[1/5] tunables: Add hooks to get and update tunables

This change adds two new tunable macros to get and update tunables
respectively called TUNABLE_GET and TUNABLE_UPDATE_VAL.

TUNABLE_GET accepts the top namespace, tunable namespace and tunable
id along with the target type as arguments and returns a value of the
target type.  For example:

    TUNABLE_GET (glibc, malloc, check, int32_t)

will return the tunable value for glibc.malloc.check in an int32_t.

TUNABLE_UPDATE_VAL accepts the top namespace, tunable namespace and
tunable id along with the value to be stored into the tunable
structure followed by the type of the value.  For example:

  TUNABLE_UPDATE_VAL (glibc, malloc, check, 1, int32_t);

will update the glibc.malloc.check tunable.

Given that the tunable structure is now relro, this is only really
possible from within the dynamic linker before it is relocated.  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.  However whenever we actually do
that, we will have to ensure that the mutable tunables are protected
with locks.

	* elf/dl-tunables.h (strval): Replace with a bool initialized.
	(TUNABLE_ENUM_NAME, TUNABLE_ENUM_NAME1): Adjust names to
	prevent collision.
	(__tunable_update_val): New function.
	(TUNABLE_GET): New macro.
	(TUNABLE_SET_VAL_FULL_WITH_CALLBACK): New macro.
	(TUNABLE_SET_VAL): Use it.
	(TUNABLE_SET_VAL_WITH_CALLBACK): Likewise.
	(TUNABLE_UPDATE_VAL): New macro.
	(tunables_strtoul): Replace strval with initialized.
	(__tunables_init): Likewise.
	(__tunable_set_val): Likewise.
	(do_tunable_update_val): New function.
	(tunables_initialize): Use it.
	(__tunable_update_val): New function.
	* README.tunables: Document the new macros.
---
 README.tunables   | 27 +++++++++++++++++++++++++++
 elf/dl-tunables.c | 43 +++++++++++++++++++++++++++++++++++--------
 elf/dl-tunables.h | 40 ++++++++++++++++++++++++++++------------
 3 files changed, 90 insertions(+), 20 deletions(-)

On 18/05/2017 17:07, Siddhesh Poyarekar wrote:
> This change adds two new tunable macros to get and update tunables
> respectively called TUNABLE_GET and TUNABLE_UPDATE_VAL.
> 
> TUNABLE_GET accepts the top namespace, tunable namespace and tunable
> id along with the target type as arguments and returns a value of the
> target type.  For example:
> 
>     TUNABLE_GET (glibc, malloc, check, int32_t)
> 
> will return the tunable value for glibc.malloc.check in an int32_t.
> 
> TUNABLE_UPDATE_VAL accepts the top namespace, tunable namespace and
> tunable id along with the value to be stored into the tunable
> structure followed by the type of the value.  For example:
> 
>   TUNABLE_UPDATE_VAL (glibc, malloc, check, 1, int32_t);
> 
> will update the glibc.malloc.check tunable.
> 
> Given that the tunable structure is now relro, this is only really
> possible from within the dynamic linker before it is relocated.  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.  However whenever we actually do
> that, we will have to ensure that the mutable tunables are protected
> with locks.
> 
> 	* elf/dl-tunables.h (strval): Replace with a bool initialized.
> 	(TUNABLE_ENUM_NAME, TUNABLE_ENUM_NAME1): Adjust names to
> 	prevent collision.
> 	(__tunable_update_val): New function.
> 	(TUNABLE_GET): New macro.
> 	(TUNABLE_SET_VAL_FULL_WITH_CALLBACK): New macro.
> 	(TUNABLE_SET_VAL): Use it.
> 	(TUNABLE_SET_VAL_WITH_CALLBACK): Likewise.
> 	(TUNABLE_UPDATE_VAL): New macro.
> 	(tunables_strtoul): Replace strval with initialized.
> 	(__tunables_init): Likewise.
> 	(__tunable_set_val): Likewise.
> 	(do_tunable_update_val): New function.
> 	(tunables_initialize): Use it.
> 	(__tunable_update_val): New function.
> 	* README.tunables: Document the new macros.

LGTM (I added just some document extension points and it is not a patch 
blocker).

> ---
>  README.tunables   | 27 +++++++++++++++++++++++++++
>  elf/dl-tunables.c | 43 +++++++++++++++++++++++++++++++++++--------
>  elf/dl-tunables.h | 40 ++++++++++++++++++++++++++++------------
>  3 files changed, 90 insertions(+), 20 deletions(-)
> 
> diff --git a/README.tunables b/README.tunables
> index 0e9b0d7..0bea2ca 100644
> --- a/README.tunables
> +++ b/README.tunables
> @@ -75,6 +75,33 @@ The list of allowed attributes are:
>     TUNABLE_SET_VALUE_WITH_CALLBACK instead and additionally pass a pointer to
>     the function that should be called if the tunable value has been set.
>  
> +GETTING A TUNABLE VALUE
> +-----------------------
> +
> +One may get the value of a tunable anywhere in glibc code using the TUNABLE_GET
> +macro as follows:
> +
> +  TUNABLE_GET (glibc, malloc, check, int32_t)
> +
> +The first three arguments of the macro are the top namespace, tunable namespace
> +and the tunable name respectively.  The last argument is the type of the
> +tunable and must match the type of the tunable.
> +
> +UPDATING A TUNABLE VALUE
> +------------------------
> +
> +The tunable list is currently stored in a relro section, i.e. it cannot be
> +modified after relocation.  As a result, one may update the value of a tunable
> +only in the dynamic linker.  The TUNABLE_UPDATE_VALUE macro can be used to do
> +this:
> +
> +    TUNABLE_UPDATE_VAL (glibc, malloc, check, val, int32_t);
> +
> +where the first thre arguments of the macro are the top namespace, tunable
> +namespace and the tunable name respectively.  The fourth argument is the value
> +to assign to the tunable and the last argument is the type of the tunable.  The
> +type in the fifth argument must match the type of the tunable.
> +

Wouldn't be worth do point where in code it updates now?  The idea would just to
add some reference point for possible future work.

