Re: [PATCH v5 2/4] man/man5/tunables.conf: Document system-wide tunables config

Alejandro Colomar <[email protected]> Fri, 7 Aug 2026 00:02:47 +0200
Newsgroups	org.kernel.vger.linux-man
Message-ID	<anUAwb8lY_u7eU3F@devuan>
Hi DJ,

> Date: 2026-08-06 16:44:30-0400
> From: DJ Delorie <[email protected]>
>
> 
> ---

You forgot to sign.

>  man/man5/tunables.conf.5 | 145 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 145 insertions(+)
>  create mode 100644 man/man5/tunables.conf.5
> 
> diff --git a/man/man5/tunables.conf.5 b/man/man5/tunables.conf.5
> new file mode 100644
> index 000000000..29beeb8f8
> --- /dev/null
> +++ b/man/man5/tunables.conf.5
> @@ -0,0 +1,145 @@
> +.TH tunables.conf 5 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +tunables.conf \- tunables configuration file
> +.SH SYNOPSIS
> +.nf
> +.B /etc/tunables.conf
> +.fi
> +.SH DESCRIPTION
> +Tunables are a feature in the GNU C Library
> +that allows application authors and distribution maintainers
> +to alter the runtime library behavior to match their workload.
> +For a list of supported tunables,
> +please consult the glibc manual
> +that corresponds to your installed version of glibc,
> +or run the following command:
> +.P
> +.in +4n
> +.EX
> +ld.so \-\-list\-tunables
> +.EE
> +.P
> +The file is parsed by
> +.BR \%ldconfig (8)
> +and the results stored in
> +.IR /etc/ld.so.cache .
> +The resulting data is read when a new process is created by
> +.IR ld.so .

Sorry for noticing this in v4; I forgot about it.  I see this unresolved
issue from earlier versions remains.

ld.so(8) runs when a new program is executed --execve(2)--, not when
a new process is created --fork(2)--, AFAIK.

	alx@devuan:~$ MANWIDTH=64 man fork | grep ld.so
	alx@devuan:~$ MANWIDTH=64 man execve | grep -C2 ld.so
	     If the executable is an a.out  dynamically  linked  binary
	     executable  containing shared‐library stubs, the Linux dy‐
	     namic linker ld.so(8) is called at the start of  execution
	     to  bring  needed  shared objects into memory and link the
	     executable with them.
	--
	     ptrace(2), exec(3), fexecve(3),  getauxval(3),  getopt(3),
	     system(3),  capabilities(7),  credentials(7),  environ(7),
	     path_resolution(7), ld.so(8)

	Linux man‐pages 6.18‐18... 2026‐02‐08                 execve(2)

> +.P
> +Each line in the file
> +.I /etc/tunables.conf
> +specifies a tunable,
> +which is specified with a string of the form
> +.IB name = value \f[R].\f[]
> +.P
> +The syntax allows lines to start with the keyword
> +.I include

s/I/B/  (since it's a literal, not a variable)


Have a lovely night!
Alex

> +followed by a
> +.BR \%glob (7)
> +pattern.
> +Files matching that pattern will be processed
> +as if their contents were included at that point.
> +.P
> +Each line may include zero or more keywords or symbols at the beginning,
> +which affect how each tunable affects each processes.
> +The keywords must be separated by whitespace,
> +but the symbols need not be.
> +.TP
> +.B overridable
> +.TQ
> +.B +
> +Allow the tunable to be overridden by the
> +.B GLIBC_TUNABLES
> +environment variable when the process runs
> +(this is the default).
> +.TP
> +.B nonoverridable
> +.TQ
> +.B \-
> +Do not allow the tunable to be overridden by the environment variable.
> +.TP
> +.B onlysecure
> +.TQ
> +.B @
> +The tunable applies only to
> +.B AT_SECURE
> +processes,
> +such as one started from a set-user-ID program,
> +or one with elevated capabilities.
> +.TP
> +.B nonsecure
> +.TQ
> +.B $
> +The tunable applies only to
> +.RB non- AT_SECURE
> +processes (this is the default).
> +.TP
> +.B anysecure
> +.TQ
> +.B *
> +The tunable applies to both
> +.B AT_SECURE
> +and
> +.RB non- AT_SECURE
> +processes.
> +.P
> +The file may also contain
> +.IR filters ,
> +which limit the tunables following it.
> +The effects of a filter are terminated by
> +any of the following:
> +.IP \[bu] 3
> +The end of the file.
> +.PD 0
> +.IP \[bu]
> +The end of an included file.
> +.IP \[bu]
> +An
> +.B include
> +directive.
> +.IP \[bu]
> +A new (possibly empty) filter.
> +.PD
> +.P
> +The syntax is:
> +.P
> +.in +4n
> +.EX
> +.BI [ filter : pattern ]
> +.EE
> +.in
> +.TP
> +.B proc
> +The
> +.B proc
> +filter limits the following tunables to processes
> +whose name matches the pattern.
> +The pattern may be an absolute path
> +or just the base name.
> +.P
> +A filter can also be empty:
> +.BR [] .
> +Such a filter has no effects.
> +.SH FILES
> +.TP
> +.I /etc/tunables.conf
> +.TP
> +.I /etc/ld.so.cache
> +cached copy of tunables
> +.SH EXAMPLES
> +Example configuration file:
> +.P
> +.in +4n
> +.EX
> +glibc.malloc.arenas_max=5
> +onlysecure glibc.malloc.arenas_max=1
> +\-glibc.pthread.rseq=1
> +[proc:/bin/bad.program]
> +\-glibc.pthread.rseq=0
> +[proc:some.program]
> +\-glibc.malloc.mmap_threshold=65536
> +.EE
> +.in
> +.SH SEE ALSO
> +.BR ld.so (8),
> +.BR ldconfig (8)
> -- 
> 2.47.3

-- 
<https://www.alejandro-colomar.es>
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