linux-kernel - Re: [PATCH] tracing/boottime: Fix kprobe multiple events

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Date:   Wed, 17 Jun 2020 11:06:17 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sascha Ortmann <sascha.ortmann@...d.uni-hannover.de>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        linux-trace-devel@...r.kernel.org, linux-kernel@...cs.fau.de,
        Maximilian Werner <maximilian.werner96@...il.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH] tracing/boottime: Fix kprobe multiple events

On Wed, 17 Jun 2020 11:05:21 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 17 Jun 2020 16:08:17 +0200
> Sascha Ortmann <sascha.ortmann@...d.uni-hannover.de> wrote:
> 
> > Fix boottime kprobe events to add multiple events even if one fails
> > and report probe generation failures.
> > 
> > As an example, when we try to set multiprobe kprobe events in
> > bootconfig like this:
> > 
> > ftrace.event.kprobes.vfsevents {
> > 	probes = "vfs_read $arg1 $arg2,,
> >                  !error! not reported;?", // leads to error
> > 		 "vfs_write $arg1 $arg2"
> > }
> > 
> > this will not work like expected. After commit
> > da0f1f4167e3af69e1d8b32d6d65195ddd2bfb64 ("tracing/boottime:
> > Fix kprobe event API usage"), the function
> > trace_boot_add_kprobe_event will not produce any error message,
> > aborting the function and stopping subsequent probes from getting
> > installed when adding a probe fails at kprobe_event_gen_cmd_start.
> > Furthermore, probes continue when kprobe_event_gen_cmd_end fails
> > (and kprobe_event_gen_cmd_start did not fail). In this case the
> > function even returns successfully when the last call to
> > kprobe_event_gen_cmd_end is successful.
> > 
> > The behaviour of reporting and aborting after failures is not
> > consistent.
> > 
> > The function trace_boot_add_kprobe_event now continues even when
> > one of the multiple events fails. Each failure is now reported
> > individually. Since the function can only return one result to the
> > caller, the function returns now the last failure (or none, if
> > nothing fails).
> > 
> > Cc: linux-kernel@...cs.fau.de
> > Signed-off-by: Maximilian Werner <maximilian.werner96@...il.com>
> > Signed-off-by: Sascha Ortmann <sascha.ortmann@...d.uni-hannover.de>  
> 
> Why the double signed off by?
> 
> Masami, I'm fine with this, but needs your review.

[ It appears that Masami wasn't in the Cc ]


> 
> -- Steve
> 
> > ---
> >  kernel/trace/trace_boot.c | 16 +++++++++++-----
> >  1 file changed, 11 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/trace/trace_boot.c b/kernel/trace/trace_boot.c
> > index 9de29bb45a27..dbb50184e060 100644
> > --- a/kernel/trace/trace_boot.c
> > +++ b/kernel/trace/trace_boot.c
> > @@ -95,18 +95,24 @@ trace_boot_add_kprobe_event(struct xbc_node *node, const char *event)
> >  	struct xbc_node *anode;
> >  	char buf[MAX_BUF_LEN];
> >  	const char *val;
> > +	int error = 0;
> >  	int ret = 0;
> >  
> >  	xbc_node_for_each_array_value(node, "probes", anode, val) {
> >  		kprobe_event_cmd_init(&cmd, buf, MAX_BUF_LEN);
> >  
> > -		ret = kprobe_event_gen_cmd_start(&cmd, event, val);
> > -		if (ret)
> > -			break;
> > +		error = kprobe_event_gen_cmd_start(&cmd, event, val);
> > +		if (error) {
> > +			pr_err("Failed to generate probe: %s\n", buf);
> > +			ret = error;
> > +			continue;
> > +		}
> >  
> > -		ret = kprobe_event_gen_cmd_end(&cmd);
> > -		if (ret)
> > +		error = kprobe_event_gen_cmd_end(&cmd);
> > +		if (error) {
> >  			pr_err("Failed to add probe: %s\n", buf);
> > +			ret = error;
> > +		}
> >  	}
> >  
> >  	return ret;  
>