linux-kernel - [PATCH v3 1/1] tracing/kprobes: Return EADDRNOTAVAIL when func matches several symbols

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Date:   Thu, 24 Aug 2023 18:08:59 +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: [PATCH v3 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 called, because
the effectively called one has no kprobes attached.

So, this commit returns EADDRNOTAVAIL when func matches several symbols.
This way, user needs to use address 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 | 61 +++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 23dba01831f7..2f393739e8cf 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 number_of_same_symbols(char *func_name)
+{
+	unsigned int count;
+
+	count = 0;
+	kallsyms_on_each_match_symbol(count_symbols, func_name, &count);
+
+	return count;
+}
+
 static int __trace_kprobe_create(int argc, const char *argv[])
 {
 	/*
@@ -836,6 +855,29 @@ static int __trace_kprobe_create(int argc, const char *argv[])
 		}
 	}
 
+	if (symbol) {
+		unsigned int count;
+
+		count = number_of_same_symbols(symbol);
+		if (count > 1) {
+			/*
+			 * Users should use ADDR to remove the ambiguity of
+			 * using KSYM only.
+			 */
+			ret = -EADDRNOTAVAIL;
+
+			goto error;
+		} else if (count == 0) {
+			/*
+			 * We can return ENOENT earlier than when register the
+			 * kprobe.
+			 */
+			ret = -ENOENT;
+
+			goto error;
+		}
+	}
+
 	trace_probe_log_set_index(0);
 	if (event) {
 		ret = traceprobe_parse_event_name(&event, &group, gbuf,
@@ -1699,6 +1741,7 @@ 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 +1752,24 @@ create_local_trace_kprobe(char *func, void *addr, unsigned long offs,
 	int ret;
 	char *event;
 
+	if (func) {
+		unsigned int count;
+
+		count = number_of_same_symbols(func);
+		if (count > 1)
+			/*
+			 * Users should use addr to remove the ambiguity of
+			 * using func only.
+			 */
+			return ERR_PTR(-EADDRNOTAVAIL);
+		else if (count == 0)
+			/*
+			 * We can return ENOENT earlier than when register the
+			 * kprobe.
+			 */
+			return ERR_PTR(-ENOENT);
+	}
+
 	/*
 	 * 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