glibc/README.tunables
Siddhesh Poyarekar ad2f35cb39 tunables: Add support for tunables of uint64_t type
Recognize the uint64_t type in addition to the current int32_t and
size_t.  This allows addition of tunables of uint64_t types.  In
addition to adding the uint64_t type, this patch also consolidates
validation and reading of integer types in tunables.

One notable change is that of overflow computation in
tunables_strtoul.  The function was lifted from __internal_strtoul,
but it does not need the boundary condition check (i.e. result ==
ULONG_MAX) since it does not need to set errno.  As a result the check
can be simplified, which I have now done.

	* elf/dl-tunable-types.h (tunable_type_code_t): New type
	TUNABLE_TYPE_UINT_64.
	* elf/dl-tunables.c (tunables_strtoul): Return uint64_t.
	Simplify computation of overflow.
	(tunable_set_val_if_valid_range_signed,
	tunable_set_val_if_valid_range_unsigned): Remove and replace
	with this...
	(TUNABLE_SET_VAL_IF_VALID_RANGE): ... New macro.
	(tunable_initialize): Adjust.  Add uint64_t support.
	(__tunable_set_val): Add uint64_t support.
	* README.tunables: Document it.
2017-05-17 13:11:55 +05:30

89 lines
3.0 KiB
Plaintext

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 two steps to adding a tunable:
1. Add a tunable ID:
Modules that wish to use the tunables interface must define the
TUNABLE_NAMESPACE macro. Following this, 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
- security_level: Specify security level of the tunable. Valid values:
SXID_ERASE: (default) Don't read for AT_SECURE binaries and
removed so that child processes can't read it.
SXID_IGNORE: Don't read for AT_SECURE binaries, but retained for
non-AT_SECURE subprocesses.
NONE: Read all the time.
2. Call either the TUNABLE_SET_VALUE and pass into it the tunable name and a
pointer to the variable that should be set with the tunable value.
If additional work needs to be done after setting the value, use the
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.
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