[v2,1/2] elf: Add a way to check if tunable is set (BZ 27069)

Message ID 20231123172915.893408-2-adhemerval.zanella@linaro.org
State Committed
Commit a4c3f5f46e850c977cda81c251036475aab8313c
Headers
Series Improve MAP_HUGETLB with glibc.malloc.hugetlb=2 |

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Commit Message

Adhemerval Zanella Nov. 23, 2023, 5:29 p.m. UTC
  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.
---
 elf/Versions             |  1 +
 elf/dl-tunable-types.h   |  1 +
 elf/dl-tunables.c        | 40 ++++++++++++++++++++++++++++++++++++++++
 elf/dl-tunables.h        | 28 ++++++++++++++++++++++++++++
 elf/dl-tunables.list     |  1 +
 scripts/gen-tunables.awk |  4 ++--
 6 files changed, 73 insertions(+), 2 deletions(-)
  

Comments

DJ Delorie Nov. 28, 2023, 9:38 p.m. UTC | #1
LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>

Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
> diff --git a/elf/Versions b/elf/Versions
> index 4614acea3e..1591031da9 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;
>    }
>  }

This is in GLIBC_PRIVATE so ok.

> diff --git a/elf/dl-tunable-types.h b/elf/dl-tunable-types.h
> index 62d6d9e629..a2048058fa 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.  */

Is this struct part of the ABI between ld.so and libc.so ?  It doesn't
look like it to me, so OK.

> diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
> index 83265bc00b..644d21d1b0 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)
> +

Ok.

> @@ -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;

Hmmm... doesn't this break the type aliasing rules?

But we do the same thing for __tunable_get_val so OK.

> +	  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)

We don't seem to have a flag that says "no default provided" but if we
assume the templates won't ever provide a default of zero, good enough?

Ok.

> diff --git a/elf/dl-tunables.h b/elf/dl-tunables.h
> index 45c191e021..0df4dde24e 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);

Ok.

> +extern void __tunable_get_default (tunable_id_t id, void *valp);

Ok.

> +rtld_hidden_proto (__tunable_is_initialized)
> +rtld_hidden_proto (__tunable_get_default)

Ok.

>  /* 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

Ok.

> +/* 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;								      \
> +})

Ok.

> diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
> index 720a8ac49c..1b23fc9473 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

Ok.

>  glibc {
> diff --git a/scripts/gen-tunables.awk b/scripts/gen-tunables.awk
> index 1e9d6b534e..9f5336381e 100644
> --- a/scripts/gen-tunables.awk
> +++ b/scripts/gen-tunables.awk
> @@ -163,8 +163,8 @@ END {
>      n = indices[2];
>      m = indices[3];
>      printf ("  {TUNABLE_NAME_S(%s, %s, %s)", t, n, m)
> -    printf (", {TUNABLE_TYPE_%s, %s, %s}, {%s}, false, %s},\n",
> -	    types[t,n,m], minvals[t,n,m], maxvals[t,n,m],
> +    printf (", {TUNABLE_TYPE_%s, %s, %s}, {%s}, {%s}, false, %s},\n",
> +	    types[t,n,m], minvals[t,n,m], maxvals[t,n,m], default_val[t,n,m],
>  	    default_val[t,n,m], env_alias[t,n,m]);
>    }
>    print "};"

Ok.
  
Adhemerval Zanella Nov. 29, 2023, 12:29 p.m. UTC | #2
On 28/11/23 18:38, DJ Delorie wrote:
> 
> LGTM
> Reviewed-by: DJ Delorie <dj@redhat.com>
> 
> Adhemerval Zanella <adhemerval.zanella@linaro.org> writes:
>> diff --git a/elf/Versions b/elf/Versions
>> index 4614acea3e..1591031da9 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;
>>    }
>>  }
> 
> This is in GLIBC_PRIVATE so ok.
> 
>> diff --git a/elf/dl-tunable-types.h b/elf/dl-tunable-types.h
>> index 62d6d9e629..a2048058fa 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.  */
> 
> Is this struct part of the ABI between ld.so and libc.so ?  It doesn't
> look like it to me, so OK.

