Merge branch 'extern-var-support'

Andrii Nakryiko says:

====================
It's often important for BPF program to know kernel version or some specific
config values (e.g., CONFIG_HZ to convert jiffies to seconds) and change or
adjust program logic based on their values. As of today, any such need has to
be resolved by recompiling BPF program for specific kernel and kernel
configuration. In practice this is usually achieved by using BCC and its
embedded LLVM/Clang. With such set up #ifdef CONFIG_XXX and similar
compile-time constructs allow to deal with kernel varieties.

With CO-RE (Compile Once – Run Everywhere) approach, this is not an option,
unfortunately. All such logic variations have to be done as a normal
C language constructs (i.e., if/else, variables, etc), not a preprocessor
directives. This patch series add support for such advanced scenarios through
C extern variables. These extern variables will be recognized by libbpf and
supplied through extra .extern internal map, similarly to global data. This
.extern map is read-only, which allows BPF verifier to track its content
precisely as constants. That gives an opportunity to have pre-compiled BPF
program, which can potentially use BPF functionality (e.g., BPF helpers) or
kernel features (types, fields, etc), that are available only on a subset of
targeted kernels, while effectively eleminating (through verifier's dead code
detection) such unsupported functionality for other kernels (typically, older
versions). Patch #3 explicitly tests a scenario of using unsupported BPF
helper, to validate the approach.

This patch set heavily relies on BTF type information emitted by compiler for
each extern variable declaration. Based on specific types, libbpf does strict
checks of config data values correctness. See patch #1 for details.

Outline of the patch set:
- patch #1 does a small clean up of internal map names contants;
- patch #2 adds all of the libbpf internal machinery for externs support,
  including setting up BTF information for .extern data section;
- patch #3 adds support for .extern into BPF skeleton;
- patch #4 adds externs selftests, as well as enhances test_skeleton.c test to
  validate mmap()-ed .extern datasection functionality.

v3->v4:
- clean up copyrights and rebase onto latest skeleton patches (Alexei);

v2->v3:
- truncate too long strings (Alexei);
- clean ups, adding comments (Alexei);

v1->v2:
- use BTF type information for externs (Alexei);
- add strings support;
- add BPF skeleton support for .extern.
====================

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

5 files changed, 741 insertions, 75 deletions

diff --git a/tools/lib/bpf/Makefile b/tools/lib/bpf/Makefile
index 23ae06c43d08..a3718cb275f2 100644
--- a/tools/lib/bpf/Makefile
+++ b/tools/lib/bpf/Makefile
@@ -56,8 +56,8 @@ ifndef VERBOSE
 endif
 
 FEATURE_USER = .libbpf
-FEATURE_TESTS = libelf libelf-mmap bpf reallocarray
-FEATURE_DISPLAY = libelf bpf
+FEATURE_TESTS = libelf libelf-mmap zlib bpf reallocarray
+FEATURE_DISPLAY = libelf zlib bpf
 
 INCLUDES = -I. -I$(srctree)/tools/include -I$(srctree)/tools/arch/$(ARCH)/include/uapi -I$(srctree)/tools/include/uapi
 FEATURE_CHECK_CFLAGS-bpf = $(INCLUDES)
@@ -160,7 +160,7 @@ all: fixdep
 
 all_cmd: $(CMD_TARGETS) check
 
-$(BPF_IN_SHARED): force elfdep bpfdep bpf_helper_defs.h
+$(BPF_IN_SHARED): force elfdep zdep bpfdep bpf_helper_defs.h
 	@(test -f ../../include/uapi/linux/bpf.h -a -f ../../../include/uapi/linux/bpf.h && ( \
 	(diff -B ../../include/uapi/linux/bpf.h ../../../include/uapi/linux/bpf.h >/dev/null) || \
 	echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/bpf.h' differs from latest version at 'include/uapi/linux/bpf.h'" >&2 )) || true
@@ -178,7 +178,7 @@ $(BPF_IN_SHARED): force elfdep bpfdep bpf_helper_defs.h
 	echo "Warning: Kernel ABI header at 'tools/include/uapi/linux/if_xdp.h' differs from latest version at 'include/uapi/linux/if_xdp.h'" >&2 )) || true
 	$(Q)$(MAKE) $(build)=libbpf OUTPUT=$(SHARED_OBJDIR) CFLAGS="$(CFLAGS) $(SHLIB_FLAGS)"
 
