linux-kernel - [RFC PATCH v2 1/1] tracing/kprobes: Return EADDRNOTAVAIL when func matches several symbols

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Date:   Thu, 24 Aug 2023 12:37:34 +0200
From:   Francis Laniel <flaniel@...ux.microsoft.com>
To:     linux-kernel@...r.kernel.org
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        linux-trace-kernel@...r.kernel.org,
        Francis Laniel <flaniel@...ux.microsoft.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [RFC PATCH v2 1/1] tracing/kprobes: Return EADDRNOTAVAIL when func matches several symbols

Previously to this commit, if func matches several symbols, a kprobe, being
either sysfs or PMU, would only be installed for the first matching address.
This could lead to some misunderstanding when some BPF code was never called
because it was attached to a function which was indeed not call, because the
effectively called one has no kprobes.

So, this commit returns EADDRNOTAVAIL when func matches several symbols.
This way, user needs to use addr to remove the ambiguity.

Suggested-by: Masami Hiramatsu <mhiramat@...nel.org>
Signed-off-by: Francis Laniel <flaniel@...ux.microsoft.com>
Link: https://lore.kernel.org/lkml/20230819101105.b0c104ae4494a7d1f2eea742@kernel.org/
---
 kernel/trace/trace_kprobe.c | 42 +++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 23dba01831f7..0c8dd6ba650b 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -705,6 +705,25 @@ static struct notifier_block trace_kprobe_module_nb = {
 	.priority = 1	/* Invoked after kprobe module callback */
 };
 
+static int count_symbols(void *data, unsigned long unused)
+{
+	unsigned int *count = data;
+
+	(*count)++;
+
+	return 0;
+}
+
+static unsigned int func_name_several_symbols(char *func_name)
+{
+	unsigned int count;
+
+	count = 0;
+	kallsyms_on_each_match_symbol(count_symbols, func_name, &count);
+
+	return count > 1;
+}
+
 static int __trace_kprobe_create(int argc, const char *argv[])
 {
 	/*
@@ -836,6 +855,18 @@ static int __trace_kprobe_create(int argc, const char *argv[])
 		}
 	}
 
+	/*
+	 * If user specifies KSYM, we check it does not correspond to several
+	 * symbols.
+	 * If this is the case, we return EADDRNOTAVAIL to indicate the user
+	 * he/she should use ADDR rather than KSYM to remove the ambiguity.
+	 */
+	if (symbol && func_name_several_symbols(symbol)) {
+		ret = -EADDRNOTAVAIL;
+
+		goto error;
+	}
+
 	trace_probe_log_set_index(0);
 	if (event) {
 		ret = traceprobe_parse_event_name(&event, &group, gbuf,
@@ -1699,6 +1730,7 @@ static int unregister_kprobe_event(struct trace_kprobe *tk)
 }
 
 #ifdef CONFIG_PERF_EVENTS
+
 /* create a trace_kprobe, but don't add it to global lists */
 struct trace_event_call *
 create_local_trace_kprobe(char *func, void *addr, unsigned long offs,
@@ -1709,6 +1741,16 @@ create_local_trace_kprobe(char *func, void *addr, unsigned long offs,
 	int ret;
 	char *event;
 
+	/*
+	 * If user specifies func, we check that function name does not
+	 * correspond to several symbols.
+	 * If this is the case, we return EADDRNOTAVAIL to indicate the user
+	 * he/she should use addr and offs rather than func to remove the
+	 * ambiguity.
+	 */
+	if (func && func_name_several_symbols(func))
+		return ERR_PTR(-EADDRNOTAVAIL);
+
 	/*
 	 * local trace_kprobes are not added to dyn_event, so they are never
 	 * searched in find_trace_kprobe(). Therefore, there is no concern of
-- 
2.34.1