Merge branch 'bpf-fix-tracing-of-kfuncs-with-implicit-args'

Ihor Solodrai says:

====================
bpf: Fix tracing of kfuncs with implicit args

Tejun reported an issue where a BPF program tracing a kfunc with
KF_IMPLICIT_ARGS can crash the kernel [1]. This is caused by a bug in
bpf_check_attach_target(): the btf_func_model for such a kfunc is
computed from a wrong BTF prototype. For more details see the commit
message of patch #1.

The second patch adds a selftest that can catch this situation.

The fix is a candidate for 7.1 backport.

[1] https://github.com/sched-ext/scx/issues/3687#issuecomment-4906694106
---

v2->v3:
  * Replace btf_kfunc_accumulated_flags() with btf_kfunc_check_flag()
    following a discussion with Eduard. Inlining the hook walk is a
    worse option than a helper, because BTF_KFUNC_HOOK_MAX and co are
    internal to btf.c and exposing them is uglier.
  * remove reduntant btf_is_func check (Jiri)
  * formatting nit (Eduard)
v2: https://lore.kernel.org/bpf/20260710192940.3020280-1-ihor.solodrai@linux.dev/

v1->v2:
  * Take a module reference in btf_attach_func_proto() around the
    btf_kfunc_accumulated_flags() call (sashiko)

v1: https://lore.kernel.org/bpf/20260710005902.2234832-1-ihor.solodrai@linux.dev/

---
====================

Link: https://patch.msgid.link/20260713235223.1639022-1-ihor.solodrai@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

include/linux/btf.h

@@ -578,6 +578,7 @@ const char *btf_str_by_offset(const struct btf *btf, u32 offset);
 struct btf *btf_parse_vmlinux(void);
 struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog);
 u32 *btf_kfunc_flags(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog);
+int btf_kfunc_check_flag(const struct btf *btf, u32 kfunc_btf_id, u32 flag);
 bool btf_kfunc_is_allowed(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_prog *prog);
 u32 *btf_kfunc_is_modify_return(const struct btf *btf, u32 kfunc_btf_id,
 				const struct bpf_prog *prog);

kernel/bpf/btf.c

@@ -9114,6 +9114,35 @@ u32 *btf_kfunc_flags(const struct btf *btf, u32 kfunc_btf_id, const struct bpf_p
 	return btf_kfunc_id_set_contains(btf, hook, kfunc_btf_id);
 }
 
+/*
+ * Check a single KF_* @flag on a kfunc across all of its hook sets.
+ * Returns:
+ *   * 1 if @flag is set
+ *   * 0 if @flag is not set
+ *   * -EINVAL if @flag is set inconsistently across the sets
+ *   * -ENOENT if kfunc_btf_id is not a registered kfunc
+ */
+int btf_kfunc_check_flag(const struct btf *btf, u32 kfunc_btf_id, u32 flag)
+{
+	enum btf_kfunc_hook hook;
+	int res = -ENOENT;
+	bool is_set;
+	u32 *flags;
+
+	for (hook = 0; hook < BTF_KFUNC_HOOK_MAX; hook++) {
+		flags = btf_kfunc_id_set_contains(btf, hook, kfunc_btf_id);
+		if (!flags)
+			continue;
+		is_set = *flags & flag;
+		if (res < 0)
+			res = is_set;
+		else if (res != is_set)
+			return -EINVAL;
+	}
+
+	return res;
+}
+
 u32 *btf_kfunc_is_modify_return(const struct btf *btf, u32 kfunc_btf_id,
 				const struct bpf_prog *prog)
 {

kernel/bpf/verifier.c

@@ -2584,24 +2584,25 @@ static struct btf *find_kfunc_desc_btf(struct bpf_verifier_env *env, s16 offset)
 
 #define KF_IMPL_SUFFIX "_impl"
 
-static const struct btf_type *find_kfunc_impl_proto(struct bpf_verifier_env *env,
+static const struct btf_type *find_kfunc_impl_proto(struct bpf_verifier_log *log,
 						    struct btf *btf,
 						    const char *func_name)
 {
-	char *buf = env->tmp_str_buf;
 	const struct btf_type *func;
+	char buf[KSYM_NAME_LEN];
 	s32 impl_id;
 	int len;
 
