Merge branch 'bpf: Support calling kernel function'

Martin KaFai says:

====================

This series adds support to allow bpf program calling kernel function.

The use case included in this set is to allow bpf-tcp-cc to directly
call some tcp-cc helper functions (e.g. "tcp_cong_avoid_ai()").  Those
functions have already been used by some kernel tcp-cc implementations.

This set will also allow the bpf-tcp-cc program to directly call the
kernel tcp-cc implementation,  For example, a bpf_dctcp may only want to
implement its own dctcp_cwnd_event() and reuse other dctcp_*() directly
from the kernel tcp_dctcp.c instead of reimplementing (or
copy-and-pasting) them.

The tcp-cc kernel functions mentioned above will be white listed
for the struct_ops bpf-tcp-cc programs to use in a later patch.
The white listed functions are not bounded to a fixed ABI contract.
Those functions have already been used by the existing kernel tcp-cc.
If any of them has changed, both in-tree and out-of-tree kernel tcp-cc
implementations have to be changed.  The same goes for the struct_ops
bpf-tcp-cc programs which have to be adjusted accordingly.

Please see individual patch for details.

v2:
- Patch 2 in v1 is removed.  No need to support extern func in kernel.
  Changed libbpf to adjust the .ksyms datasec for extern func
  in patch 11. (Andrii)
- Name change: btf_check_func_arg_match() and btf_check_subprog_arg_match()
  in patch 2. (Andrii)
- Always set unreliable on any error in patch 2 since it does not
  matter. (Andrii)
- s/kern_func/kfunc/ and s/descriptor/desc/ in this set. (Andrii)
- Remove some unnecessary changes in disasm.h and disasm.c
  in patch 3.  In particular, no need to change the function
  signature in bpf_insn_revmap_call_t.  Also, removed the changes
  in print_bpf_insn().
- Fixed an issue in check_kfunc_call() when the calling kernel function
  returns a pointer in patch 3.  Added a selftest.
- Adjusted the verifier selftests due to the changes in the verifier log
  in patch 3.
- Fixed a comparison issue in kfunc_desc_cmp_by_imm() in patch 3. (Andrii)
- Name change: is_ldimm64_insn(),
  new helper: is_call_insn() in patch 10 (Andrii)
- Move btf_func_linkage() from btf.h to libbpf.c in patch 11. (Andrii)
- Fixed the linker error when CONFIG_BPF_SYSCALL is not defined.
  Moved the check_kfunc_call from filter.c to test_run.c in patch 14.
  (kernel test robot)
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 288 insertions, 101 deletions

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index cebb0e852cf8..7aad78dbb4b4 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -185,7 +185,8 @@ enum reloc_type {
 	RELO_LD64,
 	RELO_CALL,
 	RELO_DATA,
-	RELO_EXTERN,
+	RELO_EXTERN_VAR,
+	RELO_EXTERN_FUNC,
 	RELO_SUBPROG_ADDR,
 };
 
@@ -573,14 +574,19 @@ static bool insn_is_subprog_call(const struct bpf_insn *insn)
 	       insn->off == 0;
 }
 
-static bool is_ldimm64(struct bpf_insn *insn)
+static bool is_ldimm64_insn(struct bpf_insn *insn)
 {
 	return insn->code == (BPF_LD | BPF_IMM | BPF_DW);
 }
 
+static bool is_call_insn(const struct bpf_insn *insn)
+{
+	return insn->code == (BPF_JMP | BPF_CALL);
+}
+
 static bool insn_is_pseudo_func(struct bpf_insn *insn)
 {
-	return is_ldimm64(insn) && insn->src_reg == BPF_PSEUDO_FUNC;
+	return is_ldimm64_insn(insn) && insn->src_reg == BPF_PSEUDO_FUNC;
 }
 
 static int
@@ -1921,9 +1927,9 @@ resolve_func_ptr(const struct btf *btf, __u32 id, __u32 *res_id)
 	return btf_is_func_proto(t) ? t : NULL;
 }
 
-static const char *btf_kind_str(const struct btf_type *t)
+static const char *__btf_kind_str(__u16 kind)
 {
-	switch (btf_kind(t)) {
+	switch (kind) {
 	case BTF_KIND_UNKN: return "void";
 	case BTF_KIND_INT: return "int";
 	case BTF_KIND_PTR: return "ptr";
@@ -1945,6 +1951,16 @@ static const char *btf_kind_str(const struct btf_type *t)
 	}
 }
 
