[PATCH 2/2] start-stop-daemon - find processes better

[Denys Vlasenko]

On Thursday 01 May 2008 13:09, Roy Marples wrote:
> > Let's see what help says, and whether it matches reality
> > (I deleted some obvious options):
> >
> > # start-stop-daemon --help
> > Usage: start-stop-daemon [OPTIONS] [--start|--stop] ... [-- arguments...]
> > Start and stop services
> > Options:
> >         -S,--start              Start
> >         -K,--stop               Stop
> >         -a,--startas pathname   Start process specified by pathname
> > WRONG? it seems to just set argv[0].
> > It's not a pathname, it's just a "name" (arbitrary)
> 
> Correct. It's a crap option. Gentoo silently remaps it to --exec

Are you suggesting to drop it? This risks breaking
things for existing users.


> >         -u,--user username|uid  Stop this user's processes
> > Clarification? Will e.g. "ssd -K -u roy" stop
> > all roy's processes?
> 
> Yes

And what does it mean for --start case?
ssd -S -u roy -x /usr/bin/sleep 1234

> >         -x,--exec executable    Program to either start or check
> > Waht does it mean "check"? "stop"?
> > How executable is searched for for "stop" case?
> 
> My view is that it searches for programs started as that.
> So if the user launches /usr/sbin/ntpd then ssd should check /proc/pid/cmdline 
> to see if it was started as /usr/sbin/ntpd and then match.

And what to do if cmdline contains just "ntpd"? or "./ntpd"?
Does that match?
Do we need to check /proc/$PID/stat? /proc/$PID/exec?

For more details, here is the comment from libbb/find_pid_by_name.c:
/*
In Linux we have three ways to determine "process name":
1. /proc/PID/stat has "...(name)...", among other things. It's so-called "comm" field.
2. /proc/PID/cmdline's first NUL-terminated string. It's argv[0] from exec syscall.
3. /proc/PID/exe symlink. Points to the running executable file.

kernel threads:
 comm: thread name
 cmdline: empty
 exe: <readlink fails>

executable
 comm: first 15 chars of base name
 (if executable is a symlink, then first 15 chars of symlink name are used)
 cmdline: argv[0] from exec syscall
 exe: points to executable (resolves symlink, unlike comm)

script (an executable with #!/path/to/interpreter):
 comm: first 15 chars of script's base name (symlinks are not resolved)
 cmdline: /path/to/interpreter (symlinks are not resolved)
 (script name is in argv[1], args are pushed into argv[2] etc)
 exe: points to interpreter's executable (symlinks are resolved)

If FEATURE_PREFER_APPLETS=y (and more so if FEATURE_SH_STANDALONE=y),
some commands started from busybox shell, xargs or find are started by
execXXX("/proc/self/exe", applet_name, params....)
and therefore comm field contains "exe".
*/


> OpenRC extends this futher and also matches daemon arguments. Thus, this works
> 
> start-stop-damon --start --exec /usr/sbin/daemon -- --hostname foo
> start-stop-damon --start --exec /usr/sbin/daemon -- --hostname bar
> start-stop-damon --stop --exec /usr/sbin/daemon -- --hostname foo
> the second daemon *should* still be running in my eyes.
> However, adding that will bloat ssd a little more, so may not be beneficial 
> for busybox.
> 
> >         -n,--name process-name  Stop processes with this name
> > Is it equivalent to "killall name"?
> > If not, what's the difference?
> 
> Should be equivalent.
> This is a bad option to use as many daemon love to change their process name 

Change process name? How?

> AND don't create a pidfile AND are scripts. Again, this is why we 
> changed --exec to look at how it was started to solve this problem.

How it is different from -x?

> >         -p,--pidfile pid-file   Save or load pid using a pid-file
> >         -m,--make-pidfile       Create the -p file and enter pid in it
> > ?? How "save" is different from "create and enter pid in it"?
> 
> In normal ssd, -p only matters when stopping. It reads the specified file and 
> just stops that pid.
> However, ssd can also background some daemons which don't by default. Or which 
> don't create a pidifle by default. So the solution is to run the daemon in 
> foreground mode in a forked process. We then store that pid in the pidfile.
> Example
> 
> start-stop-daemon --start --exec /usr/sbin/daemon --pidfile /var/run/daemon.pid --make-pidfile -- --foreground
> --foreground is an option specific to daemon and not start-stop-daemon.

But --make-pidfile seems to be superfluous here... --start + --pidfile FILE
imply pidfile creation.
--
vda