-$(BPF_IN_STATIC): force elfdep bpfdep bpf_helper_defs.h
+$(BPF_IN_STATIC): force elfdep zdep bpfdep bpf_helper_defs.h
 	$(Q)$(MAKE) $(build)=libbpf OUTPUT=$(STATIC_OBJDIR)
 
 bpf_helper_defs.h: $(srctree)/tools/include/uapi/linux/bpf.h
@@ -190,7 +190,7 @@ $(OUTPUT)libbpf.so: $(OUTPUT)libbpf.so.$(LIBBPF_VERSION)
 $(OUTPUT)libbpf.so.$(LIBBPF_VERSION): $(BPF_IN_SHARED)
 	$(QUIET_LINK)$(CC) $(LDFLAGS) \
 		--shared -Wl,-soname,libbpf.so.$(LIBBPF_MAJOR_VERSION) \
-		-Wl,--version-script=$(VERSION_SCRIPT) $^ -lelf -o $@
+		-Wl,--version-script=$(VERSION_SCRIPT) $^ -lelf -lz -o $@
 	@ln -sf $(@F) $(OUTPUT)libbpf.so
 	@ln -sf $(@F) $(OUTPUT)libbpf.so.$(LIBBPF_MAJOR_VERSION)
 
@@ -279,12 +279,15 @@ clean:
 
 
 
-PHONY += force elfdep bpfdep cscope tags
+PHONY += force elfdep zdep bpfdep cscope tags
 force:
 
 elfdep:
 	@if [ "$(feature-libelf)" != "1" ]; then echo "No libelf found"; exit 1 ; fi
 
+zdep:
+	@if [ "$(feature-zlib)" != "1" ]; then echo "No zlib found"; exit 1 ; fi
+
 bpfdep:
 	@if [ "$(feature-bpf)" != "1" ]; then echo "BPF API too old"; exit 1 ; fi
 
diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index 0c7d28292898..aa46700075e1 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -25,6 +25,9 @@
 #ifndef __always_inline
 #define __always_inline __attribute__((always_inline))
 #endif
+#ifndef __weak
+#define __weak __attribute__((weak))
+#endif
 
 /*
  * Helper structure used by eBPF C program
@@ -44,4 +47,10 @@ enum libbpf_pin_type {
 	LIBBPF_PIN_BY_NAME,
 };
 
+enum libbpf_tristate {
+	TRI_NO = 0,
+	TRI_YES = 1,
+	TRI_MODULE = 2,
+};
+
 #endif
diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 84fe82f27bef..520021939d81 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -578,6 +578,12 @@ static int btf_fixup_datasec(struct bpf_object *obj, struct btf *btf,
 		return -ENOENT;
 	}
 
+	/* .extern datasec size and var offsets were set correctly during
+	 * extern collection step, so just skip straight to sorting variables
+	 */
+	if (t->size)
+		goto sort_vars;
+
 	ret = bpf_object__section_size(obj, name, &size);
 	if (ret || !size || (t->size && t->size != size)) {
 		pr_debug("Invalid size for section %s: %u bytes\n", name, size);
@@ -614,7 +620,8 @@ static int btf_fixup_datasec(struct bpf_object *obj, struct btf *btf,
 		vsi->offset = off;
 	}
 
-	qsort(t + 1, vars, sizeof(*vsi), compare_vsi_off);
+sort_vars:
+	qsort(btf_var_secinfos(t), vars, sizeof(*vsi), compare_vsi_off);
 	return 0;
 }
 