-	len = snprintf(buf, TMP_STR_BUF_LEN, "%s%s", func_name, KF_IMPL_SUFFIX);
-	if (len < 0 || len >= TMP_STR_BUF_LEN) {
-		verbose(env, "function name %s%s is too long\n", func_name, KF_IMPL_SUFFIX);
+	len = snprintf(buf, sizeof(buf), "%s%s", func_name, KF_IMPL_SUFFIX);
+	if (len < 0 || len >= sizeof(buf)) {
+		bpf_log(log, "function name %s%s is too long\n",
+			func_name, KF_IMPL_SUFFIX);
 		return NULL;
 	}
 
 	impl_id = btf_find_by_name_kind(btf, buf, BTF_KIND_FUNC);
 	if (impl_id <= 0) {
-		verbose(env, "cannot find function %s in BTF\n", buf);
+		bpf_log(log, "cannot find function %s in BTF\n", buf);
 		return NULL;
 	}
 
@@ -2653,7 +2654,7 @@ static int fetch_kfunc_meta(struct bpf_verifier_env *env,
 	 * can be found through the counterpart _impl kfunc.
 	 */
 	if (kfunc_flags && (*kfunc_flags & KF_IMPLICIT_ARGS))
-		func_proto = find_kfunc_impl_proto(env, btf, func_name);
+		func_proto = find_kfunc_impl_proto(&env->log, btf, func_name);
 	else
 		func_proto = btf_type_by_id(btf, func->type);
 
@@ -18880,6 +18881,47 @@ static int btf_id_allow_sleepable(u32 btf_id, unsigned long addr, const struct b
 	return -EINVAL;
 }
 
+/*
+ * Resolve the prototype describing a trace target's real ABI. A
+ * KF_IMPLICIT_ARGS kfunc has its injected args stripped from the public
+ * prototype, so use the _impl prototype; other targets use their own.
+ */
+static const struct btf_type *
+btf_attach_func_proto(struct bpf_verifier_log *log, struct btf *btf, u32 func_id)
+{
+	const struct btf_type *func;
+	struct module *mod = NULL;
+	const char *name;
+	int implicit;
+
+	func = btf_type_by_id(btf, func_id);
+	if (!func || !btf_type_is_func(func))
+		return NULL;
+	name = btf_name_by_offset(btf, func->name_off);
+
+	/*
+	 * btf_kfunc_check_flag() reads kfunc_set_tab, which for a module is
+	 * stable only once it is live; hold a module ref across the read to
+	 * exclude a concurrent module load.
+	 */
+	if (btf_is_module(btf)) {
+		mod = btf_try_get_module(btf);
+		if (!mod)
+			return NULL;
+	}
+	implicit = btf_kfunc_check_flag(btf, func_id, KF_IMPLICIT_ARGS);
+	module_put(mod);
+
+	if (implicit == -EINVAL) {
+		bpf_log(log, "kfunc %s has inconsistent KF_IMPLICIT_ARGS\n", name);
+		return NULL;
+	}
+	if (implicit > 0)
+		return find_kfunc_impl_proto(log, btf, name);
+
+	return btf_type_by_id(btf, func->type);
+}
+
 int bpf_check_attach_target(struct bpf_verifier_log *log,
 			    const struct bpf_prog *prog,
 			    const struct bpf_prog *tgt_prog,
@@ -19128,8 +19170,8 @@ int bpf_check_attach_target(struct bpf_verifier_log *log,
 		if (prog_extension &&
 		    btf_check_type_match(log, prog, btf, t))
 			return -EINVAL;
-		t = btf_type_by_id(btf, t->type);
-		if (!btf_type_is_func_proto(t))
+		t = btf_attach_func_proto(log, btf, btf_id);
+		if (!t || !btf_type_is_func_proto(t))
 			return -EINVAL;
 
 		if ((prog->aux->saved_dst_prog_type || prog->aux->saved_dst_attach_type) &&
@@ -19412,10 +19454,8 @@ int bpf_check_attach_btf_id_multi(struct btf *btf, struct bpf_prog *prog, u32 bt
 	tname = btf_name_by_offset(btf, t->name_off);
 	if (!tname)
 		return -EINVAL;
-	if (!btf_type_is_func(t))
-		return -EINVAL;
-	t = btf_type_by_id(btf, t->type);
-	if (!btf_type_is_func_proto(t))
+	t = btf_attach_func_proto(NULL, btf, btf_id);
+	if (!t || !btf_type_is_func_proto(t))
 		return -EINVAL;
 	err = btf_distill_func_proto(NULL, btf, t, tname, &tgt_info->fmodel);
 	if (err < 0)

