[PATCH bpf-next v2 4/6] bpf: Use KF_DEPRECATED to emit verifier warnings

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

As documented in kfuncs.rst, introduce a KF_DEPRECATED annotation for
kfuncs and apply it to all existing impl-suffixed kfuncs that are
preserved for backwards compatibility for a few kernel releases, after
we introduced versions with KF_IMPLICIT_ARGS.

Use the kfunc flag to decide whether to emit warnings to the verifier
log or not. Be helpful and point out the file:line in addition to the
instruction when the information is available while emitting the
deprecation warning.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/btf.h                           |  1 +
 kernel/bpf/helpers.c                          | 16 ++++-----
 kernel/bpf/verifier.c                         | 34 +++++++++++++++++++
 .../selftests/bpf/test_kmods/bpf_testmod.c    |  2 +-
 4 files changed, 44 insertions(+), 9 deletions(-)

diff --git a/include/linux/btf.h b/include/linux/btf.h
index 48108471c5b1..86006a2d8b2a 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -79,6 +79,7 @@
 #define KF_ARENA_ARG1   (1 << 14) /* kfunc takes an arena pointer as its first argument */
 #define KF_ARENA_ARG2   (1 << 15) /* kfunc takes an arena pointer as its second argument */
 #define KF_IMPLICIT_ARGS (1 << 16) /* kfunc has implicit arguments supplied by the verifier */
+#define KF_DEPRECATED   (1 << 17) /* kfunc is deprecated and should warn when used */
 
 /*
  * Tag marking a kernel function as a kfunc. This is meant to minimize the
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index bb95e287b0dc..190c0b382f11 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -4685,19 +4685,19 @@ BTF_KFUNCS_START(generic_btf_ids)
 BTF_ID_FLAGS(func, crash_kexec, KF_DESTRUCTIVE)
 #endif
 BTF_ID_FLAGS(func, bpf_obj_new, KF_ACQUIRE | KF_RET_NULL | KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_obj_new_impl, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_obj_new_impl, KF_ACQUIRE | KF_RET_NULL | KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_percpu_obj_new, KF_ACQUIRE | KF_RET_NULL | KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_percpu_obj_new_impl, KF_ACQUIRE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_percpu_obj_new_impl, KF_ACQUIRE | KF_RET_NULL | KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_obj_drop, KF_RELEASE | KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_obj_drop_impl, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_obj_drop_impl, KF_RELEASE | KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_percpu_obj_drop, KF_RELEASE | KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_percpu_obj_drop_impl, KF_RELEASE)
+BTF_ID_FLAGS(func, bpf_percpu_obj_drop_impl, KF_RELEASE | KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_refcount_acquire, KF_ACQUIRE | KF_RET_NULL | KF_RCU | KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_refcount_acquire_impl, KF_ACQUIRE | KF_RET_NULL | KF_RCU)
+BTF_ID_FLAGS(func, bpf_refcount_acquire_impl, KF_ACQUIRE | KF_RET_NULL | KF_RCU | KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_list_push_front, KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_list_push_front_impl)
+BTF_ID_FLAGS(func, bpf_list_push_front_impl, KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_list_push_back, KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_list_push_back_impl)
+BTF_ID_FLAGS(func, bpf_list_push_back_impl, KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_pop_back, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_front, KF_RET_NULL)
@@ -4706,7 +4706,7 @@ BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_rbtree_remove, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_rbtree_add, KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_rbtree_add_impl)
+BTF_ID_FLAGS(func, bpf_rbtree_add_impl, KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_rbtree_first, KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_rbtree_root, KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_rbtree_left, KF_RET_NULL)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 86d77d5f7f83..267ab3c36f6f 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3229,6 +3229,7 @@ struct bpf_kfunc_desc {
 	u32 func_id;
 	s32 imm;
 	u16 offset;
+	bool warned_deprecated;
 	unsigned long addr;
 };
 
@@ -12426,6 +12427,11 @@ static bool is_kfunc_destructive(struct bpf_kfunc_call_arg_meta *meta)
 	return meta->kfunc_flags & KF_DESTRUCTIVE;
 }
 
+static bool is_kfunc_deprecated(struct bpf_kfunc_call_arg_meta *meta)
+{
+	return meta->kfunc_flags & KF_DEPRECATED;
+}
+
 static bool is_kfunc_rcu(struct bpf_kfunc_call_arg_meta *meta)
 {
 	return meta->kfunc_flags & KF_RCU;
@@ -14583,6 +14589,32 @@ static int check_special_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_ca
 
 static int check_return_code(struct bpf_verifier_env *env, int regno, const char *reg_name);
 
+static void warn_for_deprecated_kfuncs(struct bpf_verifier_env *env,
+				       struct bpf_kfunc_call_arg_meta *meta,
+				       int insn_idx, s16 offset)
+{
+	const struct bpf_line_info *linfo;
+	struct bpf_kfunc_desc *desc;
+	const char *file;
+	int line_num;
+
+	desc = find_kfunc_desc(env->prog, meta->func_id, offset);
+	if (!desc || desc->warned_deprecated || !is_kfunc_deprecated(meta) || !env->prog->aux->btf)
+		return;
+
+	linfo = bpf_find_linfo(env->prog, insn_idx);
+	if (linfo) {
+		bpf_get_linfo_file_line(env->prog->aux->btf, linfo, &file, NULL, &line_num);
+		warn(env, "%s:%d (insn #%d) uses deprecated kfunc %s(), which will be removed.\n",
+		     file, line_num, insn_idx, meta->func_name);
+	} else {
+		warn(env, "(insn #%d) uses deprecated kfunc %s(), which will be removed.\n",
+		     insn_idx, meta->func_name);
+	}
+
+	desc->warned_deprecated = true;
+}
+
 static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 			    int *insn_idx_p)
 {
@@ -14612,6 +14644,8 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 
 	insn_aux->is_iter_next = is_iter_next_kfunc(&meta);
 
+	warn_for_deprecated_kfuncs(env, &meta, insn_idx, insn->off);
+
 	if (!insn->off &&
 	    (insn->imm == special_kfunc_list[KF_bpf_res_spin_lock] ||
 	     insn->imm == special_kfunc_list[KF_bpf_res_spin_lock_irqsave])) {
diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
index d876314a4d67..6ad7b2cdfd05 100644
--- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
@@ -1326,7 +1326,7 @@ BTF_ID_FLAGS(func, bpf_kfunc_multi_st_ops_test_1)
 BTF_ID_FLAGS(func, bpf_kfunc_multi_st_ops_test_1_assoc, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg, KF_IMPLICIT_ARGS)
 BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy, KF_IMPLICIT_ARGS)
-BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy_impl)
+BTF_ID_FLAGS(func, bpf_kfunc_implicit_arg_legacy_impl, KF_DEPRECATED)
 BTF_ID_FLAGS(func, bpf_kfunc_trigger_ctx_check)
 BTF_KFUNCS_END(bpf_testmod_check_kfunc_ids)
 
-- 
2.52.0