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index a1a902fa6e0c..c9f90bfd2f0d 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -44,6 +44,7 @@
 #include <tools/libc_compat.h>
 #include <libelf.h>
 #include <gelf.h>
+#include <zlib.h>
 
 #include "libbpf.h"
 #include "bpf.h"
@@ -139,6 +140,20 @@ struct bpf_capabilities {
 	__u32 array_mmap:1;
 };
 
+enum reloc_type {
+	RELO_LD64,
+	RELO_CALL,
+	RELO_DATA,
+	RELO_EXTERN,
+};
+
+struct reloc_desc {
+	enum reloc_type type;
+	int insn_idx;
+	int map_idx;
+	int sym_off;
+};
+
 /*
  * bpf_prog should be a better name but it has been used in
  * linux/filter.h.
@@ -157,16 +172,7 @@ struct bpf_program {
 	size_t insns_cnt, main_prog_cnt;
 	enum bpf_prog_type type;
 
-	struct reloc_desc {
-		enum {
-			RELO_LD64,
-			RELO_CALL,
-			RELO_DATA,
-		} type;
-		int insn_idx;
-		int map_idx;
-		int sym_off;
-	} *reloc_desc;
+	struct reloc_desc *reloc_desc;
 	int nr_reloc;
 	int log_level;
 
@@ -195,17 +201,24 @@ struct bpf_program {
 	__u32 prog_flags;
 };
 
+#define DATA_SEC ".data"
+#define BSS_SEC ".bss"
+#define RODATA_SEC ".rodata"
+#define EXTERN_SEC ".extern"
+
 enum libbpf_map_type {
 	LIBBPF_MAP_UNSPEC,
 	LIBBPF_MAP_DATA,
 	LIBBPF_MAP_BSS,
 	LIBBPF_MAP_RODATA,
+	LIBBPF_MAP_EXTERN,
 };
 
 static const char * const libbpf_type_to_btf_name[] = {
-	[LIBBPF_MAP_DATA]	= ".data",
-	[LIBBPF_MAP_BSS]	= ".bss",
-	[LIBBPF_MAP_RODATA]	= ".rodata",
+	[LIBBPF_MAP_DATA]	= DATA_SEC,
+	[LIBBPF_MAP_BSS]	= BSS_SEC,
+	[LIBBPF_MAP_RODATA]	= RODATA_SEC,
+	[LIBBPF_MAP_EXTERN]	= EXTERN_SEC,
 };
 
 struct bpf_map {
@@ -227,6 +240,28 @@ struct bpf_map {
 	bool reused;
 };
 
+enum extern_type {
+	EXT_UNKNOWN,
+	EXT_CHAR,
+	EXT_BOOL,
+	EXT_INT,
+	EXT_TRISTATE,
+	EXT_CHAR_ARR,
+};
+
+struct extern_desc {
+	const char *name;
+	int sym_idx;
+	int btf_id;
+	enum extern_type type;
+	int sz;
+	int align;
+	int data_off;
+	bool is_signed;
+	bool is_weak;
+	bool is_set;
+};
+
 static LIST_HEAD(bpf_objects_list);
 
 struct bpf_object {
@@ -240,6 +275,11 @@ struct bpf_object {
 	size_t nr_maps;
 	size_t maps_cap;
 