+static const char *btf_kind_str(const struct btf_type *t)
+{
+	return __btf_kind_str(btf_kind(t));
+}
+
+static enum btf_func_linkage btf_func_linkage(const struct btf_type *t)
+{
+	return (enum btf_func_linkage)BTF_INFO_VLEN(t->info);
+}
+
 /*
  * Fetch integer attribute of BTF map definition. Such attributes are
  * represented using a pointer to an array, in which dimensionality of array
@@ -3009,7 +3025,7 @@ static bool sym_is_subprog(const GElf_Sym *sym, int text_shndx)
 static int find_extern_btf_id(const struct btf *btf, const char *ext_name)
 {
 	const struct btf_type *t;
-	const char *var_name;
+	const char *tname;
 	int i, n;
 
 	if (!btf)
@@ -3019,14 +3035,18 @@ static int find_extern_btf_id(const struct btf *btf, const char *ext_name)
 	for (i = 1; i <= n; i++) {
 		t = btf__type_by_id(btf, i);
 
-		if (!btf_is_var(t))
+		if (!btf_is_var(t) && !btf_is_func(t))
 			continue;
 
-		var_name = btf__name_by_offset(btf, t->name_off);
-		if (strcmp(var_name, ext_name))
+		tname = btf__name_by_offset(btf, t->name_off);
+		if (strcmp(tname, ext_name))
 			continue;
 
-		if (btf_var(t)->linkage != BTF_VAR_GLOBAL_EXTERN)
+		if (btf_is_var(t) &&
+		    btf_var(t)->linkage != BTF_VAR_GLOBAL_EXTERN)
+			return -EINVAL;
+
+		if (btf_is_func(t) && btf_func_linkage(t) != BTF_FUNC_EXTERN)
 			return -EINVAL;
 
 		return i;
@@ -3139,12 +3159,48 @@ static int find_int_btf_id(const struct btf *btf)
 	return 0;
 }
 
+static int add_dummy_ksym_var(struct btf *btf)
+{
+	int i, int_btf_id, sec_btf_id, dummy_var_btf_id;
+	const struct btf_var_secinfo *vs;
+	const struct btf_type *sec;
+
+	sec_btf_id = btf__find_by_name_kind(btf, KSYMS_SEC,
+					    BTF_KIND_DATASEC);
+	if (sec_btf_id < 0)
+		return 0;
+
+	sec = btf__type_by_id(btf, sec_btf_id);
+	vs = btf_var_secinfos(sec);
+	for (i = 0; i < btf_vlen(sec); i++, vs++) {
+		const struct btf_type *vt;
+
+		vt = btf__type_by_id(btf, vs->type);
+		if (btf_is_func(vt))
+			break;
+	}
+
+	/* No func in ksyms sec.  No need to add dummy var. */
+	if (i == btf_vlen(sec))
+		return 0;
+
+	int_btf_id = find_int_btf_id(btf);
+	dummy_var_btf_id = btf__add_var(btf,
+					"dummy_ksym",
+					BTF_VAR_GLOBAL_ALLOCATED,
+					int_btf_id);
+	if (dummy_var_btf_id < 0)
+		pr_warn("cannot create a dummy_ksym var\n");
+
+	return dummy_var_btf_id;
+}
+
 static int bpf_object__collect_externs(struct bpf_object *obj)
 {
 	struct btf_type *sec, *kcfg_sec = NULL, *ksym_sec = NULL;
 	const struct btf_type *t;
 	struct extern_desc *ext;
-	int i, n, off;
+	int i, n, off, dummy_var_btf_id;
 	const char *ext_name, *sec_name;
 	Elf_Scn *scn;
 	GElf_Shdr sh;
@@ -3156,6 +3212,10 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 	if (elf_sec_hdr(obj, scn, &sh))
 		return -LIBBPF_ERRNO__FORMAT;
 
+	dummy_var_btf_id = add_dummy_ksym_var(obj->btf);
+	if (dummy_var_btf_id < 0)
+		return dummy_var_btf_id;
+
 	n = sh.sh_size / sh.sh_entsize;
 	pr_debug("looking for externs among %d symbols...\n", n);
 
