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

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Date:   Wed, 17 Jun 2020 16:08:17 +0200
From:   Sascha Ortmann <sascha.ortmann@...d.uni-hannover.de>
To:     linux-kernel@...r.kernel.org
Cc:     rostedt@...dmis.org, mingo@...hat.com,
        linux-trace-devel@...r.kernel.org,
        Sascha Ortmann <sascha.ortmann@...d.uni-hannover.de>,
        linux-kernel@...cs.fau.de,
        Maximilian Werner <maximilian.werner96@...il.com>
Subject: [PATCH] tracing/boottime: Fix kprobe multiple events

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>
---
 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;
-- 
2.17.1