glibc/manual/README.tunables

			TUNABLE FRAMEWORK
			=================

Tunables is a feature in the GNU C Library that allows application authors and
distribution maintainers to alter the runtime library behaviour to match their
workload.

The tunable framework allows modules within glibc to register variables that
may be tweaked through an environment variable.  It aims to enforce a strict
namespace rule to bring consistency to naming of these tunable environment
variables across the project.  This document is a guide for glibc developers to
add tunables to the framework.

ADDING A NEW TUNABLE
--------------------

The TOP_NAMESPACE macro is defined by default as 'glibc'.  If distributions
intend to add their own tunables, they should do so in a different top
namespace by overriding the TOP_NAMESPACE macro for that tunable.  Downstream
implementations are discouraged from using the 'glibc' top namespace for
tunables they don't already have consensus to push upstream.

There are three steps to adding a tunable:

1. Add a tunable to the list and fully specify its properties:

For each tunable you want to add, make an entry in elf/dl-tunables.list.  The
format of the file is as follows:

TOP_NAMESPACE {
  NAMESPACE1 {
    TUNABLE1 {
      # tunable attributes, one per line
    }
    # A tunable with default attributes, i.e. string variable.
    TUNABLE2
    TUNABLE3 {
      # its attributes
    }
  }
  NAMESPACE2 {
    ...
  }
}

The list of allowed attributes are:

- type:			Data type.  Defaults to STRING.  Allowed types are:
			INT_32, UINT_64, SIZE_T and STRING.  Numeric types may
			be in octal or hexadecimal format too.

- minval:		Optional minimum acceptable value.  For a string type
			this is the minimum length of the value.

- maxval:		Optional maximum acceptable value.  For a string type
			this is the maximum length of the value.

- default:		Specify an optional default value for the tunable.

- env_alias:		An alias environment variable

2. Use TUNABLE_GET/TUNABLE_SET/TUNABLE_SET_WITH_BOUNDS to get and set tunables.

3. OPTIONAL: If tunables in a namespace are being used multiple times within a
   specific module, set the TUNABLE_NAMESPACE macro to reduce the amount of
   typing.

GETTING AND SETTING TUNABLES
----------------------------

When the TUNABLE_NAMESPACE macro is defined, one may get tunables in that
module using the TUNABLE_GET macro as follows:

  val = TUNABLE_GET (check, int32_t, TUNABLE_CALLBACK (check_callback))

where 'check' is the tunable name, 'int32_t' is the C type of the tunable and
'check_callback' is the function to call if the tunable got initialized to a
non-default value.  The macro returns the value as type 'int32_t'.

The callback function should be defined as follows:

  void
  TUNABLE_CALLBACK (check_callback) (int32_t *valp)
  {
  ...
  }

where it can expect the tunable value to be passed in VALP.

Tunables in the module can be updated using:

  TUNABLE_SET (check, val)

where 'check' is the tunable name and 'val' is a value of same type.

To get and set tunables in a different namespace from that module, use the full
form of the macros as follows:

  val = TUNABLE_GET_FULL (glibc, malloc, mmap_max, int32_t, NULL)

  TUNABLE_SET_FULL (glibc, malloc, mmap_max, val)

where 'glibc' is the top namespace, 'malloc' is the tunable namespace and the
remaining arguments are the same as the short form macros.

The minimum and maximum values can updated together with the tunable value
using:

  TUNABLE_SET_WITH_BOUNDS (check, val, min, max)

where 'check' is the tunable name, 'val' is a value of same type, 'min' and
'max' are the minimum and maximum values of the tunable.

To set the minimum and maximum values of tunables in a different namespace
from that module, use the full form of the macros as follows:

  val = TUNABLE_GET_FULL (glibc, malloc, mmap_max, int32_t, NULL)

  TUNABLE_SET_WITH_BOUNDS_FULL (glibc, malloc, mmap_max, val, min, max)

where 'glibc' is the top namespace, 'malloc' is the tunable namespace and the
remaining arguments are the same as the short form macros.

When TUNABLE_NAMESPACE is not defined in a module, TUNABLE_GET is equivalent to
TUNABLE_GET_FULL, so you will need to provide full namespace information for
both macros.  Likewise for TUNABLE_SET, TUNABLE_SET_FULL,
TUNABLE_SET_WITH_BOUNDS and TUNABLE_SET_WITH_BOUNDS_FULL.

** IMPORTANT NOTE **

The tunable list is set as read-only after the dynamic linker relocates itself,
so setting tunable values must be limited only to tunables within the dynamic
linker, that too before relocation.

STRING TUNABLES ARE STARTUP-ONLY
--------------------------------

The value of a STRING tunable usually points into the GLIBC_TUNABLES (or
alias) environment variable, which lives in the environment block the kernel
places on the initial stack.  That memory is owned by the application, which
may overwrite it (e.g. setproctitle), so the value is only valid while no
code outside of startup has run.

The values are therefore sealed by __tunable_seal_strings, called from
dl_main for dynamically linked programs and from __libc_start_main for
static ones.  Both calls happen after init_cpu_features, the only consumer
of string tunables, and before audit modules, IFUNC resolvers or
constructors can run.  Reading or setting a string tunable after that point
is a glibc internal error and terminates the process.

FUTURE WORK
-----------

The framework currently only allows a one-time initialization of variables
through environment variables and in some cases, modification of variables via
an API call.  A future goals for this project include:

- Setting system-wide and user-wide defaults for tunables through some
  mechanism like a configuration file.

- Allow tweaking of some tunables at runtime