reloc* reloc,
   rtems_rtl_obj_sect* sect;
   bool                is_rela;
   Elf_Word            symvalue;
+  rtems_rtl_obj*      sobj;
 
-  is_rela =reloc->flags & 1;
+  is_rela = reloc->flags & 1;
 
   sect = rtems_rtl_obj_find_section_by_index (reloc->obj, reloc->sect);
   if (!sect)
@@ -329,13 +463,20 @@ rtems_rtl_obj_relocate_unresolved (rtems_rtl_unresolv_reloc* reloc,
       return false;
   }
 
-  if (reloc->obj->unresolved)
+  if (reloc->obj->unresolved > 0)
   {
     --reloc->obj->unresolved;
-    if (!reloc->obj->unresolved)
+    if (reloc->obj->unresolved == 0)
       reloc->obj->flags &= ~RTEMS_RTL_OBJ_UNRESOLVED;
   }
 
+  sobj = rtems_rtl_find_obj_with_symbol (sym);
+  if (sobj != NULL)
+  {
+    if (rtems_rtl_obj_add_dependent (reloc->obj, sobj))
+      rtems_rtl_obj_inc_reference (sobj);
+  }
+
   return true;
 }
 
@@ -402,6 +543,24 @@ rtems_rtl_elf_common (rtems_rtl_obj*      obj,
 }
 
 static bool
+rtems_rtl_elf_dependents (rtems_rtl_obj* obj, rtems_rtl_elf_reloc_data* reloc)
+{
+  /*
+   * If there are dependencies and no unresolved externals allocate and size
+   * the dependency table to the number of dependent object files. If there are
+   * unresolved externals the number of dependencies is unknown at this point
+   * in time so use dynamic allocation to allocate the block size number of
+   * entries when the entries are added.
+   */
+  if (reloc->dependents > 0 && reloc->unresolved == 0)
+  {
+    if (!rtems_rtl_obj_alloc_dependents (obj, reloc->dependents))
+      return false;
+  }
+  return true;
+}
+
+static bool
 rtems_rtl_elf_symbols (rtems_rtl_obj*      obj,
                        int                 fd,
                        rtems_rtl_obj_sect* sect,
@@ -1024,7 +1183,8 @@ rtems_rtl_elf_file_load (rtems_rtl_obj* obj, int fd)
 {
   rtems_rtl_obj_cache*      header;
   Elf_Ehdr                  ehdr;
-  rtems_rtl_elf_common_data common = { .size = 0, .alignment = 0 };
+  rtems_rtl_elf_reloc_data  relocs = { 0 };
+  rtems_rtl_elf_common_data common = { 0 };
 
   rtems_rtl_obj_caches (&header, NULL, NULL);
 
@@ -1102,10 +1262,16 @@ rtems_rtl_elf_file_load (rtems_rtl_obj* obj, int fd)
   if (!rtems_rtl_obj_load_sections (obj, fd, rtems_rtl_elf_loader, &ehdr))
     return false;
 
+  if (!rtems_rtl_obj_relocate (obj, fd, rtems_rtl_elf_relocs_parser, &relocs))
+    return false;
+
+  if (!rtems_rtl_elf_dependents (obj, &relocs))
+    return false;
+
   if (!rtems_rtl_obj_load_symbols (obj, fd, rtems_rtl_elf_symbols, &ehdr))
     return false;
 
-  if (!rtems_rtl_obj_relocate (obj, fd, rtems_rtl_elf_relocator, &ehdr))
+  if (!rtems_rtl_obj_relocate (obj, fd, rtems_rtl_elf_relocs_locator, &ehdr))
     return false;
 
   rtems_rtl_obj_synchronize_cache (obj);
diff --git a/cpukit/libdl/rtl-elf.h b/cpukit/libdl/rtl-elf.h
index 5d560a0b34..1f5f82eb89 100644
--- a/cpukit/libdl/rtl-elf.h
+++ b/cpukit/libdl/rtl-elf.h
@@ -123,28 +123,6 @@ bool rtems_rtl_elf_relocate_rela (const rtems_rtl_obj*      obj,
                                   const Elf_Word            symvalue);
 