@@ -3200,6 +3260,11 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 		sec_name = btf__name_by_offset(obj->btf, sec->name_off);
 
 		if (strcmp(sec_name, KCONFIG_SEC) == 0) {
+			if (btf_is_func(t)) {
+				pr_warn("extern function %s is unsupported under %s section\n",
+					ext->name, KCONFIG_SEC);
+				return -ENOTSUP;
+			}
 			kcfg_sec = sec;
 			ext->type = EXT_KCFG;
 			ext->kcfg.sz = btf__resolve_size(obj->btf, t->type);
@@ -3221,6 +3286,11 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 				return -ENOTSUP;
 			}
 		} else if (strcmp(sec_name, KSYMS_SEC) == 0) {
+			if (btf_is_func(t) && ext->is_weak) {
+				pr_warn("extern weak function %s is unsupported\n",
+					ext->name);
+				return -ENOTSUP;
+			}
 			ksym_sec = sec;
 			ext->type = EXT_KSYM;
 			skip_mods_and_typedefs(obj->btf, t->type,
@@ -3247,7 +3317,14 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 		 * extern variables in DATASEC
 		 */
 		int int_btf_id = find_int_btf_id(obj->btf);
+		/* For extern function, a dummy_var added earlier
+		 * will be used to replace the vs->type and
+		 * its name string will be used to refill
+		 * the missing param's name.
+		 */
+		const struct btf_type *dummy_var;
 
+		dummy_var = btf__type_by_id(obj->btf, dummy_var_btf_id);
 		for (i = 0; i < obj->nr_extern; i++) {
 			ext = &obj->externs[i];
 			if (ext->type != EXT_KSYM)
@@ -3266,12 +3343,32 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 			ext_name = btf__name_by_offset(obj->btf, vt->name_off);
 			ext = find_extern_by_name(obj, ext_name);
 			if (!ext) {
-				pr_warn("failed to find extern definition for BTF var '%s'\n",
-					ext_name);
+				pr_warn("failed to find extern definition for BTF %s '%s'\n",
+					btf_kind_str(vt), ext_name);
 				return -ESRCH;
 			}
-			btf_var(vt)->linkage = BTF_VAR_GLOBAL_ALLOCATED;
-			vt->type = int_btf_id;
+			if (btf_is_func(vt)) {
+				const struct btf_type *func_proto;
+				struct btf_param *param;
+				int j;
+
+				func_proto = btf__type_by_id(obj->btf,
+							     vt->type);
+				param = btf_params(func_proto);
+				/* Reuse the dummy_var string if the
+				 * func proto does not have param name.
+				 */
+				for (j = 0; j < btf_vlen(func_proto); j++)
+					if (param[j].type && !param[j].name_off)
+						param[j].name_off =
+							dummy_var->name_off;
+				vs->type = dummy_var_btf_id;
+				vt->info &= ~0xffff;
+				vt->info |= BTF_FUNC_GLOBAL;
+			} else {
+				btf_var(vt)->linkage = BTF_VAR_GLOBAL_ALLOCATED;
+				vt->type = int_btf_id;
+			}
 			vs->offset = off;
 			vs->size = sizeof(int);
 		}
@@ -3403,31 +3500,7 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
 
 	reloc_desc->processed = false;
 
-	/* sub-program call relocation */
-	if (insn->code == (BPF_JMP | BPF_CALL)) {
-		if (insn->src_reg != BPF_PSEUDO_CALL) {
-			pr_warn("prog '%s': incorrect bpf_call opcode\n", prog->name);
-			return -LIBBPF_ERRNO__RELOC;
-		}
-		/* text_shndx can be 0, if no default "main" program exists */
-		if (!shdr_idx || shdr_idx != obj->efile.text_shndx) {
-			sym_sec_name = elf_sec_name(obj, elf_sec_by_idx(obj, shdr_idx));
-			pr_warn("prog '%s': bad call relo against '%s' in section '%s'\n",
-				prog->name, sym_name, sym_sec_name);
-			return -LIBBPF_ERRNO__RELOC;
-		}
-		if (sym->st_value % BPF_INSN_SZ) {
-			pr_warn("prog '%s': bad call relo against '%s' at offset %zu\n",
-				prog->name, sym_name, (size_t)sym->st_value);
-			return -LIBBPF_ERRNO__RELOC;
-		}
-		reloc_desc->type = RELO_CALL;
-		reloc_desc->insn_idx = insn_idx;
-		reloc_desc->sym_off = sym->st_value;
-		return 0;
-	}
-
-	if (!is_ldimm64(insn)) {
+	if (!is_call_insn(insn) && !is_ldimm64_insn(insn)) {
 		pr_warn("prog '%s': invalid relo against '%s' for insns[%d].code 0x%x\n",
 			prog->name, sym_name, insn_idx, insn->code);
 		return -LIBBPF_ERRNO__RELOC;
@@ -3450,12 +3523,39 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
 		}
 		pr_debug("prog '%s': found extern #%d '%s' (sym %d) for insn #%u\n",
 			 prog->name, i, ext->name, ext->sym_idx, insn_idx);
-		reloc_desc->type = RELO_EXTERN;
+		if (insn->code == (BPF_JMP | BPF_CALL))
+			reloc_desc->type = RELO_EXTERN_FUNC;
+		else
+			reloc_desc->type = RELO_EXTERN_VAR;
 		reloc_desc->insn_idx = insn_idx;
 		reloc_desc->sym_off = i; /* sym_off stores extern index */
 		return 0;
 	}
 