+	char *kconfig_path;
+	struct extern_desc *externs;
+	int nr_extern;
+	int extern_map_idx;
+
 	bool loaded;
 	bool has_pseudo_calls;
 	bool relaxed_core_relocs;
@@ -267,6 +307,7 @@ struct bpf_object {
 		int maps_shndx;
 		int btf_maps_shndx;
 		int text_shndx;
+		int symbols_shndx;
 		int data_shndx;
 		int rodata_shndx;
 		int bss_shndx;
@@ -538,6 +579,7 @@ static struct bpf_object *bpf_object__new(const char *path,
 	obj->efile.data_shndx = -1;
 	obj->efile.rodata_shndx = -1;
 	obj->efile.bss_shndx = -1;
+	obj->extern_map_idx = -1;
 
 	obj->kern_version = get_kernel_version();
 	obj->loaded = false;
@@ -736,13 +778,13 @@ int bpf_object__section_size(const struct bpf_object *obj, const char *name,
 	*size = 0;
 	if (!name) {
 		return -EINVAL;
-	} else if (!strcmp(name, ".data")) {
+	} else if (!strcmp(name, DATA_SEC)) {
 		if (obj->efile.data)
 			*size = obj->efile.data->d_size;
-	} else if (!strcmp(name, ".bss")) {
+	} else if (!strcmp(name, BSS_SEC)) {
 		if (obj->efile.bss)
 			*size = obj->efile.bss->d_size;
-	} else if (!strcmp(name, ".rodata")) {
+	} else if (!strcmp(name, RODATA_SEC)) {
 		if (obj->efile.rodata)
 			*size = obj->efile.rodata->d_size;
 	} else {
@@ -862,7 +904,8 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
 	def->key_size = sizeof(int);
 	def->value_size = data_sz;
 	def->max_entries = 1;
-	def->map_flags = type == LIBBPF_MAP_RODATA ? BPF_F_RDONLY_PROG : 0;
+	def->map_flags = type == LIBBPF_MAP_RODATA || type == LIBBPF_MAP_EXTERN
+			 ? BPF_F_RDONLY_PROG : 0;
 	def->map_flags |= BPF_F_MMAPABLE;
 
 	pr_debug("map '%s' (global data): at sec_idx %d, offset %zu, flags %x.\n",
@@ -879,7 +922,7 @@ bpf_object__init_internal_map(struct bpf_object *obj, enum libbpf_map_type type,
 		return err;
 	}
 
-	if (type != LIBBPF_MAP_BSS)
+	if (data)
 		memcpy(map->mmaped, data, data_sz);
 
 	pr_debug("map %td is \"%s\"\n", map - obj->maps, map->name);
@@ -920,6 +963,271 @@ static int bpf_object__init_global_data_maps(struct bpf_object *obj)
 	return 0;
 }
 
