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

Alejandro Colomar <[email protected]> Fri, 7 Aug 2026 02:11:01 +0200
Newsgroups	org.kernel.vger.linux-man
Message-ID	<anUiQua2QnWnzBeR@devuan>
> Date: 2026-08-07 02:07:00+0200
> From: Alejandro Colomar <[email protected]>
>
> Hi DJ,
> 
> > Date: 2026-08-06 18:15:15-0400
> > From: DJ Delorie <[email protected]>
> >
> > Alejandro Colomar <[email protected]> writes:
> > > You forgot to sign.
> > 
> > I will never remember that... :-P
> > 
> > >> +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.
> > 
> > A program is a file on disk.  A process is a running memory image.
> 
> Yup.  fork(3) creates a process (produces a new PID).  execve(2)
> replaces the image of the current process with a program read from disk
> (or in short, executes a program in the current process).
> 
> > 
> > Neither fork() nor exec() cause the tunables to be loaded, the process
> > itself has to do that.  For dynamic ELF programs/processes built with
> > glibc, ld.so does it before it passes control to the ELF image.  There
> > are other types of programs/processes that do not use ld.so and thus do
> > not read the tunables cache (which is called "ld.so.cache" for a reason ;-)
> 
> Hmmmmm, let's say you call execve(2) several times without any fork(2)s.
> Assuming a that the program paths that you pass to execve(2) are all
> dynamic ELF programs built with glibc, I guess for every time you call
> execve(2), ld.so(8) will be run, and the tunables stored in ld.so.cache
> will be used.  Is this correct?
> 
> 	#include <unistd.h>
> 	int
> 	main(int, char *argv[])
> 	{
> 		sleep(1);
> 		execve("proc/self/exe", argv, NULL);
> 	}

Oh, I thought this would work as a recursive infinite-loop program.  It
doesn't seem to work.  I guess there's some race condition?  :D

	alx@devuan:~$ cd tmp/
	alx@devuan:~/tmp$ cat ex.c 
	#include <unistd.h>
	int
	main(int, char *argv[])
	{
		sleep(1);
		execve("proc/self/exe", argv, NULL);
	}

	alx@devuan:~/tmp$ gcc -Wall -Wextra ex.c 
	alx@devuan:~/tmp$ time ./a.out 

	real	0m1.003s
	user	0m0.003s
	sys	0m0.000s


Cheers,
Alex

> 
> Let's also say you fork(2) several times, without any execve(2) calls.
> I suppose that won't trigger any of this, right?
> 
> 	#include <unistd.h>
> 	int
> 	main(void)
> 	{
> 		for (int i = 0; i < 10; i++) {
> 			sleep(1);
> 			fork();
> 		}
> 	}
> 
> From my understanding, only the first program will trigger this more
> than once, right?
> 
> If so, I'd use the same wording that execve(2) uses:
> 
> 	execve() executes the program referred to by path.
> 
> That is, execve(2) executes programs, and ld.so.cache is read when
> a program is executed (in the current process).
> 
> > 
> > I don't know how to say this clearly with fewer words...
> > 
> > >> +The syntax allows lines to start with the keyword
> > >> +.I include
> > >
> > > s/I/B/  (since it's a literal, not a variable)
> > 
> > Fixed.
> > 
> 
> 
> Cheers,
> Alex
> 
> -- 
> <https://www.alejandro-colomar.es>



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