glibc/manual
Siddhesh Poyarekar 6c2b579962 Reword description of SXID_* tunable properties
The SXID_* tunable properties only influence processes that are
AT_SECURE, so make that a bit more explicit in the documentation and
comment.

Revisiting the code after a few years I managed to confuse myself, so
I imagine there could be others who may have incorrectly assumed like
I did that the SXID_ERASE tunables are not inherited by children of
non-AT_SECURE processes.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
2020-10-22 13:52:38 +05:30
..
examples manual: Fix backtraces code example [BZ #10441] 2020-05-29 10:08:08 -03:00
argp.texi manual: Complete @standards in argp.texi. 2017-06-16 01:19:30 -07:00
arith.texi Use C2x return value from getpayload of non-NaN (bug 26073). 2020-07-06 16:18:02 +00:00
charset.texi mbstowcs: Document, test, and fix null pointer dst semantics (Bug 25219) 2020-06-01 12:26:32 -04:00
check-safety.sh Update copyright dates with scripts/update-copyrights. 2020-01-01 00:14:33 +00:00
conf.texi [manual] Job control is no longer optional. 2018-10-17 14:10:51 -04:00
contrib.texi Add more contributors to the manual 2020-02-01 17:16:54 +05:30
creature.texi Add feature test macro _ISOC2X_SOURCE. 2019-08-13 11:26:00 +00:00
crypt.texi Add GRND_INSECURE from Linux 5.6 to sys/random.h 2020-04-09 21:21:16 +00:00
ctype.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
debug.texi Add manual documentation for threads.h 2018-07-24 14:07:31 -03:00
dir
errno.texi manual: Put the istrerrorname_np and strerrordesc_np return type in braces 2020-08-07 17:14:49 -03:00
fdl-1.3.texi
filesys.texi Revise the documentation of simple calendar time. 2019-10-30 17:11:10 -03:00
freemanuals.texi Prefer https to http for gnu.org and fsf.org URLs 2019-09-07 02:43:31 -07:00
getopt.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
header.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
install-plain.texi BZ #15941: Fix INSTALL file regeneration failure with makeinfo 5.x 2013-12-05 09:58:20 +05:30
install.texi Linux: Require properly configured /dev/pts for PTYs 2020-10-07 14:55:50 +02:00
intro.texi manual: Revise crypt.texi. 2018-06-29 16:53:37 +02:00
io.texi
ipc.texi manual/ipc.texi: Fix AC-safety notes. 2014-04-08 17:12:15 -04:00
job.texi manual/jobs.texi: remove unused var from example code 2020-06-03 18:09:45 -04:00
lang.texi manual: Rewrite the section on widths of integer types. 2017-08-10 20:28:28 -07:00
lgpl-2.1.texi
libc-texinfo.sh Remove add-ons mechanism. 2017-10-05 15:58:13 +00:00
libc.texinfo manual: Show copyright information not just in the printed manual 2020-07-03 10:06:24 +02:00
libcbook.texi
libdl.texi * manual/libdl.texi: New. 2014-01-31 23:23:59 -02:00
llio.texi manual: Document the O_NOFOLLOW open flag 2020-05-05 09:23:53 +02:00
locale.texi Use STRFMON_LDBL_IS_DBL instead of __ldbl_is_dbl. 2018-11-16 09:21:14 -02:00
macros.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
maint.texi Fix the manual for old texinfo 2019-01-04 11:45:13 +00:00
Makefile Update copyright dates with scripts/update-copyrights. 2020-01-01 00:14:33 +00:00
math.texi manual: Use Unicode instead HTML entities for characters (bug 19737) 2020-07-16 10:17:31 +02:00
memory.texi manual: correct the spelling of "MALLOC_PERTURB_" [BZ #23015] 2020-10-13 14:58:16 +02:00
message.texi manual: Use @code{errno} instead of @var{errno} [BZ #24063] 2019-01-07 11:42:04 +01:00
nss.texi Remove --enable-obsolete-rpc configure flag 2020-07-13 19:36:35 +02:00
nsswitch.texi Remove --enable-obsolete-nsl configure flag 2020-07-08 17:25:57 +02:00
