Don't leak old code when loading a modules with an on_load function

Normally, calling code:delete/1 before re-loading the code for a
module is unnecessary but causes no problem.

But there will be be problems if the new code has an on_load function.
Code with an on_load function will always be loaded as old code
to allowed it to be easily purged if the on_load function would fail.
If the on_load function succeeds, the old and current code will be
swapped.

So in the scenario where code:delete/1 has been called explicitly,
there is old code but no current code. Loading code with an
on_load function will cause the reference to the old code to be
overwritten. That will at best cause a memory leak, and at worst
an emulator crash (especially if NIFs are involved).

To avoid that situation, we will put the code with the on_load
function in a special, third slot in Module.

ERL-240

6 files changed, 76 insertions, 24 deletions

diff --git a/erts/emulator/beam/atom.names b/erts/emulator/beam/atom.names
index 9dae67cb2d..2b66bf6f4e 100644
--- a/erts/emulator/beam/atom.names
+++ b/erts/emulator/beam/atom.names
@@ -505,6 +505,7 @@ atom port_limit
 atom port_op
 atom positive
 atom prepare
+atom prepare_on_load
 atom print
 atom priority
 atom private
diff --git a/erts/emulator/beam/beam_bif_load.c b/erts/emulator/beam/beam_bif_load.c
index dddcfbb77d..5969197168 100644
--- a/erts/emulator/beam/beam_bif_load.c
+++ b/erts/emulator/beam/beam_bif_load.c
@@ -53,6 +53,7 @@ static struct {
     ErlFunEntry *def_funs[10];
     Uint fe_size;
     Uint fe_ix;
+    struct erl_module_instance saved_old;
 } purge_state;
 
 Process *erts_code_purger = NULL;
@@ -105,6 +106,8 @@ init_purge_state(void)
     purge_state.fe_size = sizeof(purge_state.def_funs);
     purge_state.fe_size /= sizeof(purge_state.def_funs[0]);
     purge_state.fe_ix = 0;
+
+    purge_state.saved_old.code_hdr = 0;
 }
 
 void
@@ -739,10 +742,13 @@ BIF_RETTYPE call_on_load_function_1(BIF_ALIST_1)
 {
     Module* modp = erts_get_module(BIF_ARG_1, erts_active_code_ix());
 
-    if (modp && modp->old.code_hdr) {
-	BIF_TRAP_CODE_PTR_0(BIF_P, modp->old.code_hdr->on_load_function_ptr);
+    if (!modp || !modp->on_load) {
+	BIF_ERROR(BIF_P, BADARG);
     }
-    else {
+    if (modp->on_load->code_hdr) {
+	BIF_TRAP_CODE_PTR_0(BIF_P,
+			    modp->on_load->code_hdr->on_load_function_ptr);
+    } else {
 	BIF_ERROR(BIF_P, BADARG);
     }
 }
@@ -765,14 +771,14 @@ BIF_RETTYPE finish_after_on_load_2(BIF_ALIST_2)
     code_ix = erts_active_code_ix();
     modp = erts_get_module(BIF_ARG_1, code_ix);
 
-    if (!modp || !modp->old.code_hdr) {
+    if (!modp || !modp->on_load || !modp->on_load->code_hdr) {
     error:
 	erts_smp_thr_progress_unblock();
         erts_smp_proc_lock(BIF_P, ERTS_PROC_LOCK_MAIN);
 	erts_release_code_write_permission();
 	BIF_ERROR(BIF_P, BADARG);
     }
-    if (modp->old.code_hdr->on_load_function_ptr == NULL) {
+    if (modp->on_load->code_hdr->on_load_function_ptr == NULL) {
 	goto error;
     }
     if (BIF_ARG_2 != am_false && BIF_ARG_2 != am_true) {
@@ -781,14 +787,17 @@ BIF_RETTYPE finish_after_on_load_2(BIF_ALIST_2)
 
     if (BIF_ARG_2 == am_true) {
 	int i;
-	struct erl_module_instance t;
 
 	/*
-	 * Swap old and new code.
+	 * Make the code with the on_load function current.
 	 */
-	t = modp->curr;
-	modp->curr = modp->old;
-	modp->old = t;
+
+	if (modp->curr.code_hdr) {
+	    modp->old = modp->curr;
+	}
+	modp->curr = *modp->on_load;
+	erts_free(ERTS_ALC_T_PREPARED_CODE, modp->on_load);
+	modp->on_load = 0;
 
 	/*
 	 * The on_load function succeded. Fix up export entries.
@@ -1731,7 +1740,8 @@ BIF_RETTYPE erts_internal_purge_module_2(BIF_ALIST_2)
 
     switch (BIF_ARG_2) {
 
-    case am_prepare: {
+    case am_prepare:
+    case am_prepare_on_load: {
 	/*
 	 * Prepare for purge by marking all fun
 	 * entries referring to the code to purge
@@ -1756,7 +1766,22 @@ BIF_RETTYPE erts_internal_purge_module_2(BIF_ALIST_2)
 	    /*
 	     * Any code to purge?
 	     */
-	    erts_rlock_old_code(code_ix);
+
+	    if (BIF_ARG_2 == am_prepare_on_load) {
+                erts_rwlock_old_code(code_ix);
+	    } else {
+		erts_rlock_old_code(code_ix);
+	    }
+
+	    if (BIF_ARG_2 == am_prepare_on_load) {
+		ASSERT(modp->on_load);
+		ASSERT(modp->on_load->code_hdr);
+		purge_state.saved_old = modp->old;
+		modp->old = *modp->on_load;
+		erts_free(ERTS_ALC_T_PREPARED_CODE, (void *) modp->on_load);
+		modp->on_load = 0;
+	    }
+
 	    if (!modp->old.code_hdr)
 		res = am_false;
 	    else {
@@ -1774,7 +1799,12 @@ BIF_RETTYPE erts_internal_purge_module_2(BIF_ALIST_2)
 		ERTS_SET_COPY_LITERAL_AREA(modp->old.code_hdr->literal_area);
 #endif
 	    }
-	    erts_runlock_old_code(code_ix);
+
+            if (BIF_ARG_2 == am_prepare_on_load) {
+                erts_rwunlock_old_code(code_ix);
+	    } else {
+                erts_runlock_old_code(code_ix);
+	    }
 	}
 	