 /**
- * Find the symbol. The symbol is passed as an ELF type symbol with the name
- * and the value returned is the absolute address of the symbol.
- *
- * If the symbol type is STT_NOTYPE the symbol references a global symbol. The
- * gobal symbol table is searched to find it and that value returned. If the
- * symbol is local to the object module the section for the symbol is located
- * and it's base added to the symbol's value giving an absolute location.
- *
- * @param obj The object the symbol is being resolved for.
- * @param sym The ELF type symbol.
- * @param symname The sym's name read from the symbol string table.
- * @param value Return the value of the symbol. Only valid if the return value
- *              is true.
- * @retval true The symbol resolved.
- * @retval false The symbol could not be result. The RTL error is set.
- */
-bool rtems_rtl_elf_find_symbol (rtems_rtl_obj* obj,
-                                const Elf_Sym* sym,
-                                const char*    symname,
-                                Elf_Word*      value);
-
-/**
  * The ELF format check handler.
  *
  * @param obj The object being checked.
diff --git a/cpukit/libdl/rtl-obj.c b/cpukit/libdl/rtl-obj.c
index 7109b86fe8..0f58bf9bda 100644
--- a/cpukit/libdl/rtl-obj.c
+++ b/cpukit/libdl/rtl-obj.c
@@ -83,6 +83,7 @@ rtems_rtl_obj_alloc (void)
      * Initialise the chains.
      */
     rtems_chain_initialize_empty (&obj->sections);
+    rtems_chain_initialize_empty (&obj->dependents);
     /*
      * No valid format.
      */
@@ -114,6 +115,8 @@ rtems_rtl_obj_free (rtems_rtl_obj* obj)
     rtems_chain_extract (&obj->link);
   rtems_rtl_alloc_module_del (&obj->text_base, &obj->const_base, &obj->eh_base,
                               &obj->data_base, &obj->bss_base);
+  rtems_rtl_obj_erase_sections (obj);
+  rtems_rtl_obj_erase_dependents (obj);
   rtems_rtl_symbol_obj_erase (obj);
   rtems_rtl_obj_free_names (obj);
   if (obj->sec_num)
@@ -550,6 +553,110 @@ rtems_rtl_obj_find_section_by_mask (const rtems_rtl_obj* obj,
   return match.sect;
 }
 