pattern.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
pipe.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
platform.texi <sys/platform/x86.h>: Add FSRCS/FSRS/FZLRM support 2020-10-09 11:52:30 -07:00
probes.texi malloc: tcache double free check 2018-11-20 13:24:09 -05:00
process.texi manual: Fix typos in the fexecve description 2020-04-29 10:15:51 +02:00
README.pretty-printers Use gen-as-const.py to process .pysym files. 2018-12-10 22:56:59 +00:00
README.tunables Reword description of SXID_* tunable properties 2020-10-22 13:52:38 +05:30
resource.texi Move vtimes to a compatibility symbol 2020-10-19 16:44:20 -03:00
search.texi manual: Adjust twalk_r documentation. 2019-05-14 15:56:56 -04:00
setjmp.texi manual: Use @code{errno} instead of @var{errno} [BZ #24063] 2019-01-07 11:42:04 +01:00
signal.texi manual: Fix sigdescr_np and sigabbrev_np return type (BZ #26343) 2020-08-08 16:51:26 -03:00
socket.texi manual: Update struct sockaddr_in, struct sockaddr_sin6 description 2019-02-01 14:15:50 +01:00
startup.texi manual: Remove warning in the documentation of the abort function 2019-10-11 20:15:24 +02:00
stdio-fp.c
stdio.texi manual: Fix typo in parse_printf_format example [BZ #24638] 2020-03-04 13:20:07 +01:00
string.texi manual: replace an obsolete collation example with a valid one 2020-10-13 14:58:16 +02:00
summary.pl Update copyright dates with scripts/update-copyrights. 2020-01-01 00:14:33 +00:00
sysinfo.texi Linux: Remove <sys/sysctl.h> and the sysctl function 2020-04-15 17:17:32 +02:00
syslog.texi manual: Replace summary.awk with summary.pl. 2017-06-15 21:26:20 -07:00
terminal.texi Remove obsolete, never-implemented XSI STREAMS declarations 2019-03-14 15:44:15 +01:00
texinfo.tex Update miscellaneous files from upstream sources. 2019-01-01 00:52:59 +00:00
texis.awk
threads.texi manual: Fix typo 2020-10-05 17:29:46 +01:00
time.texi manual: Fix some @code/@var formatting glitches chapter Date And Time 2020-08-05 09:22:21 +02:00
tsort.awk Update copyright dates with scripts/update-copyrights. 2020-01-01 00:14:33 +00:00
tunables.texi Reversing calculation of __x86_shared_non_temporal_threshold 2020-09-28 22:10:39 +00:00
users.texi login: Remove utmp backend jump tables [BZ #23518] 2019-08-05 15:55:05 +02:00
xtract-typefun.awk Make shebang interpreter directives consistent 2016-01-07 04:03:21 -05:00

			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

- security_level:	Specify security level of the tunable for AT_SECURE
			binaries.  Valid values are:

			SXID_ERASE: (default) Do not read and do not pass on to
			child processes.
			SXID_IGNORE: Do not read, but retain for non-AT_SECURE
			child processes.
			NONE: Read all the time.

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, int32_t, val)

where 'check' is the tunable name, 'int32_t' is the C type of the tunable 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, cpu, hwcap_mask, uint64_t, NULL)

  TUNABLE_SET_FULL (glibc, cpu, hwcap_mask, uint64_t, val)

where 'glibc' is the top namespace, 'cpu' 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, int32_t, val, min, max)

where 'check' is the tunable name, 'int32_t' is the C type of the tunable,
'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, cpu, hwcap_mask, uint64_t, NULL)

  TUNABLE_SET_WITH_BOUNDS_FULL (glibc, cpu, hwcap_mask, uint64_t, val, min, max)

where 'glibc' is the top namespace, 'cpu' 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.

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