+	/* sub-program call relocation */
+	if (is_call_insn(insn)) {
+		if (insn->src_reg != BPF_PSEUDO_CALL) {
+			pr_warn("prog '%s': incorrect bpf_call opcode\n", prog->name);
+			return -LIBBPF_ERRNO__RELOC;
+		}
+		/* text_shndx can be 0, if no default "main" program exists */
+		if (!shdr_idx || shdr_idx != obj->efile.text_shndx) {
+			sym_sec_name = elf_sec_name(obj, elf_sec_by_idx(obj, shdr_idx));
+			pr_warn("prog '%s': bad call relo against '%s' in section '%s'\n",
+				prog->name, sym_name, sym_sec_name);
+			return -LIBBPF_ERRNO__RELOC;
+		}
+		if (sym->st_value % BPF_INSN_SZ) {
+			pr_warn("prog '%s': bad call relo against '%s' at offset %zu\n",
+				prog->name, sym_name, (size_t)sym->st_value);
+			return -LIBBPF_ERRNO__RELOC;
+		}
+		reloc_desc->type = RELO_CALL;
+		reloc_desc->insn_idx = insn_idx;
+		reloc_desc->sym_off = sym->st_value;
+		return 0;
+	}
+
 	if (!shdr_idx || shdr_idx >= SHN_LORESERVE) {
 		pr_warn("prog '%s': invalid relo against '%s' in special section 0x%x; forgot to initialize global var?..\n",
 			prog->name, sym_name, shdr_idx);
@@ -5695,7 +5795,7 @@ poison:
 		/* poison second part of ldimm64 to avoid confusing error from
 		 * verifier about "unknown opcode 00"
 		 */
-		if (is_ldimm64(insn))
+		if (is_ldimm64_insn(insn))
 			bpf_core_poison_insn(prog, relo_idx, insn_idx + 1, insn + 1);
 		bpf_core_poison_insn(prog, relo_idx, insn_idx, insn);
 		return 0;
@@ -5771,7 +5871,7 @@ poison:
 	case BPF_LD: {
 		__u64 imm;
 
-		if (!is_ldimm64(insn) ||
+		if (!is_ldimm64_insn(insn) ||
 		    insn[0].src_reg != 0 || insn[0].off != 0 ||
 		    insn_idx + 1 >= prog->insns_cnt ||
 		    insn[1].code != 0 || insn[1].dst_reg != 0 ||
@@ -6213,7 +6313,7 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
 			insn[0].imm = obj->maps[relo->map_idx].fd;
 			relo->processed = true;
 			break;
-		case RELO_EXTERN:
+		case RELO_EXTERN_VAR:
 			ext = &obj->externs[relo->sym_off];
 			if (ext->type == EXT_KCFG) {
 				insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
@@ -6231,6 +6331,12 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
 			}
 			relo->processed = true;
 			break;
+		case RELO_EXTERN_FUNC:
+			ext = &obj->externs[relo->sym_off];
+			insn[0].src_reg = BPF_PSEUDO_KFUNC_CALL;
+			insn[0].imm = ext->ksym.kernel_btf_id;
+			relo->processed = true;
+			break;
 		case RELO_SUBPROG_ADDR:
 			insn[0].src_reg = BPF_PSEUDO_FUNC;
 			/* will be handled as a follow up pass */
@@ -7351,6 +7457,7 @@ static int bpf_object__read_kallsyms_file(struct bpf_object *obj)
 {
 	char sym_type, sym_name[500];
 	unsigned long long sym_addr;
+	const struct btf_type *t;
 	struct extern_desc *ext;
 	int ret, err = 0;
 	FILE *f;
@@ -7377,6 +7484,10 @@ static int bpf_object__read_kallsyms_file(struct bpf_object *obj)
 		if (!ext || ext->type != EXT_KSYM)
 			continue;
 
