Message-Id: <20250708072140.945296-1-dongml2@chinatelecom.cn>
Date: Tue,  8 Jul 2025 15:21:40 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: ast@...nel.org,
	daniel@...earbox.net
Cc: john.fastabend@...il.com,
	andrii@...nel.org,
	martin.lau@...ux.dev,
	eddyz87@...il.com,
	song@...nel.org,
	yonghong.song@...ux.dev,
	kpsingh@...nel.org,
	sdf@...ichev.me,
	haoluo@...gle.com,
	jolsa@...nel.org,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Menglong Dong <dongml2@...natelecom.cn>
Subject: [PATCH bpf-next v2] bpf: make the attach target more accurate

For now, we lookup the address of the attach target in
bpf_check_attach_target() with find_kallsyms_symbol_value or
kallsyms_lookup_name, which is not accurate in some cases.

For example, we want to attach to the target "t_next", but there are
multiple symbols with the name "t_next" exist in the kallsyms. The one
that kallsyms_lookup_name() returned may have no ftrace record, which
makes the attach target not available. So we want the one that has ftrace
record to be returned.

Meanwhile, there may be multiple symbols with the name "t_next" in ftrace
record. In this case, the attach target is ambiguous, so the attach should
fail.

Introduce the function bpf_lookup_attach_addr() to do the address lookup,
which is able to solve this problem.

Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
---
v2:
- Lookup both vmlinux and modules symbols when mod is NULL, just like
  kallsyms_lookup_name().

  If the btf is not a modules, shouldn't we lookup on the vmlinux only?
  I'm not sure if we should keep the same logic with
  kallsyms_lookup_name().

- Return the kernel symbol that don't have ftrace location if the symbols
  with ftrace location are not available
---
 kernel/bpf/verifier.c | 77 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 72 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 53007182b46b..4bacd0abf207 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -23476,6 +23476,73 @@ static int check_non_sleepable_error_inject(u32 btf_id)
 	return btf_id_set_contains(&btf_non_sleepable_error_inject, btf_id);
 }
 
+struct symbol_lookup_ctx {
+	const char *name;
+	unsigned long addr;
+	bool ftrace_addr;
+};
+
+static int symbol_callback(void *data, unsigned long addr)
+{
+	struct symbol_lookup_ctx *ctx = data;
+
+	ctx->addr = addr;
+	if (!ftrace_location(addr))
+		return 0;
+
+	if (ctx->ftrace_addr)
+		return -EADDRNOTAVAIL;
+	ctx->ftrace_addr = true;
+
+	return 0;
+}
+
+static int symbol_mod_callback(void *data, const char *name, unsigned long addr)
+{
+	if (strcmp(((struct symbol_lookup_ctx *)data)->name, name) != 0)
+		return 0;
+
+	return symbol_callback(data, addr);
+}
+
+/**
+ * bpf_lookup_attach_addr: Lookup address for a symbol
+ *
+ * @mod: kernel module to lookup the symbol, NULL means to lookup both vmlinux
+ * and modules symbols
+ * @sym: the symbol to resolve
+ * @addr: pointer to store the result
+ *
+ * Lookup the address of the symbol @sym. If multiple symbols with the name
+ * @sym exist, the one that has ftrace location is preferred. If more
+ * than 1 has ftrace location, -EADDRNOTAVAIL will be returned.
+ *
+ * Returns: 0 on success, -errno otherwise.
+ */
+static int bpf_lookup_attach_addr(const struct module *mod, const char *sym,
+				  unsigned long *addr)
+{
+	struct symbol_lookup_ctx ctx = { .addr = 0, .name = sym };
+	const char *mod_name = NULL;
+	int err = 0;
+
+#ifdef CONFIG_MODULES
+	mod_name = mod ? mod->name : NULL;
+#endif
+	if (!mod_name)
+		err = kallsyms_on_each_match_symbol(symbol_callback, sym, &ctx);
+
+	if (!err && !ctx.addr)
+		err = module_kallsyms_on_each_symbol(mod_name, symbol_mod_callback,
+						     &ctx);
+
+	if (!ctx.addr)
+		err = -ENOENT;
+	*addr = err ? 0 : ctx.addr;
+
+	return err;
+}
+
 int bpf_check_attach_target(struct bpf_verifier_log *log,
 			    const struct bpf_prog *prog,
 			    const struct bpf_prog *tgt_prog,
@@ -23729,18 +23796,18 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 			if (btf_is_module(btf)) {
 				mod = btf_try_get_module(btf);
 				if (mod)
-					addr = find_kallsyms_symbol_value(mod, tname);
+					ret = bpf_lookup_attach_addr(mod, tname, &addr);
 				else
-					addr = 0;
+					ret = -ENOENT;
 			} else {
-				addr = kallsyms_lookup_name(tname);
+				ret = bpf_lookup_attach_addr(NULL, tname, &addr);
 			}
-			if (!addr) {
+			if (ret) {
 				module_put(mod);
 				bpf_log(log,
 					"The address of function %s cannot be found\n",
 					tname);
-				return -ENOENT;
+				return ret;
 			}
 		}
 
-- 
2.39.5

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