tunables: Clean up hooks to get and set tunables

The TUNABLE_SET_VALUE and family of macros (and my later attempt to
add a TUNABLE_GET) never quite went together very well because the
overall interface was not clearly defined.  This patch is an attempt
to do just that.

This patch consolidates the API to two simple sets of macros,
TUNABLE_GET* and TUNABLE_SET*.  If TUNABLE_NAMESPACE is defined,
TUNABLE_GET takes just the tunable name, type and a (optionally NULL)
callback function to get the value of the tunable.  The callback
function, if non-NULL, is called if the tunable was externally set
(i.e. via GLIBC_TUNABLES or any future mechanism).  For example:

    val = TUNABLE_GET (check, int32_t, check_callback)

returns the value of the glibc.malloc.check tunable (assuming
TUNABLE_NAMESPACE is set to malloc) as an int32_t into VAL after
calling check_callback.

Likewise, TUNABLE_SET can be used to set the value of the tunable,
although this is currently possible only in the dynamic linker before
it relocates itself.  For example:

  TUNABLE_SET (check, int32_t, 2)

will set glibc.malloc.check to 2.  Of course, this is not possible
since we set (or read) glibc.malloc.check long after it is relocated.

To access or set a tunable outside of TUNABLE_NAMESPACE, use the
TUNABLE_GET_FULL and TUNABLE_SET_FULL macros, which have the following
prototype:

  TUNABLE_GET_FULL (glibc, tune, hwcap_mask, uint64_t, NULL)
  TUNABLE_SET_FULL (glibc, tune, hwcap_mask, uint64_t, 0xffff)

In future the tunable list may get split into mutable and immutable
tunables where mutable tunables can be modified by the library and
userspace after relocation as well and TUNABLE_SET will be more useful
than it currently is.  However whenever we actually do that split, we
will have to ensure that the mutable tunables are protected with
locks.

	* elf/Versions (__tunable_set_val): Rename to __tunable_get_val.
	* elf/dl-tunables.c: Likewise.
	(do_tunable_update_val): New function.
	(__tunable_set_val): New function.
	(__tunable_get_val): Call CB only if the tunable was externally
	initialized.
	(tunables_strtoul): Replace strval with initialized.
	* elf/dl-tunables.h (strval): Replace with a bool initialized.
	(TUNABLE_ENUM_NAME, TUNABLE_ENUM_NAME1): Adjust names to
	prevent collision.
	(__tunable_set_val): New function.
	(TUNABLE_GET, TUNABLE_GET_FULL): New macros.
	(TUNABLE_SET, TUNABLE_SET_FULL): Likewise.
	(TUNABLE_SET_VAL): Remove.
	(TUNABLE_SET_VAL_WITH_CALLBACK): Likewise.
	* README.tunables: Document the new macros.
	* malloc/arena.c (ptmalloc_init): Adjust.

elf/dl-tunables.c

@@ -184,19 +184,17 @@ tunables_strtoul (const char *nptr)
   if ((__type) (__val) >= min && (__type) (val) <= max)			      \
     {									      \
       (__cur)->val.numval = val;					      \
-      (__cur)->strval = strval;						      \
+      (__cur)->initialized = true;					      \
     }									      \
 })
 
-/* Validate range of the input value and initialize the tunable CUR if it looks
-   good.  */
 static void
-tunable_initialize (tunable_t *cur, const char *strval)
+do_tunable_update_val (tunable_t *cur, const void *valp)
 {
   uint64_t val;
 
   if (cur->type.type_code != TUNABLE_TYPE_STRING)
-    val = tunables_strtoul (strval);
+    val = *((int64_t *) valp);
 
   switch (cur->type.type_code)
     {
@@ -217,7 +215,7 @@ tunable_initialize (tunable_t *cur, const char *strval)
 	}
     case TUNABLE_TYPE_STRING:
 	{
-	  cur->val.strval = cur->strval = strval;
+	  cur->val.strval = valp;
 	  break;
 	}
     default:
@@ -225,6 +223,35 @@ tunable_initialize (tunable_t *cur, const char *strval)
     }
 }
 
+/* Validate range of the input value and initialize the tunable CUR if it looks
+   good.  */
+static void
+tunable_initialize (tunable_t *cur, const char *strval)
+{
+  uint64_t val;
+  const void *valp;
+
+  if (cur->type.type_code != TUNABLE_TYPE_STRING)
+    {
+      val = tunables_strtoul (strval);
+      valp = &val;
+    }
+  else
+    {
+      cur->initialized = true;
+      valp = strval;
+    }
+  do_tunable_update_val (cur, valp);
+}
+
+void
+__tunable_set_val (tunable_id_t id, void *valp)
+{
+  tunable_t *cur = &tunable_list[id];
+
+  do_tunable_update_val (cur, valp);
+}
+
 #if TUNABLES_FRONTEND == TUNABLES_FRONTEND_valstring
 /* Parse the tunable string TUNESTR and adjust it to drop any tunables that may
    be unsafe for AT_SECURE processes so that it can be used as the new
@@ -375,7 +402,7 @@ __tunables_init (char **envp)
 
 	  /* Skip over tunables that have either been set already or should be
 	     skipped.  */
-	  if (cur->strval != NULL || cur->env_alias == NULL)
+	  if (cur->initialized || cur->env_alias == NULL)
 	    continue;
 
 	  const char *name = cur->env_alias;
@@ -426,20 +453,10 @@ __tunables_init (char **envp)
 /* Set the tunable value.  This is called by the module that the tunable exists
    in. */
 void
-__tunable_set_val (tunable_id_t id, void *valp, tunable_callback_t callback)
+__tunable_get_val (tunable_id_t id, void *valp, tunable_callback_t callback)
 {
   tunable_t *cur = &tunable_list[id];
 
-  /* Don't do anything if our tunable was not set during initialization or if
-     it failed validation.  */
-  if (cur->strval == NULL)
-    return;
-
-  /* Caller does not need the value, just call the callback with our tunable
-     value.  */
-  if (valp == NULL)
-    goto cb;
-
   switch (cur->type.type_code)
     {
     case TUNABLE_TYPE_UINT_64:
@@ -466,9 +483,8 @@ __tunable_set_val (tunable_id_t id, void *valp, tunable_callback_t callback)
       __builtin_unreachable ();
     }
 
-cb:
-  if (callback)
+  if (cur->initialized && callback != NULL)
     callback (&cur->val);
 }
 
-rtld_hidden_def (__tunable_set_val)
+rtld_hidden_def (__tunable_get_val)