+		t = btf__type_by_id(obj->btf, ext->btf_id);
+		if (!btf_is_var(t))
+			continue;
+
 		if (ext->is_set && ext->ksym.addr != sym_addr) {
 			pr_warn("extern (ksym) '%s' resolution is ambiguous: 0x%llx or 0x%llx\n",
 				sym_name, ext->ksym.addr, sym_addr);
@@ -7395,75 +7506,151 @@ out:
 	return err;
 }
 
-static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
+static int find_ksym_btf_id(struct bpf_object *obj, const char *ksym_name,
+			    __u16 kind, struct btf **res_btf,
+			    int *res_btf_fd)
 {
-	struct extern_desc *ext;
+	int i, id, btf_fd, err;
 	struct btf *btf;
-	int i, j, id, btf_fd, err;
 
-	for (i = 0; i < obj->nr_extern; i++) {
-		const struct btf_type *targ_var, *targ_type;
-		__u32 targ_type_id, local_type_id;
-		const char *targ_var_name;
-		int ret;
+	btf = obj->btf_vmlinux;
+	btf_fd = 0;
+	id = btf__find_by_name_kind(btf, ksym_name, kind);
 
-		ext = &obj->externs[i];
-		if (ext->type != EXT_KSYM || !ext->ksym.type_id)
-			continue;
-
-		btf = obj->btf_vmlinux;
-		btf_fd = 0;
-		id = btf__find_by_name_kind(btf, ext->name, BTF_KIND_VAR);
-		if (id == -ENOENT) {
-			err = load_module_btfs(obj);
-			if (err)
-				return err;
+	if (id == -ENOENT) {
+		err = load_module_btfs(obj);
+		if (err)
+			return err;
 
-			for (j = 0; j < obj->btf_module_cnt; j++) {
-				btf = obj->btf_modules[j].btf;
-				/* we assume module BTF FD is always >0 */
-				btf_fd = obj->btf_modules[j].fd;
-				id = btf__find_by_name_kind(btf, ext->name, BTF_KIND_VAR);
-				if (id != -ENOENT)
-					break;
-			}
-		}
-		if (id <= 0) {
-			pr_warn("extern (ksym) '%s': failed to find BTF ID in kernel BTF(s).\n",
-				ext->name);
-			return -ESRCH;
+		for (i = 0; i < obj->btf_module_cnt; i++) {
+			btf = obj->btf_modules[i].btf;
+			/* we assume module BTF FD is always >0 */
+			btf_fd = obj->btf_modules[i].fd;
+			id = btf__find_by_name_kind(btf, ksym_name, kind);
+			if (id != -ENOENT)
+				break;
 		}
+	}
+	if (id <= 0) {
+		pr_warn("extern (%s ksym) '%s': failed to find BTF ID in kernel BTF(s).\n",
+			__btf_kind_str(kind), ksym_name);
+		return -ESRCH;
+	}
+
+	*res_btf = btf;
+	*res_btf_fd = btf_fd;
+	return id;
+}
 
-		/* find local type_id */
-		local_type_id = ext->ksym.type_id;
+static int bpf_object__resolve_ksym_var_btf_id(struct bpf_object *obj,
+					       struct extern_desc *ext)
+{
+	const struct btf_type *targ_var, *targ_type;
+	__u32 targ_type_id, local_type_id;
+	const char *targ_var_name;
+	int id, btf_fd = 0, err;
+	struct btf *btf = NULL;
 
-		/* find target type_id */
-		targ_var = btf__type_by_id(btf, id);
-		targ_var_name = btf__name_by_offset(btf, targ_var->name_off);
-		targ_type = skip_mods_and_typedefs(btf, targ_var->type, &targ_type_id);
+	id = find_ksym_btf_id(obj, ext->name, BTF_KIND_VAR, &btf, &btf_fd);
+	if (id < 0)
+		return id;
 
-		ret = bpf_core_types_are_compat(obj->btf, local_type_id,
-						btf, targ_type_id);
-		if (ret <= 0) {
-			const struct btf_type *local_type;
-			const char *targ_name, *local_name;
+	/* find local type_id */
+	local_type_id = ext->ksym.type_id;
 
