elf: Add a way to check if tunable is set (BZ 27069)

The patch adds two new macros, TUNABLE_GET_DEFAULT and TUNABLE_IS_INITIALIZED,
here the former get the default value with a signature similar to
TUNABLE_GET, while the later returns whether the tunable was set by
the environment variable.

Checked on x86_64-linux-gnu.
Reviewed-by: DJ Delorie <dj@redhat.com>
Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>

5 files changed, 71 insertions, 0 deletions

diff --git a/elf/Versions b/elf/Versions
index 4614ace..1591031 100644
--- a/elf/Versions
+++ b/elf/Versions
@@ -77,6 +77,7 @@ ld {
     _dl_signal_error;
 
     # Set value of a tunable.
+    __tunable_is_initialized;
     __tunable_get_val;
   }
 }
diff --git a/elf/dl-tunable-types.h b/elf/dl-tunable-types.h
index 62d6d9e..a204805 100644
--- a/elf/dl-tunable-types.h
+++ b/elf/dl-tunable-types.h
@@ -61,6 +61,7 @@ struct _tunable
 {
   const char name[TUNABLE_NAME_MAX];	/* Internal name of the tunable.  */
   tunable_type_t type;			/* Data type of the tunable.  */
+  const tunable_val_t def;		/* The value.  */
   tunable_val_t val;			/* The value.  */
   bool initialized;			/* Flag to indicate that the tunable is
 					   initialized.  */
diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
index 83265bc..644d21d 100644
--- a/elf/dl-tunables.c
+++ b/elf/dl-tunables.c
@@ -145,6 +145,13 @@ tunable_initialize (tunable_t *cur, const char *strval)
   do_tunable_update_val (cur, &val, NULL, NULL);
 }
 
+bool
+__tunable_is_initialized (tunable_id_t id)
+{
+  return tunable_list[id].initialized;
+}
+rtld_hidden_def (__tunable_is_initialized)
+
 void
 __tunable_set_val (tunable_id_t id, tunable_val_t *valp, tunable_num_t *minp,
 		   tunable_num_t *maxp)
@@ -333,6 +340,39 @@ __tunables_print (void)
     }
 }
 
+void
+__tunable_get_default (tunable_id_t id, void *valp)
+{
+  tunable_t *cur = &tunable_list[id];
+
+  switch (cur->type.type_code)
+    {
+    case TUNABLE_TYPE_UINT_64:
+	{
+	  *((uint64_t *) valp) = (uint64_t) cur->def.numval;
+	  break;
+	}
+    case TUNABLE_TYPE_INT_32:
+	{
+	  *((int32_t *) valp) = (int32_t) cur->def.numval;
+	  break;
+	}
+    case TUNABLE_TYPE_SIZE_T:
+	{
+	  *((size_t *) valp) = (size_t) cur->def.numval;
+	  break;
+	}
+    case TUNABLE_TYPE_STRING:
+	{
+	  *((const char **)valp) = cur->def.strval;
+	  break;
+	}
+    default:
+      __builtin_unreachable ();
+    }
+}
+rtld_hidden_def (__tunable_get_default)
+
 /* Set the tunable value.  This is called by the module that the tunable exists
    in. */
 void
diff --git a/elf/dl-tunables.h b/elf/dl-tunables.h
index 45c191e..0df4dde 100644
--- a/elf/dl-tunables.h
+++ b/elf/dl-tunables.h
@@ -45,18 +45,26 @@ typedef void (*tunable_callback_t) (tunable_val_t *);
 
 extern void __tunables_init (char **);
 extern void __tunables_print (void);
+extern bool __tunable_is_initialized (tunable_id_t);
 extern void __tunable_get_val (tunable_id_t, void *, tunable_callback_t);
 extern void __tunable_set_val (tunable_id_t, tunable_val_t *, tunable_num_t *,
 			       tunable_num_t *);
+extern void __tunable_get_default (tunable_id_t id, void *valp);
 rtld_hidden_proto (__tunables_init)
 rtld_hidden_proto (__tunables_print)
+rtld_hidden_proto (__tunable_is_initialized)
 rtld_hidden_proto (__tunable_get_val)
 rtld_hidden_proto (__tunable_set_val)
+rtld_hidden_proto (__tunable_get_default)
 
 /* Define TUNABLE_GET and TUNABLE_SET in short form if TOP_NAMESPACE and
    TUNABLE_NAMESPACE are defined.  This is useful shorthand to get and set
    tunables within a module.  */
 #if defined TOP_NAMESPACE && defined TUNABLE_NAMESPACE
+# define TUNABLE_IS_INITIALIZED(__id) \
+  TUNABLE_IS_INITIALIZED_FULL(TOP_NAMESPACE, TUNABLE_NAMESPACE, __id)
+# define TUNABLE_GET_DEFAULT(__id, __type) \
+  TUNABLE_GET_DEFAULT_FULL(TOP_NAMESPACE, TUNABLE_NAMESPACE,__id, __type)
 # define TUNABLE_GET(__id, __type, __cb) \
   TUNABLE_GET_FULL (TOP_NAMESPACE, TUNABLE_NAMESPACE, __id, __type, __cb)
 # define TUNABLE_SET(__id, __val) \
@@ -65,6 +73,10 @@ rtld_hidden_proto (__tunable_set_val)
   TUNABLE_SET_WITH_BOUNDS_FULL (TOP_NAMESPACE, TUNABLE_NAMESPACE, __id, \
 				__val, __min, __max)
 #else
+# define TUNABLE_IS_INITIALIZED(__top, __ns, __id) \
+  TUNABLE_IS_INITIALIZED_FULL(__top, __ns, __id)
+# define TUNABLE_GET_DEFAULT(__top, __ns, __type) \
+  TUNABLE_GET_DEFAULT_FULL(__top, __ns, __id, __type)
 # define TUNABLE_GET(__top, __ns, __id, __type, __cb) \
   TUNABLE_GET_FULL (__top, __ns, __id, __type, __cb)
 # define TUNABLE_SET(__top, __ns, __id, __val) \
@@ -73,6 +85,22 @@ rtld_hidden_proto (__tunable_set_val)
   TUNABLE_SET_WITH_BOUNDS_FULL (__top, __ns, __id, __val, __min, __max)
 #endif
 
+/* Return whether the tunable was initialized by the environment variable.  */
+#define TUNABLE_IS_INITIALIZED_FULL(__top, __ns, __id) \
+({									      \
+  tunable_id_t id = TUNABLE_ENUM_NAME (__top, __ns, __id);		      \
+  __tunable_is_initialized (id);					      \
+})
+
+/* Return the default value of the tunable.  */
+#define TUNABLE_GET_DEFAULT_FULL(__top, __ns, __id, __type) \
+({									      \
+  tunable_id_t id = TUNABLE_ENUM_NAME (__top, __ns, __id);		      \
+  __type __ret;								      \
+  __tunable_get_default (id, &__ret);					      \
+  __ret;								      \
+})
+
 /* Get and return a tunable value.  If the tunable was set externally and __CB
    is defined then call __CB before returning the value.  */
 #define TUNABLE_GET_FULL(__top, __ns, __id, __type, __cb) \
diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
index 720a8ac..1b23fc9 100644
--- a/elf/dl-tunables.list
+++ b/elf/dl-tunables.list
@@ -20,6 +20,7 @@
 # type: Defaults to STRING
 # minval: Optional minimum acceptable value
 # maxval: Optional maximum acceptable value
+# default: Optional default value (if not specified it will be 0 or "")
 # env_alias: An alias environment variable
 
 glibc {