>  FUTURE WORK
>  -----------
>  
> diff --git a/elf/dl-tunables.c b/elf/dl-tunables.c
> index 8d72e26..abf10e5 100644
> --- a/elf/dl-tunables.c
> +++ b/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_update_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;
> @@ -432,7 +459,7 @@ __tunable_set_val (tunable_id_t id, void *valp, tunable_callback_t callback)
>  
>    /* Don't do anything if our tunable was not set during initialization or if
>       it failed validation.  */
> -  if (cur->strval == NULL)
> +  if (!cur->initialized)
>      return;
>  
>    /* Caller does not need the value, just call the callback with our tunable
> diff --git a/elf/dl-tunables.h b/elf/dl-tunables.h
> index f33adfb..f465370 100644
> --- a/elf/dl-tunables.h
> +++ b/elf/dl-tunables.h
> @@ -39,8 +39,8 @@ struct _tunable
>    const char *name;			/* Internal name of the tunable.  */
>    tunable_type_t type;			/* Data type of the tunable.  */
>    tunable_val_t val;			/* The value.  */
> -  const char *strval;			/* The string containing the value,
> -					   points into envp.  */
> +  bool initialized;			/* Flag to indicate that the tunable is
> +					   initialized.  */
>    tunable_seclevel_t security_level;	/* Specify the security level for the
>  					   tunable with respect to AT_SECURE
>  					   programs.  See description of
> @@ -61,29 +61,45 @@ typedef struct _tunable tunable_t;
>  /* Full name for a tunable is top_ns.tunable_ns.id.  */
>  # define TUNABLE_NAME_S(top,ns,id) #top "." #ns "." #id
>  
> -# define TUNABLE_ENUM_NAME(top,ns,id) TUNABLE_ENUM_NAME1 (top,ns,id)
> -# define TUNABLE_ENUM_NAME1(top,ns,id) top ## _ ## ns ## _ ## id
> +# define TUNABLE_ENUM_NAME(__top,__ns,__id) TUNABLE_ENUM_NAME1 (__top,__ns,__id)
> +# define TUNABLE_ENUM_NAME1(__top,__ns,__id) __top ## _ ## __ns ## _ ## __id
>  
>  # include "dl-tunable-list.h"
>  
>  extern void __tunables_init (char **);
>  extern void __tunable_set_val (tunable_id_t, void *, tunable_callback_t);
> +extern void __tunable_update_val (tunable_id_t, void *);
> +
> +/* Get and return a tunable value.  */
> +# define TUNABLE_GET(__top, __ns, __id, __type) \
> +({									      \
> +  tunable_id_t id = TUNABLE_ENUM_NAME (__top, __ns, __id);		      \
> +  __type ret;								      \
> +  TUNABLE_SET_VAL_FULL_WITH_CALLBACK (id, &ret, NULL);			      \
> +  ret;									      \
> +})
>  
>  /* Check if the tunable has been set to a non-default value and if it is, copy
>     it over into __VAL.  */
>  # define TUNABLE_SET_VAL(__id,__val) \
> -({									      \
> -  __tunable_set_val							      \
> -   (TUNABLE_ENUM_NAME (TOP_NAMESPACE, TUNABLE_NAMESPACE, __id), (__val),      \
> -    NULL);								      \
> -})
> +  TUNABLE_SET_VAL_FULL_WITH_CALLBACK (TUNABLE_ENUM_NAME (TOP_NAMESPACE,       \
> +							 TUNABLE_NAMESPACE,   \
> +							 __id),	(__val), NULL)
>  
>  /* Same as TUNABLE_SET_VAL, but also call the callback function __CB.  */
>  # define TUNABLE_SET_VAL_WITH_CALLBACK(__id,__val,__cb) \
> +  TUNABLE_SET_VAL_FULL_WITH_CALLBACK (TUNABLE_ENUM_NAME (TOP_NAMESPACE,       \
> +							 TUNABLE_NAMESPACE,   \
> +							 __id),		      \
> +				      (__val), DL_TUNABLE_CALLBACK (__cb))
> +
> +# define TUNABLE_SET_VAL_FULL_WITH_CALLBACK(__id,__val,__cb)\
> +  __tunable_set_val (__id, (__val), (__cb))
> +
> +# define TUNABLE_UPDATE_VAL(__top, __ns, __id, __val, __type) \
>  ({									      \
> -  __tunable_set_val							      \
> -   (TUNABLE_ENUM_NAME (TOP_NAMESPACE, TUNABLE_NAMESPACE, __id), (__val),      \
> -    DL_TUNABLE_CALLBACK (__cb));					      \
> +  __tunable_update_val (TUNABLE_ENUM_NAME (__top, __ns, __id),		      \
> +			& (__type) {__val});				      \
>  })
>  
>  /* Namespace sanity for callback functions.  Use this macro to keep the
>