[PATCH bpf-next v1 4/8] bpf/libbpf: BTF support for typed ksyms

[Hao Luo]

If a ksym is defined with a type, libbpf will try to find the ksym's btf
information from kernel btf. If a valid btf entry for the ksym is found,
libbpf can pass in the found btf id to the verifier, which validates the
ksym's type and value.

Typeless ksyms (i.e. those defined as 'void') will not have such btf_id,
but it has the symbol's address (read from kallsyms) and its value is
treated as a raw pointer.

Signed-off-by: Hao Luo <haoluo@xxxxxxxxxx>
---
 tools/lib/bpf/libbpf.c | 130 ++++++++++++++++++++++++++++++++++++-----
 1 file changed, 114 insertions(+), 16 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 4a81c6b2d21b..94eff612c7c2 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -357,7 +357,16 @@ struct extern_desc {
 			bool is_signed;
 		} kcfg;
 		struct {
-			unsigned long long addr;
+			/*
+			 *  1. If ksym is typeless, the field 'addr' is valid.
+			 *  2. If ksym is typed, the field 'vmlinux_btf_id' is
+			 *     valid.
+			 */
+			bool is_typeless;
+			union {
+				unsigned long long addr;
+				int vmlinux_btf_id;
+			};
 		} ksym;
 	};
 };
@@ -382,6 +391,7 @@ struct bpf_object {
 
 	bool loaded;
 	bool has_pseudo_calls;
+	bool has_typed_ksyms;
 
 	/*
 	 * Information when doing elf related work. Only valid if fd
@@ -2521,6 +2531,10 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj)
 	if (obj->btf_ext && obj->btf_ext->core_relo_info.len)
 		need_vmlinux_btf = true;
 
+	/* Support for typed ksyms needs kernel BTF */
+	if (obj->has_typed_ksyms)
+		need_vmlinux_btf = true;
+
 	bpf_object__for_each_program(prog, obj) {
 		if (!prog->load)
 			continue;
@@ -2975,10 +2989,10 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 			ext->type = EXT_KSYM;
 
 			vt = skip_mods_and_typedefs(obj->btf, t->type, NULL);
-			if (!btf_is_void(vt)) {
-				pr_warn("extern (ksym) '%s' is not typeless (void)\n", ext_name);
-				return -ENOTSUP;
-			}
+			ext->ksym.is_typeless = btf_is_void(vt);
+
+			if (!obj->has_typed_ksyms && !ext->ksym.is_typeless)
+				obj->has_typed_ksyms = true;
 		} else {
 			pr_warn("unrecognized extern section '%s'\n", sec_name);
 			return -ENOTSUP;
@@ -2992,9 +3006,9 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 	/* sort externs by type, for kcfg ones also by (align, size, name) */
 	qsort(obj->externs, obj->nr_extern, sizeof(*ext), cmp_externs);
 
-	/* for .ksyms section, we need to turn all externs into allocated
-	 * variables in BTF to pass kernel verification; we do this by
-	 * pretending that each extern is a 8-byte variable
+	/* for .ksyms section, we need to turn all typeless externs into
+	 * allocated variables in BTF to pass kernel verification; we do
+	 * this by pretending that each typeless extern is a 8-byte variable
 	 */
 	if (ksym_sec) {
 		/* find existing 4-byte integer type in BTF to use for fake
@@ -3012,7 +3026,7 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 
 		sec = ksym_sec;
 		n = btf_vlen(sec);
-		for (i = 0, off = 0; i < n; i++, off += sizeof(int)) {
+		for (i = 0, off = 0; i < n; i++) {
 			struct btf_var_secinfo *vs = btf_var_secinfos(sec) + i;
 			struct btf_type *vt;
 
@@ -3025,9 +3039,14 @@ static int bpf_object__collect_externs(struct bpf_object *obj)
 				return -ESRCH;
 			}
 			btf_var(vt)->linkage = BTF_VAR_GLOBAL_ALLOCATED;
-			vt->type = int_btf_id;
+			if (ext->ksym.is_typeless) {
+				vt->type = int_btf_id;
+				vs->size = sizeof(int);
+			}
 			vs->offset = off;
-			vs->size = sizeof(int);
+			off += vs->size;
+			pr_debug("ksym var_secinfo: var '%s', type #%d, size %d, offset %d\n",
+				 ext->name, vt->type, vs->size, vs->offset);
 		}
 		sec->size = off;
 	}
@@ -5300,8 +5319,13 @@ bpf_program__relocate(struct bpf_program *prog, struct bpf_object *obj)
 				insn[0].imm = obj->maps[obj->kconfig_map_idx].fd;
 				insn[1].imm = ext->kcfg.data_off;
 			} else /* EXT_KSYM */ {
-				insn[0].imm = (__u32)ext->ksym.addr;
-				insn[1].imm = ext->ksym.addr >> 32;
+				if (ext->ksym.is_typeless) { /* typelss ksyms */
+					insn[0].imm = (__u32)ext->ksym.addr;
+					insn[1].imm = ext->ksym.addr >> 32;
+				} else { /* typed ksyms */
+					insn[0].src_reg = BPF_PSEUDO_BTF_ID;
+					insn[0].imm = ext->ksym.vmlinux_btf_id;
+				}
 			}
 			break;
 		case RELO_CALL:
@@ -6019,6 +6043,10 @@ static int bpf_object__read_kallsyms_file(struct bpf_object *obj)
 		if (!ext || ext->type != EXT_KSYM)
 			continue;
 