+bool
+rtems_rtl_obj_alloc_dependents (rtems_rtl_obj* obj, size_t dependents)
+{
+  rtems_rtl_obj_depends* depends;
+  size_t                 size;
+
+  size = sizeof (rtems_rtl_obj_depends) + sizeof (rtems_rtl_obj*) * dependents;
+
+  depends = rtems_rtl_alloc_new (RTEMS_RTL_ALLOC_OBJECT,
+                                 size,
+                                 true);
+  if (depends == NULL)
+  {
+    rtems_rtl_set_error (ENOMEM, "no memory for the dependency");
+  }
+  else
+  {
+    depends->dependents = dependents;
+    rtems_chain_append (&obj->dependents, &depends->node);
+  }
+
+  return depends != NULL;
+}
+
+void
+rtems_rtl_obj_erase_dependents (rtems_rtl_obj* obj)
+{
+  rtems_chain_node* node = rtems_chain_first (&obj->dependents);
+  while (!rtems_chain_is_tail (&obj->dependents, node))
+  {
+    rtems_rtl_obj_depends* depends = (rtems_rtl_obj_depends*) node;
+    rtems_chain_node*      next_node = rtems_chain_next (node);
+    rtems_chain_extract (node);
+    rtems_rtl_alloc_del (RTEMS_RTL_ALLOC_OBJECT, depends);
+    node = next_node;
+  }
+}
+
+bool
+rtems_rtl_obj_add_dependent (rtems_rtl_obj* obj, rtems_rtl_obj* dependent)
+{
+  rtems_rtl_obj**   free_slot;
+  rtems_chain_node* node;
+  if (obj == dependent || dependent == rtems_rtl_baseimage ())
+    return false;
+  free_slot = NULL;
+  node = rtems_chain_first (&obj->dependents);
+  while (!rtems_chain_is_tail (&obj->dependents, node))
+  {
+    rtems_rtl_obj_depends* depends = (rtems_rtl_obj_depends*) node;
+    size_t                 d;
+    for (d = 0; d < depends->dependents; ++d)
+    {
+      if (free_slot == NULL && depends->depends[d] == NULL)
+        free_slot = &(depends->depends[d]);
+      if (depends->depends[d] == dependent)
+        return false;
+    }
+    node = rtems_chain_next (node);
+  }
+  if (free_slot == NULL)
+  {
+    if (rtems_rtl_obj_alloc_dependents (obj,
+                                        RTEMS_RTL_DEPENDENCY_BLOCK_SIZE))
+    {
+      rtems_rtl_obj_depends* depends;
+      node = rtems_chain_last (&obj->dependents);
+      depends = (rtems_rtl_obj_depends*) node;
+      free_slot = &(depends->depends[0]);
+      if (*free_slot != NULL)
+      {
+        rtems_rtl_set_error (EINVAL, "new dependency node not empty");
+        free_slot = NULL;
+      }
+    }
+  }
+  if (free_slot != NULL)
+    *free_slot = dependent;
+  return free_slot != NULL;
+}
+
+bool
+rtems_rtl_obj_iterate_dependents (rtems_rtl_obj*                 obj,
+                                  rtems_rtl_obj_depends_iterator iterator,
+                                  void*                          data)
+{
+  rtems_chain_node* node = rtems_chain_first (&obj->dependents);
+  while (!rtems_chain_is_tail (&obj->dependents, node))
+  {
+    rtems_rtl_obj_depends* depends = (rtems_rtl_obj_depends*) node;
+    size_t                 d;
+    for (d = 0; d < depends->dependents; ++d)
+    {
+      if (depends->depends[d])
+      {
+        if (iterator (obj, depends->depends[d], data))
+          return true;
+      }
+    }
+    node = rtems_chain_next (node);
+  }
+  return false;
+}
+
 size_t
 rtems_rtl_obj_text_size (const rtems_rtl_obj* obj)
 {
@@ -1220,10 +1327,42 @@ static bool
 rtems_rtl_obj_file_unload (rtems_rtl_obj* obj)
 {
   if (obj->format >= 0 && obj->format < RTEMS_RTL_LOADERS)
-      return loaders[obj->format].unload (obj);
+  {
+    rtems_chain_node* node;
+
+    if (!loaders[obj->format].unload (obj))
+      return false;
+
+    node = rtems_chain_first (&obj->dependents);
+    while (!rtems_chain_is_tail (&obj->dependents, node))
+    {
+      rtems_rtl_obj_depends* depends = (rtems_rtl_obj_depends*) node;
+      size_t                 d;
+      for (d = 0; d < depends->dependents; ++d)
+      {
+        if (depends->depends[d] != NULL)
+          rtems_rtl_obj_dec_reference (depends->depends[d]);
+      }
+      node = rtems_chain_next (node);
+    }
+  }
+
   return false;
 }
 
+void
+rtems_rtl_obj_inc_reference (rtems_rtl_obj* obj)
+{
+  ++obj->refs;
+}
+
+void
+rtems_rtl_obj_dec_reference (rtems_rtl_obj* obj)
+{
+  if (obj->refs)
+    --obj->refs;
+}
+
 bool
 rtems_rtl_obj_load (rtems_rtl_obj* obj)
 {
diff --git a/cpukit/libdl/rtl-shell.c b/cpukit/libdl/rtl-shell.c
index 5c144977a0..f3471d8d8e 100644
--- a/cpukit/libdl/rtl-shell.c
+++ b/cpukit/libdl/rtl-shell.c
@@ -117,13 +117,14 @@ rtems_rtl_shell_status (rtems_rtl_data* rtl, int argc, char *argv[])
  */
 typedef struct
 {
-  rtems_rtl_data* rtl;        /**< The RTL data. */
-  int             indent;     /**< Spaces to indent. */
-  bool            oname;      /**< Print object names. */
-  bool            names;      /**< Print details of all names. */
-  bool            memory_map; /**< Print the memory map. */
-  bool            symbols;    /**< Print the global symbols. */
-  bool            base;       /**< Include the base object file. */
+  rtems_rtl_data* rtl;          /**< The RTL data. */
+  int             indent;       /**< Spaces to indent. */
+  bool            oname;        /**< Print object names. */
+  bool            names;        /**< Print details of all names. */
+  bool            memory_map;   /**< Print the memory map. */
+  bool            symbols;      /**< Print the global symbols. */
+  bool            dependencies; /**< Print any dependencies. */
+  bool            base;         /**< Include the base object file. */
 } rtems_rtl_obj_print;
 