tools/testing/selftests/bpf/prog_tests/kfunc_implicit_args_tracing.c

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include "kfunc_implicit_args_tracing.skel.h"
+
+void test_kfunc_implicit_args_tracing(void)
+{
+	struct kfunc_implicit_args_tracing *skel;
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+	int err, fd;
+
+	skel = kfunc_implicit_args_tracing__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "open_and_load"))
+		return;
+
+	err = kfunc_implicit_args_tracing__attach(skel);
+	if (!ASSERT_OK(err, "attach"))
+		goto cleanup;
+
+	fd = bpf_program__fd(skel->progs.trigger_implicit_arg);
+	err = bpf_prog_test_run_opts(fd, &topts);
+	if (!ASSERT_OK(err, "test_run"))
+		goto cleanup;
+
+	ASSERT_EQ(topts.retval, 5, "kfunc_retval");
+	ASSERT_EQ(skel->bss->fentry_arg_cnt, 2, "fentry_arg_cnt");
+	ASSERT_NEQ(skel->bss->fentry_aux_arg, 0, "fentry_aux_arg");
+	ASSERT_EQ(skel->bss->fentry_result, 1, "fentry_result");
+	ASSERT_EQ(skel->bss->fexit_arg_cnt, 2, "fexit_arg_cnt");
+	ASSERT_NEQ(skel->bss->fexit_aux_arg, 0, "fexit_aux_arg");
+	ASSERT_EQ(skel->bss->fexit_result, 1, "fexit_result");
+
+cleanup:
+	kfunc_implicit_args_tracing__destroy(skel);
+}

tools/testing/selftests/bpf/progs/kfunc_implicit_args_tracing.c

@@ -0,0 +1,77 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2026 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <errno.h>
+
+extern int bpf_kfunc_implicit_arg(int a) __weak __ksym;
+
+char _license[] SEC("license") = "GPL";
+
+/* Shared arg checks; reports arg count and aux, returns 1 on success. */
+static __always_inline __u64
+check_implicit_args(void *ctx, __u64 *arg_cnt, __u64 *aux_arg)
+{
+	__u64 a = 0, aux = 0, z = 0;
+	__u64 result;
+	__s64 err;
+
+	*arg_cnt = bpf_get_func_arg_cnt(ctx);
+	result = *arg_cnt == 2;
+
+	err = bpf_get_func_arg(ctx, 0, &a);
+	result &= err == 0 && (int)a == 5;
+
+	err = bpf_get_func_arg(ctx, 1, &aux);
+	*aux_arg = aux;
+	result &= err == 0 && aux != 0;
+
+	err = bpf_get_func_arg(ctx, 2, &z);
+	result &= err == -EINVAL;
+
+	return result;
+}
+
+__u64 fentry_result;
+__u64 fentry_arg_cnt;
+__u64 fentry_aux_arg;
+
+SEC("fentry/bpf_kfunc_implicit_arg")
+int BPF_PROG(trace_implicit_arg_fentry)
+{
+	__u64 ret = 0;
+	__s64 err;
+
+	fentry_result = check_implicit_args(ctx, &fentry_arg_cnt, &fentry_aux_arg);
+
+	err = bpf_get_func_ret(ctx, &ret);
+	fentry_result &= err == -EOPNOTSUPP;
+
+	return 0;
+}
+
+__u64 fexit_result;
+__u64 fexit_arg_cnt;
+__u64 fexit_aux_arg;
+
+SEC("fexit/bpf_kfunc_implicit_arg")
+int BPF_PROG(trace_implicit_arg_fexit)
+{
+	__u64 ret = 0;
+	__s64 err;
+
+	fexit_result = check_implicit_args(ctx, &fexit_arg_cnt, &fexit_aux_arg);
+
+	err = bpf_get_func_ret(ctx, &ret);
+	fexit_result &= err == 0 && ret == 5;
+
+	return 0;
+}
+
+SEC("syscall")
+int trigger_implicit_arg(void *ctx)
+{
+	return bpf_kfunc_implicit_arg(5);
+}