[DTrace-devel] [PATCH 4/4] Do not register handlers when not executing probe clauses

[Kris Van Hees]

On Wed, Jun 10, 2020 at 09:14:34AM -0700, Eugene Loh wrote:
> This raises a question I've been curious about.  Why do we even compile 
> clauses if, say, we're in "list" mode?  In that case, we only care about 
> the probe descriptions, and the clauses don't matter.

Excellent question and one I have pondered for quite a long time myself...
And the main reason seems to be that -l is an operation mode where all probes
that are used by scripts (and/or command line clauses) are listed.  A script
may include other scripts and thereby end up with a larger probe set than can
be seen from just the script itself.  That is one example where compilation
(or in this case at least pre-processing) matters in terms of being able to
capture the complete set of probes.

> On 06/10/2020 08:23 AM, Kris Van Hees wrote:
> > The various handlers that are used by consumers only apply to the
> > execution of probe clauses.  For operating modes that do not execute
> > clauses there is no need to register handlers.  Even worse, if e.g.
> > the error handler is being registered with the default error clause
> > and -xnolibs is used (which is common with the -h option to generate
> > a header file), an error will occur because bpf_get_bvar() cannot be
> > found.
> >
> > Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
> > ---
> >   cmd/dtrace.c | 6 +++++-
> >   1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/cmd/dtrace.c b/cmd/dtrace.c
> > index cef86ef2..17d5692a 100644
> > --- a/cmd/dtrace.c
> > +++ b/cmd/dtrace.c
> > @@ -1337,7 +1337,11 @@ main(int argc, char *argv[])
> >   	for (i = 0; i < g_cmdc; i++)
> >   		g_cmdv[i].dc_func(&g_cmdv[i]);
> >   
> > -	if (g_mode != DMODE_LIST) {
> > +	/*
> > +	 * We only need to register handler if we are going to execute probe
> > +	 * clauses.
> > +	 */
> > +	if (g_mode == DMODE_EXEC) {
> >   		if (dtrace_handle_err(g_dtp, &errhandler, NULL) == -1)
> >   			dfatal("failed to establish error handler");
> >   
> 
> 
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