 /**
@@ -169,6 +170,29 @@ rtems_rtl_symbols_arg (int argc, char *argv[])
 }
 
 /**
+ * Dependenncies printer.
+ */
+typedef struct
+{
+  bool   first;   /**< Is this the first line printed. */
+  size_t indent;  /**< The indent. */
+} rtems_rtl_dep_data;
+
+static bool
+rtems_rtl_dependencies (rtems_rtl_obj* obj,
+                        rtems_rtl_obj* dependent,
+                        void*          data)
+{
+  rtems_rtl_dep_data* dd = (rtems_rtl_dep_data*) data;
+  if (!dd->first)
+    printf ("\n%-*c: ", dd->indent, ' ');
+  else
+    dd->first = false;
+  printf ("%s", dependent->oname);
+  return false;
+}
+
+/**
  * Object printer.
  */
 static bool
@@ -214,7 +238,9 @@ rtems_rtl_obj_printer (rtems_rtl_obj_print* print, rtems_rtl_obj* obj)
     printf ("%-*cbss base      : %p (%zi)\n", print->indent, ' ',
             obj->bss_base, obj->bss_size);
   }
-  printf ("%-*cunresolved    : %" PRIu32 "\n", print->indent, ' ', obj->unresolved);
+  printf ("%-*cunresolved    : %zu\n", print->indent, ' ', obj->unresolved);
+  printf ("%-*cusers         : %zu\n", print->indent, ' ', obj->users);
+  printf ("%-*creferences    : %zu\n", print->indent, ' ', obj->refs);
   printf ("%-*csymbols       : %zi\n", print->indent, ' ', obj->global_syms);
   printf ("%-*csymbol memory : %zi\n", print->indent, ' ', obj->global_size);
   if (print->symbols)
@@ -231,6 +257,16 @@ rtems_rtl_obj_printer (rtems_rtl_obj_print* print, rtems_rtl_obj* obj)
       printf ("%-*c%-*s = %p\n", print->indent + 2, ' ',
               max_len, obj->global_table[s].name, obj->global_table[s].value);
   }
+  if (print->dependencies)
+  {
+    rtems_rtl_dep_data dd = {
+      .first = true,
+      .indent = strlen ("dependencies :") + print->indent
+    };
+    printf ("%-*cdependencies  : ", print->indent, ' ');
+    rtems_rtl_obj_iterate_dependents (obj, rtems_rtl_dependencies, &dd);
+    printf ("\n");
+  }
   printf ("\n");
   return true;
 }
@@ -269,6 +305,7 @@ rtems_rtl_shell_list (rtems_rtl_data* rtl, int argc, char *argv[])
   print.names = true;
   print.memory_map = true;
   print.symbols = rtems_rtl_symbols_arg (argc, argv);
+  print.dependencies = true;
   print.base = false;
   rtems_rtl_chain_iterate (&rtl->objects,
                            rtems_rtl_obj_print_iterator,
@@ -286,6 +323,7 @@ rtems_rtl_shell_sym (rtems_rtl_data* rtl, int argc, char *argv[])
   print.names = false;
   print.memory_map = false;
   print.symbols = true;
+  print.dependencies = false;
   print.base = rtems_rtl_base_arg (argc, argv);
   rtems_rtl_chain_iterate (&rtl->objects,
                            rtems_rtl_obj_print_iterator,
diff --git a/cpukit/libdl/rtl-unresolved.c b/cpukit/libdl/rtl-unresolved.c
index 4c0f6b3bf1..4992c3bb4a 100644
--- a/cpukit/libdl/rtl-unresolved.c
+++ b/cpukit/libdl/rtl-unresolved.c
@@ -171,7 +171,7 @@ rtems_rtl_unresolved_resolve_reloc (rtems_rtl_unresolv_rec* rec,
     rtems_rtl_unresolved_reloc_data* rd;
     rd = (rtems_rtl_unresolved_reloc_data*) data;
 
