linux-kernel - [PATCH v6 2/2] bpftool: Add CET-aware symbol matching for x86

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Message-Id: <20250815025227.6204-3-chenyuan_fl@163.com>
Date: Fri, 15 Aug 2025 03:52:27 +0100
From: chenyuan_fl@....com
To: yonghong.song@...ux.dev,
	olsajiri@...il.com
Cc: aef2617b-ce03-4830-96a7-39df0c93aaad@...nel.org,
	andrii@...nel.org,
	ast@...nel.org,
	bpf@...r.kernel.org,
	chenyuan@...inos.cn,
	daniel@...earbox.net,
	linux-kernel@...r.kernel.org,
	qmo@...nel.org
Subject: [PATCH v6 2/2] bpftool: Add CET-aware symbol matching for x86_64 architectures

From: Yuan Chen <chenyuan@...inos.cn>

Adjust symbol matching logic to account for Control-flow Enforcement
Technology (CET) on x86_64 systems. CET prefixes functions with
a 4-byte 'endbr' instruction, shifting the actual hook entry point to
symbol + 4.

Signed-off-by: Yuan Chen <chenyuan@...inos.cn>
---
 tools/bpf/bpftool/link.c | 50 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 48 insertions(+), 2 deletions(-)

diff --git a/tools/bpf/bpftool/link.c b/tools/bpf/bpftool/link.c
index a773e05d5ade..6787971d3167 100644
--- a/tools/bpf/bpftool/link.c
+++ b/tools/bpf/bpftool/link.c
@@ -282,11 +282,52 @@ get_addr_cookie_array(__u64 *addrs, __u64 *cookies, __u32 count)
 	return data;
 }
 
+static bool is_x86_ibt_enabled(void)
+{
+#if defined(__x86_64__)
+	struct kernel_config_option options[] = {
+		{ "CONFIG_X86_KERNEL_IBT", },
+	};
+	char *values[ARRAY_SIZE(options)] = { };
+	bool ret;
+
+	if (read_kernel_config(options, ARRAY_SIZE(options), values, NULL))
+		return false;
+
+	ret = !!values[0];
+	free(values[0]);
+	return ret;
+#else
+	return false;
+#endif
+}
+
+static bool
+symbol_matches_target(__u64 sym_addr, __u64 target_addr, bool is_ibt_enabled)
+{
+	if (sym_addr == target_addr)
+		return true;
+
+	/*
+	 * On x86_64 architectures with CET (Control-flow Enforcement Technology),
+	 * function entry points have a 4-byte 'endbr' instruction prefix.
+	 * This causes kprobe hooks to target the address *after* 'endbr'
+	 * (symbol address + 4), preserving the CET instruction.
+	 * Here we check if the symbol address matches the hook target address
+	 * minus 4, indicating a CET-enabled function entry point.
+	 */
+	if (is_ibt_enabled && sym_addr == target_addr - 4)
+		return true;
+
+	return false;
+}
+
 static void
 show_kprobe_multi_json(struct bpf_link_info *info, json_writer_t *wtr)
 {
 	struct addr_cookie *data;
 	__u32 i, j = 0;
+	bool is_ibt_enabled;
 
 	jsonw_bool_field(json_wtr, "retprobe",
 			 info->kprobe_multi.flags & BPF_F_KPROBE_MULTI_RETURN);
@@ -306,8 +347,10 @@ show_kprobe_multi_json(struct bpf_link_info *info, json_writer_t *wtr)
 	if (!dd.sym_count)
 		goto error;
 
+	is_ibt_enabled = is_x86_ibt_enabled();
 	for (i = 0; i < dd.sym_count; i++) {
-		if (dd.sym_mapping[i].address != data[j].addr)
+		if (!symbol_matches_target(dd.sym_mapping[i].address,
+					   data[j].addr, is_ibt_enabled))
 			continue;
 		jsonw_start_object(json_wtr);
 		jsonw_uint_field(json_wtr, "addr", dd.sym_mapping[i].address);
@@ -719,6 +762,7 @@ static void show_kprobe_multi_plain(struct bpf_link_info *info)
 {
 	struct addr_cookie *data;
 	__u32 i, j = 0;
+	bool is_ibt_enabled;
 
 	if (!info->kprobe_multi.count)
 		return;
@@ -742,9 +786,11 @@ static void show_kprobe_multi_plain(struct bpf_link_info *info)
 	if (!dd.sym_count)
 		goto error;
 
+	is_ibt_enabled = is_x86_ibt_enabled();
 	printf("\n\t%-16s %-16s %s", "addr", "cookie", "func [module]");
 	for (i = 0; i < dd.sym_count; i++) {
-		if (dd.sym_mapping[i].address != data[j].addr)
+		if (!symbol_matches_target(dd.sym_mapping[i].address,
+					   data[j].addr, is_ibt_enabled))
 			continue;
 		printf("\n\t%016lx %-16llx %s",
 		       dd.sym_mapping[i].address, data[j].cookie, dd.sym_mapping[i].name);
-- 
2.39.5