I don't think so, libc.so will issue the macros TUNABLES_* that will maps to
tunable_id_t.  The tunable_t is used only on dl-tunables.c.

> 
>> diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
>> index 83265bc00b..644d21d1b0 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)
>> +
> 
> Ok.
> 
>> @@ -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;
> 
> Hmmm... doesn't this break the type aliasing rules?
> 
> But we do the same thing for __tunable_get_val so OK.

Indeed, I will send a patch to fix it.

> 
>> +	  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)
> 
> We don't seem to have a flag that says "no default provided" but if we
> assume the templates won't ever provide a default of zero, good enough?

Right, but I think it would be easier to add once we actually require it.

> 
> Ok.
> 
>> diff --git a/elf/dl-tunables.h b/elf/dl-tunables.h
>> index 45c191e021..0df4dde24e 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);
> 
> Ok.
> 
>> +extern void __tunable_get_default (tunable_id_t id, void *valp);
> 
> Ok.
> 
>> +rtld_hidden_proto (__tunable_is_initialized)
>> +rtld_hidden_proto (__tunable_get_default)
> 
> Ok.
> 
>>  /* 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
> 
> Ok.
> 
>> +/* 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;								      \
>> +})
> 
> Ok.
> 
>> diff --git a/elf/dl-tunables.list b/elf/dl-tunables.list
>> index 720a8ac49c..1b23fc9473 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
> 
> Ok.
> 
>>  glibc {
>> diff --git a/scripts/gen-tunables.awk b/scripts/gen-tunables.awk
>> index 1e9d6b534e..9f5336381e 100644
>> --- a/scripts/gen-tunables.awk
>> +++ b/scripts/gen-tunables.awk
>> @@ -163,8 +163,8 @@ END {
>>      n = indices[2];
>>      m = indices[3];
>>      printf ("  {TUNABLE_NAME_S(%s, %s, %s)", t, n, m)
>> -    printf (", {TUNABLE_TYPE_%s, %s, %s}, {%s}, false, %s},\n",
>> -	    types[t,n,m], minvals[t,n,m], maxvals[t,n,m],
>> +    printf (", {TUNABLE_TYPE_%s, %s, %s}, {%s}, {%s}, false, %s},\n",
>> +	    types[t,n,m], minvals[t,n,m], maxvals[t,n,m], default_val[t,n,m],
>>  	    default_val[t,n,m], env_alias[t,n,m]);
>>    }
>>    print "};"
> 
> Ok.
>
  

Patch

diff --git a/elf/Versions b/elf/Versions
index 4614acea3e..1591031da9 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 62d6d9e629..a2048058fa 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 83265bc00b..644d21d1b0 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 45c191e021..0df4dde24e 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 720a8ac49c..1b23fc9473 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 {
diff --git a/scripts/gen-tunables.awk b/scripts/gen-tunables.awk
index 1e9d6b534e..9f5336381e 100644
--- a/scripts/gen-tunables.awk
+++ b/scripts/gen-tunables.awk
@@ -163,8 +163,8 @@  END {
     n = indices[2];
     m = indices[3];
     printf ("  {TUNABLE_NAME_S(%s, %s, %s)", t, n, m)
-    printf (", {TUNABLE_TYPE_%s, %s, %s}, {%s}, false, %s},\n",
-	    types[t,n,m], minvals[t,n,m], maxvals[t,n,m],
+    printf (", {TUNABLE_TYPE_%s, %s, %s}, {%s}, {%s}, false, %s},\n",
+	    types[t,n,m], minvals[t,n,m], maxvals[t,n,m], default_val[t,n,m],
 	    default_val[t,n,m], env_alias[t,n,m]);
   }
   print "};"