+
+static struct extern_desc *find_extern_by_name(const struct bpf_object *obj,
+					       const void *name)
+{
+	int i;
+
+	for (i = 0; i < obj->nr_extern; i++) {
+		if (strcmp(obj->externs[i].name, name) == 0)
+			return &obj->externs[i];
+	}
+	return NULL;
+}
+
+static int set_ext_value_tri(struct extern_desc *ext, void *ext_val,
+			     char value)
+{
+	switch (ext->type) {
+	case EXT_BOOL:
+		if (value == 'm') {
+			pr_warn("extern %s=%c should be tristate or char\n",
+				ext->name, value);
+			return -EINVAL;
+		}
+		*(bool *)ext_val = value == 'y' ? true : false;
+		break;
+	case EXT_TRISTATE:
+		if (value == 'y')
+			*(enum libbpf_tristate *)ext_val = TRI_YES;
+		else if (value == 'm')
+			*(enum libbpf_tristate *)ext_val = TRI_MODULE;
+		else /* value == 'n' */
+			*(enum libbpf_tristate *)ext_val = TRI_NO;
+		break;
+	case EXT_CHAR:
+		*(char *)ext_val = value;
+		break;
+	case EXT_UNKNOWN:
+	case EXT_INT:
+	case EXT_CHAR_ARR:
+	default:
+		pr_warn("extern %s=%c should be bool, tristate, or char\n",
+			ext->name, value);
+		return -EINVAL;
+	}
+	ext->is_set = true;
+	return 0;
+}
+
+static int set_ext_value_str(struct extern_desc *ext, char *ext_val,
+			     const char *value)
+{
+	size_t len;
+
+	if (ext->type != EXT_CHAR_ARR) {
+		pr_warn("extern %s=%s should char array\n", ext->name, value);
+		return -EINVAL;
+	}
+
+	len = strlen(value);
+	if (value[len - 1] != '"') {
+		pr_warn("extern '%s': invalid string config '%s'\n",
+			ext->name, value);
+		return -EINVAL;
+	}
+
+	/* strip quotes */
+	len -= 2;
+	if (len >= ext->sz) {
+		pr_warn("extern '%s': long string config %s of (%zu bytes) truncated to %d bytes\n",
+			ext->name, value, len, ext->sz - 1);
+		len = ext->sz - 1;
+	}
+	memcpy(ext_val, value + 1, len);
+	ext_val[len] = '\0';
+	ext->is_set = true;
+	return 0;
+}
+
+static int parse_u64(const char *value, __u64 *res)
+{
+	char *value_end;
+	int err;
+
+	errno = 0;
+	*res = strtoull(value, &value_end, 0);
+	if (errno) {
+		err = -errno;
+		pr_warn("failed to parse '%s' as integer: %d\n", value, err);
+		return err;
+	}
+	if (*value_end) {
+		pr_warn("failed to parse '%s' as integer completely\n", value);
+		return -EINVAL;
+	}
+	return 0;
+}
+
+static bool is_ext_value_in_range(const struct extern_desc *ext, __u64 v)
+{
+	int bit_sz = ext->sz * 8;
+
+	if (ext->sz == 8)
+		return true;
+
+	/* Validate that value stored in u64 fits in integer of `ext->sz`
+	 * bytes size without any loss of information. If the target integer
+	 * is signed, we rely on the following limits of integer type of
+	 * Y bits and subsequent transformation:
+	 *
+	 *     -2^(Y-1) <= X           <= 2^(Y-1) - 1
+	 *            0 <= X + 2^(Y-1) <= 2^Y - 1
+	 *            0 <= X + 2^(Y-1) <  2^Y
+	 *
+	 *  For unsigned target integer, check that all the (64 - Y) bits are
+	 *  zero.
+	 */
+	if (ext->is_signed)
+		return v + (1ULL << (bit_sz - 1)) < (1ULL << bit_sz);
+	else
+		return (v >> bit_sz) == 0;
+}
+
+static int set_ext_value_num(struct extern_desc *ext, void *ext_val,
+			     __u64 value)
+{
+	if (ext->type != EXT_INT && ext->type != EXT_CHAR) {
+		pr_warn("extern %s=%llu should be integer\n",
+			ext->name, value);
+		return -EINVAL;
+	}
+	if (!is_ext_value_in_range(ext, value)) {
+		pr_warn("extern %s=%llu value doesn't fit in %d bytes\n",
+			ext->name, value, ext->sz);
+		return -ERANGE;
+	}