 #ifndef ERTS_SMP
@@ -1911,8 +1941,14 @@ BIF_RETTYPE erts_internal_purge_module_2(BIF_ALIST_2)
 		modp->old.code_length = 0;
 		modp->old.catches = BEAM_CATCHES_NIL;
 		erts_remove_from_ranges(code);
+
 		ERTS_BIF_PREP_RET(ret, am_true);
 	    }
+
+	    if (purge_state.saved_old.code_hdr) {
+		modp->old = purge_state.saved_old;
+		purge_state.saved_old.code_hdr = 0;
+	    }
 	    erts_rwunlock_old_code(code_ix);
 	}
 	if (is_blocking) {
diff --git a/erts/emulator/beam/beam_load.c b/erts/emulator/beam/beam_load.c
index f63addb309..0afdedf6c2 100644
--- a/erts/emulator/beam/beam_load.c
+++ b/erts/emulator/beam/beam_load.c
@@ -833,11 +833,20 @@ erts_finish_loading(Binary* magic, Process* c_p,
 
     size = stp->loaded_size;
     erts_total_code_size += size;
-    if (stp->on_load) {
-	inst_p = &mod_tab_p->old;
-    } else {
+
+    if (!stp->on_load) {
 	inst_p = &mod_tab_p->curr;
+    } else {
+	mod_tab_p->on_load =
+	    (struct erl_module_instance *)
+	    erts_alloc(ERTS_ALC_T_PREPARED_CODE,
+		       sizeof(struct erl_module_instance));
+	inst_p = mod_tab_p->on_load;
+	inst_p->nif = 0;
+        inst_p->num_breakpoints = 0;
+        inst_p->num_traced_exports = 0;
     }
+
     inst_p->code_hdr = stp->hdr;
     inst_p->code_length = size;
 
diff --git a/erts/emulator/beam/erl_nif.c b/erts/emulator/beam/erl_nif.c
index 559b4017e7..3a547982da 100644
--- a/erts/emulator/beam/erl_nif.c
+++ b/erts/emulator/beam/erl_nif.c
@@ -3203,18 +3203,20 @@ BIF_RETTYPE load_nif_2(BIF_ALIST_2)
     if (init_func != NULL)
       handle = init_func;
 
+    this_mi = &module_p->curr;
+    prev_mi = &module_p->old;
     if (in_area(caller, module_p->old.code_hdr, module_p->old.code_length)) {
-	if (module_p->old.code_hdr->on_load_function_ptr) {
-	    this_mi = &module_p->old;
+	ret = load_nif_error(BIF_P, "old_code", "Calling load_nif from old "
+			     "module '%T' not allowed", mod_atom);
+	goto error;
+    } else if (module_p->on_load) {
+	ASSERT(module_p->on_load->code_hdr->on_load_function_ptr);
+	if (module_p->curr.code_hdr) {
 	    prev_mi = &module_p->curr;
 	} else {
-	    ret = load_nif_error(BIF_P, "old_code", "Calling load_nif from old "
-				 "module '%T' not allowed", mod_atom);
-	    goto error;
+	    prev_mi = &module_p->old;
 	}
-    } else {
-	this_mi = &module_p->curr;
-	prev_mi = &module_p->old;
+	this_mi = module_p->on_load;
     }
 
     if (init_func == NULL &&
diff --git a/erts/emulator/beam/module.c b/erts/emulator/beam/module.c
index 3eb11f1693..4f36377450 100644
--- a/erts/emulator/beam/module.c
+++ b/erts/emulator/beam/module.c
@@ -85,6 +85,7 @@ static Module* module_alloc(Module* tmpl)
     obj->old.num_breakpoints  = 0;
     obj->curr.num_traced_exports = 0;
     obj->old.num_traced_exports = 0;
+    obj->on_load = 0;
     return obj;
 }
 
@@ -201,6 +202,7 @@ void module_start_staging(void)
 
 	dst_mod->curr = src_mod->curr;
 	dst_mod->old = src_mod->old;
+	dst_mod->on_load = src_mod->on_load;
     }
 
     /*
@@ -214,6 +216,7 @@ void module_start_staging(void)
 
 	dst_mod->curr = src_mod->curr;
 	dst_mod->old = src_mod->old;
+	dst_mod->on_load = src_mod->on_load;
     }
     newsz = index_table_sz(dst);
     erts_smp_atomic_add_nob(&tot_module_bytes, (newsz - oldsz));
diff --git a/erts/emulator/beam/module.h b/erts/emulator/beam/module.h
index 5a60bc90d9..1c1afc8461 100644
--- a/erts/emulator/beam/module.h
+++ b/erts/emulator/beam/module.h
@@ -39,6 +39,7 @@ typedef struct erl_module {
 
     struct erl_module_instance curr;
     struct erl_module_instance old; /* protected by "old_code" rwlock */
+    struct erl_module_instance* on_load;
 } Module; 
 
 Module* erts_get_module(Eterm mod, ErtsCodeIndex code_ix);