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SubjectRe: [PATCH 2/3] tracing/boottime: Fix kprobe event API usage
On Sat, 25 Apr 2020 10:00:20 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Sat, 25 Apr 2020 14:49:17 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
>
> > Fix boottime kprobe events to use API correctly for
> > multiple events.
> >
> > For example, when we set a multiprobe kprobe events in
> > bootconfig like below,
> >
> > ftrace.event.kprobes.myevent {
> > probes = "vfs_read $arg1 $arg2", "vfs_write $arg1 $arg2"
> > }
> >
> > This cause an error;
> >
> > trace_boot: Failed to add probe: p:kprobes/myevent (null) vfs_read $arg1 $arg2 vfs_write $arg1 $arg2
> >
> > This shows the 1st argument becomes NULL and multiprobes
> > are merged to 1 probe.
> >
> > Fixes: 29a154810546 ("tracing: Change trace_boot to use kprobe_event interface")
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: stable@vger.kernel.org
> > ---
> > kernel/trace/trace_boot.c | 20 ++++++++------------
> > 1 file changed, 8 insertions(+), 12 deletions(-)
> >
> > diff --git a/kernel/trace/trace_boot.c b/kernel/trace/trace_boot.c
> > index 06d7feb5255f..9de29bb45a27 100644
> > --- a/kernel/trace/trace_boot.c
> > +++ b/kernel/trace/trace_boot.c
> > @@ -95,24 +95,20 @@ 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 ret;
> > + int ret = 0;
> >
> > - kprobe_event_cmd_init(&cmd, buf, MAX_BUF_LEN);
> > + 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, NULL);
> > - if (ret)
> > - return ret;
> > + ret = kprobe_event_gen_cmd_start(&cmd, event, val);
> > + if (ret)
> > + break;
>
> Should we break here? What about just printing an error message and
> continuing to the next probe. If I start up something with a typo in
> the first element, I lose all events. But if I have a typo in the last
> one, I get all but that one. I rather have it just fail on the ones that
> don't parse properly.

This kprobe_event_gen_cmd_start() causes an error only if there is
a program bug or out of memory, because it never evaluate given probe
definition, but kprobe_event_gen_cmd_end() does. Thus I think this is
correct way to handle the error.

IOW, if you typo a probe, it will be handled by
kprobe_event_gen_cmd_end() and it shows an error message and continue
to process other probe definitions. See below,

> > - xbc_node_for_each_array_value(node, "probes", anode, val) {
> > - ret = kprobe_event_add_field(&cmd, val);
> > + ret = kprobe_event_gen_cmd_end(&cmd);
> > if (ret)
> > - return ret;
> > + pr_err("Failed to add probe: %s\n", buf);
> > }

This continues to next probe ;-)

Thank you,

> >
> > - ret = kprobe_event_gen_cmd_end(&cmd);
> > - if (ret)
> > - pr_err("Failed to add probe: %s\n", buf);
> > -
> > return ret;
> > }
> > #else
>


--
Masami Hiramatsu <mhiramat@kernel.org>

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