+	switch (ext->sz) {
+		case 1: *(__u8 *)ext_val = value; break;
+		case 2: *(__u16 *)ext_val = value; break;
+		case 4: *(__u32 *)ext_val = value; break;
+		case 8: *(__u64 *)ext_val = value; break;
+		default:
+			return -EINVAL;
+	}
+	ext->is_set = true;
+	return 0;
+}
+
+static int bpf_object__read_kernel_config(struct bpf_object *obj,
+					  const char *config_path,
+					  void *data)
+{
+	char buf[PATH_MAX], *sep, *value;
+	struct extern_desc *ext;
+	int len, err = 0;
+	void *ext_val;
+	__u64 num;
+	gzFile file;
+
+	if (config_path) {
+		file = gzopen(config_path, "r");
+	} else {
+		struct utsname uts;
+
+		uname(&uts);
+		len = snprintf(buf, PATH_MAX, "/boot/config-%s", uts.release);
+		if (len < 0)
+			return -EINVAL;
+		else if (len >= PATH_MAX)
+			return -ENAMETOOLONG;
+		/* gzopen also accepts uncompressed files. */
+		file = gzopen(buf, "r");
+		if (!file)
+			file = gzopen("/proc/config.gz", "r");
+	}
+	if (!file) {
+		pr_warn("failed to read kernel config at '%s'\n", config_path);
+		return -ENOENT;
+	}
+
+	while (gzgets(file, buf, sizeof(buf))) {
+		if (strncmp(buf, "CONFIG_", 7))
+			continue;
+
+		sep = strchr(buf, '=');
+		if (!sep) {
+			err = -EINVAL;
+			pr_warn("failed to parse '%s': no separator\n", buf);
+			goto out;
+		}
+		/* Trim ending '\n' */
+		len = strlen(buf);
+		if (buf[len - 1] == '\n')
+			buf[len - 1] = '\0';
+		/* Split on '=' and ensure that a value is present. */
+		*sep = '\0';
+		if (!sep[1]) {
+			err = -EINVAL;
+			*sep = '=';
+			pr_warn("failed to parse '%s': no value\n", buf);
+			goto out;
+		}
+
+		ext = find_extern_by_name(obj, buf);
+		if (!ext)
+			continue;
+		if (ext->is_set) {
+			err = -EINVAL;
+			pr_warn("re-defining extern '%s' not allowed\n", buf);
+			goto out;
+		}
+
+		ext_val = data + ext->data_off;
+		value = sep + 1;
+
+		switch (*value) {
+		case 'y': case 'n': case 'm':
+			err = set_ext_value_tri(ext, ext_val, *value);
+			break;
+		case '"':
+			err = set_ext_value_str(ext, ext_val, value);
+			break;
+		default:
+			/* assume integer */
+			err = parse_u64(value, &num);
+			if (err) {
+				pr_warn("extern %s=%s should be integer\n",
+					ext->name, value);
+				goto out;
+			}
+			err = set_ext_value_num(ext, ext_val, num);
+			break;
+		}
+		if (err)
+			goto out;
+		pr_debug("extern %s=%s\n", ext->name, value);
+	}
+
+out:
+	gzclose(file);
+	return err;
+}
+
+static int bpf_object__init_extern_map(struct bpf_object *obj)
+{
+	struct extern_desc *last_ext;
+	size_t map_sz;
+	int err;
+
+	if (obj->nr_extern == 0)
+		return 0;
+
+	last_ext = &obj->externs[obj->nr_extern - 1];
+	map_sz = last_ext->data_off + last_ext->sz;
+
+	err = bpf_object__init_internal_map(obj, LIBBPF_MAP_EXTERN,
+					    obj->efile.symbols_shndx,
+					    NULL, map_sz);
+	if (err)
+		return err;
+
+	obj->extern_map_idx = obj->nr_maps - 1;
+
+	return 0;
+}
+
 static int bpf_object__init_user_maps(struct bpf_object *obj, bool strict)
 {
 	Elf_Data *symbols = obj->efile.symbols;
@@ -1397,19 +1705,17 @@ static int bpf_object__init_user_btf_maps(struct bpf_object *obj, bool strict,
 static int bpf_object__init_maps(struct bpf_object *obj,
 				 const struct bpf_object_open_opts *opts)
 {
-	const char *pin_root_path = OPTS_GET(opts, pin_root_path, NULL);
-	bool strict = !OPTS_GET(opts, relaxed_maps, false);
+	const char *pin_root_path;
+	bool strict;
 	int err;
 