+		/* Typed ksyms have the verifier to fill their addresses. */
+		if (!ext->ksym.is_typeless)
+			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);
@@ -6037,10 +6065,72 @@ static int bpf_object__read_kallsyms_file(struct bpf_object *obj)
 	return err;
 }
 
+static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
+{
+	struct extern_desc *ext;
+	int i, id;
+
+	if (!obj->btf_vmlinux) {
+		pr_warn("support of typed ksyms needs kernel btf.\n");
+		return -ENOENT;
+	}
+
+	for (i = 0; i < obj->nr_extern; i++) {
+		const struct btf_type *v, *vx; /* VARs in object and vmlinux btf */
+		const struct btf_type *t, *tx; /* TYPEs in btf */
+		__u32 vt, vtx; /* btf_ids of TYPEs */
+
+		ext = &obj->externs[i];
+		if (ext->type != EXT_KSYM)
+			continue;
+
+		if (ext->ksym.is_typeless)
+			continue;
+
+		if (ext->is_set) {
+			pr_warn("typed ksym '%s' resolved as typeless ksyms.\n",
+				ext->name);
+			return -EFAULT;
+		}
+
+		id = btf__find_by_name_kind(obj->btf_vmlinux, ext->name,
+					    BTF_KIND_VAR);
+		if (id <= 0) {
+			pr_warn("no btf entry for ksym '%s' in vmlinux.\n",
+				ext->name);
+			return -ESRCH;
+		}
+
+		vx = btf__type_by_id(obj->btf_vmlinux, id);
+		tx = skip_mods_and_typedefs(obj->btf_vmlinux, vx->type, &vtx);
+
+		v = btf__type_by_id(obj->btf, ext->btf_id);
+		t = skip_mods_and_typedefs(obj->btf, v->type, &vt);
+
+		if (!btf_ksym_type_match(obj->btf_vmlinux, vtx, obj->btf, vt)) {
+			const char *tname, *txname; /* names of TYPEs */
+
+			txname = btf__name_by_offset(obj->btf_vmlinux, tx->name_off);
+			tname = btf__name_by_offset(obj->btf, t->name_off);
+
+			pr_warn("ksym '%s' expects type '%s' (vmlinux_btf_id: #%d), "
+				"but got '%s' (btf_id: #%d)\n", ext->name,
+				txname, vtx, tname, vt);
+			return -EINVAL;
+		}
+
+		ext->is_set = true;
+		ext->ksym.vmlinux_btf_id = id;
+		pr_debug("extern (ksym) %s=vmlinux_btf_id(#%d)\n", ext->name, id);
+	}
+	return 0;
+}
+
 static int bpf_object__resolve_externs(struct bpf_object *obj,
 				       const char *extra_kconfig)
 {
-	bool need_config = false, need_kallsyms = false;
+	bool need_config = false;
+	bool need_kallsyms = false, need_vmlinux_btf = false;
 	struct extern_desc *ext;
 	void *kcfg_data = NULL;
 	int err, i;
@@ -6071,7 +6161,10 @@ static int bpf_object__resolve_externs(struct bpf_object *obj,
 			   strncmp(ext->name, "CONFIG_", 7) == 0) {
 			need_config = true;
 		} else if (ext->type == EXT_KSYM) {
-			need_kallsyms = true;
+			if (ext->ksym.is_typeless)
+				need_kallsyms = true;
+			else
+				need_vmlinux_btf = true;
 		} else {
 			pr_warn("unrecognized extern '%s'\n", ext->name);
 			return -EINVAL;
@@ -6100,6 +6193,11 @@ static int bpf_object__resolve_externs(struct bpf_object *obj,
 		if (err)
 			return -EINVAL;
 	}
+	if (need_vmlinux_btf) {
+		err = bpf_object__resolve_ksyms_btf_id(obj);
+		if (err)
+			return -EINVAL;
+	}
 	for (i = 0; i < obj->nr_extern; i++) {
 		ext = &obj->externs[i];
 
@@ -6132,10 +6230,10 @@ int bpf_object__load_xattr(struct bpf_object_load_attr *attr)
 	}
 
 	err = bpf_object__probe_loading(obj);
+	err = err ? : bpf_object__load_vmlinux_btf(obj);
 	err = err ? : bpf_object__resolve_externs(obj, obj->kconfig);
 	err = err ? : bpf_object__sanitize_and_load_btf(obj);
 	err = err ? : bpf_object__sanitize_maps(obj);
-	err = err ? : bpf_object__load_vmlinux_btf(obj);
 	err = err ? : bpf_object__init_kern_struct_ops_maps(obj);
 	err = err ? : bpf_object__create_maps(obj);
 	err = err ? : bpf_object__relocate(obj, attr->target_btf_path);
-- 
2.28.0.220.ged08abb693-goog