-			local_type = btf__type_by_id(obj->btf, local_type_id);
-			local_name = btf__name_by_offset(obj->btf, local_type->name_off);
-			targ_name = btf__name_by_offset(btf, targ_type->name_off);
+	/* find target type_id */
+	targ_var = btf__type_by_id(btf, id);
+	targ_var_name = btf__name_by_offset(btf, targ_var->name_off);
+	targ_type = skip_mods_and_typedefs(btf, targ_var->type, &targ_type_id);
 
-			pr_warn("extern (ksym) '%s': incompatible types, expected [%d] %s %s, but kernel has [%d] %s %s\n",
-				ext->name, local_type_id,
-				btf_kind_str(local_type), local_name, targ_type_id,
-				btf_kind_str(targ_type), targ_name);
-			return -EINVAL;
-		}
+	err = bpf_core_types_are_compat(obj->btf, local_type_id,
+					btf, targ_type_id);
+	if (err <= 0) {
+		const struct btf_type *local_type;
+		const char *targ_name, *local_name;
+
+		local_type = btf__type_by_id(obj->btf, local_type_id);
+		local_name = btf__name_by_offset(obj->btf, local_type->name_off);
+		targ_name = btf__name_by_offset(btf, targ_type->name_off);
+
+		pr_warn("extern (var ksym) '%s': incompatible types, expected [%d] %s %s, but kernel has [%d] %s %s\n",
+			ext->name, local_type_id,
+			btf_kind_str(local_type), local_name, targ_type_id,
+			btf_kind_str(targ_type), targ_name);
+		return -EINVAL;
+	}
+
+	ext->is_set = true;
+	ext->ksym.kernel_btf_obj_fd = btf_fd;
+	ext->ksym.kernel_btf_id = id;
+	pr_debug("extern (var ksym) '%s': resolved to [%d] %s %s\n",
+		 ext->name, id, btf_kind_str(targ_var), targ_var_name);
+
+	return 0;
+}
+
+static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
+						struct extern_desc *ext)
+{
+	int local_func_proto_id, kfunc_proto_id, kfunc_id;
+	const struct btf_type *kern_func;
+	struct btf *kern_btf = NULL;
+	int ret, kern_btf_fd = 0;
+
+	local_func_proto_id = ext->ksym.type_id;
+
+	kfunc_id = find_ksym_btf_id(obj, ext->name, BTF_KIND_FUNC,
+				    &kern_btf, &kern_btf_fd);
+	if (kfunc_id < 0) {
+		pr_warn("extern (func ksym) '%s': not found in kernel BTF\n",
+			ext->name);
+		return kfunc_id;
+	}
+
+	if (kern_btf != obj->btf_vmlinux) {
+		pr_warn("extern (func ksym) '%s': function in kernel module is not supported\n",
+			ext->name);
+		return -ENOTSUP;
+	}
+
+	kern_func = btf__type_by_id(kern_btf, kfunc_id);
+	kfunc_proto_id = kern_func->type;
+
+	ret = bpf_core_types_are_compat(obj->btf, local_func_proto_id,
+					kern_btf, kfunc_proto_id);
+	if (ret <= 0) {
+		pr_warn("extern (func ksym) '%s': func_proto [%d] incompatible with kernel [%d]\n",
+			ext->name, local_func_proto_id, kfunc_proto_id);
+		return -EINVAL;
+	}
 
-		ext->is_set = true;
-		ext->ksym.kernel_btf_obj_fd = btf_fd;
-		ext->ksym.kernel_btf_id = id;
-		pr_debug("extern (ksym) '%s': resolved to [%d] %s %s\n",
-			 ext->name, id, btf_kind_str(targ_var), targ_var_name);
+	ext->is_set = true;
+	ext->ksym.kernel_btf_obj_fd = kern_btf_fd;
+	ext->ksym.kernel_btf_id = kfunc_id;
+	pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
+		 ext->name, kfunc_id);
+
+	return 0;
+}
+
+static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
+{
+	const struct btf_type *t;
+	struct extern_desc *ext;
+	int i, err;
+
+	for (i = 0; i < obj->nr_extern; i++) {
+		ext = &obj->externs[i];
+		if (ext->type != EXT_KSYM || !ext->ksym.type_id)
+			continue;
+
+		t = btf__type_by_id(obj->btf, ext->btf_id);
+		if (btf_is_var(t))
+			err = bpf_object__resolve_ksym_var_btf_id(obj, ext);
+		else
+			err = bpf_object__resolve_ksym_func_btf_id(obj, ext);
+		if (err)
+			return err;
 	}
 	return 0;
 }