-    if (rec->rec.reloc.name == rd->name)
+    if (rec->rec.reloc.name == rd->name && rec->rec.reloc.obj != NULL)
     {
       if (rtems_rtl_trace (RTEMS_RTL_TRACE_UNRESOLVED))
         printf ("rtl: unresolv: resolve reloc: %s\n", rd->name_rec->rec.name.name);
@@ -185,7 +185,7 @@ rtems_rtl_unresolved_resolve_reloc (rtems_rtl_unresolv_rec* rec,
        * names with a reference count of 0.
        */
       rec->rec.reloc.obj = NULL;
-      if (rd->name_rec && rd->name_rec->rec.name.refs)
+      if (rd->name_rec != NULL && rd->name_rec->rec.name.refs > 0)
         --rd->name_rec->rec.name.refs;
     }
   }
diff --git a/cpukit/libdl/rtl.c b/cpukit/libdl/rtl.c
index 628fc771bc..fdf4c1229f 100644
--- a/cpukit/libdl/rtl.c
+++ b/cpukit/libdl/rtl.c
@@ -216,6 +216,11 @@ rtems_rtl_data_init (void)
        */
       rtl->base->oname = rtems_rtl_strdup ("rtems-kernel");
 
+      /*
+       * Lock the base image and flag it as the base image.
+       */
+      rtl->base->flags |= RTEMS_RTL_OBJ_LOCKED | RTEMS_RTL_OBJ_BASE;
+
       rtems_chain_append (&rtl->objects, &rtl->base->link);
     }
 
@@ -312,6 +317,21 @@ rtems_rtl_obj_decompress (rtems_rtl_obj_comp** decomp,
   }
 }
 
+void
+rtems_rtl_obj_update_flags (uint32_t clear, uint32_t set)
+{
+  rtems_chain_node* node  = rtems_chain_first (&rtl->objects);
+  while (!rtems_chain_is_tail (&rtl->objects, node))
+  {
+    rtems_rtl_obj* obj = (rtems_rtl_obj*) node;
+    if (clear != 0)
+      obj->flags &= ~clear;
+    if (set != 0)
+      obj->flags |= set;
+    node = rtems_chain_next (node);
+  }
+}
+
 rtems_rtl_data*
 rtems_rtl_lock (void)
 {
@@ -386,6 +406,21 @@ rtems_rtl_find_obj (const char* name)
 }
 
 rtems_rtl_obj*
+rtems_rtl_find_obj_with_symbol (const rtems_rtl_obj_sym* sym)
+{
+  rtems_chain_node* node = rtems_chain_first (&rtl->objects);
+  while (!rtems_chain_is_tail (&rtl->objects, node))
+  {
+    rtems_rtl_obj* obj = (rtems_rtl_obj*) node;
+    if (sym >= obj->global_table &&
+        sym < (obj->global_table + obj->global_syms))
+      return obj;
+    node = rtems_chain_next (node);
+  }
+  return NULL;
+}
+
+rtems_rtl_obj*
 rtems_rtl_load_object (const char* name, int mode)
 {
   rtems_rtl_obj* obj;
@@ -489,6 +524,11 @@ rtems_rtl_unload_object (rtems_rtl_obj* obj)
 
   if (obj->users == 0)
   {
+    if (obj->refs != 0)
+    {
+      rtems_rtl_set_error (EBUSY, "object file referenced");
+      return false;
+    }
     obj->flags |= RTEMS_RTL_OBJ_LOCKED;
     rtems_rtl_unlock ();
     rtems_rtl_obj_run_dtors (obj);
@@ -618,5 +658,11 @@ rtems_rtl_base_sym_global_add (const unsigned char* esyms,
 rtems_rtl_obj*
 rtems_rtl_baseimage (void)
 {
-  return NULL;
+  rtems_rtl_obj* base = NULL;
+  if (rtems_rtl_lock ())
+  {
+    base = rtl->base;
+    rtems_rtl_unlock ();
+  }
+  return base;
 }