-	err = bpf_object__init_user_maps(obj, strict);
-	if (err)
-		return err;
+	strict = !OPTS_GET(opts, relaxed_maps, false);
+	pin_root_path = OPTS_GET(opts, pin_root_path, NULL);
 
-	err = bpf_object__init_user_btf_maps(obj, strict, pin_root_path);
-	if (err)
-		return err;
-
-	err = bpf_object__init_global_data_maps(obj);
+	err = bpf_object__init_user_maps(obj, strict);
+	err = err ?: bpf_object__init_user_btf_maps(obj, strict, pin_root_path);
+	err = err ?: bpf_object__init_global_data_maps(obj);
+	err = err ?: bpf_object__init_extern_map(obj);
 	if (err)
 		return err;
 
@@ -1528,11 +1834,6 @@ static int bpf_object__init_btf(struct bpf_object *obj,
 				BTF_ELF_SEC, err);
 			goto out;
 		}
-		err = btf__finalize_data(obj, obj->btf);
-		if (err) {
-			pr_warn("Error finalizing %s: %d.\n", BTF_ELF_SEC, err);
-			goto out;
-		}
 	}
 	if (btf_ext_data) {
 		if (!obj->btf) {
@@ -1566,6 +1867,30 @@ out:
 	return 0;
 }
 
+static int bpf_object__finalize_btf(struct bpf_object *obj)
+{
+	int err;
+
+	if (!obj->btf)
+		return 0;
+
+	err = btf__finalize_data(obj, obj->btf);
+	if (!err)
+		return 0;
+
+	pr_warn("Error finalizing %s: %d.\n", BTF_ELF_SEC, err);
+	btf__free(obj->btf);
+	obj->btf = NULL;
+	btf_ext__free(obj->btf_ext);
+	obj->btf_ext = NULL;
+
+	if (bpf_object__is_btf_mandatory(obj)) {
+		pr_warn("BTF is required, but is missing or corrupted.\n");
+		return -ENOENT;
+	}
+	return 0;
+}
+
 static int bpf_object__sanitize_and_load_btf(struct bpf_object *obj)
 {
 	int err = 0;
@@ -1666,6 +1991,7 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 				return -LIBBPF_ERRNO__FORMAT;
 			}
 			obj->efile.symbols = data;
+			obj->efile.symbols_shndx = idx;
 			obj->efile.strtabidx = sh.sh_link;
 		} else if (sh.sh_type == SHT_PROGBITS && data->d_size > 0) {
 			if (sh.sh_flags & SHF_EXECINSTR) {
@@ -1684,10 +2010,10 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 						name, obj->path, cp);
 					return err;
 				}
-			} else if (strcmp(name, ".data") == 0) {
+			} else if (strcmp(name, DATA_SEC) == 0) {
 				obj->efile.data = data;
 				obj->efile.data_shndx = idx;
-			} else if (strcmp(name, ".rodata") == 0) {
+			} else if (strcmp(name, RODATA_SEC) == 0) {
 				obj->efile.rodata = data;
 				obj->efile.rodata_shndx = idx;
 			} else {
@@ -1717,7 +2043,8 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 
 			obj->efile.reloc_sects[nr_sects].shdr = sh;
 			obj->efile.reloc_sects[nr_sects].data = data;
-		} else if (sh.sh_type == SHT_NOBITS && strcmp(name, ".bss") == 0) {
+		} else if (sh.sh_type == SHT_NOBITS &&
+			   strcmp(name, BSS_SEC) == 0) {
 			obj->efile.bss = data;
 			obj->efile.bss_shndx = idx;
 		} else {
@@ -1732,6 +2059,216 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 	return bpf_object__init_btf(obj, btf_data, btf_ext_data);
 }
 
+static bool sym_is_extern(const GElf_Sym *sym)
+{
+	int bind = GELF_ST_BIND(sym->st_info);
+	/* externs are symbols w/ type=NOTYPE, bind=GLOBAL|WEAK, section=UND */
+	return sym->st_shndx == SHN_UNDEF &&
+	       (bind == STB_GLOBAL || bind == STB_WEAK) &&
+	       GELF_ST_TYPE(sym->st_info) == STT_NOTYPE;
+}
+
+static int find_extern_btf_id(const struct btf *btf, const char *ext_name)
+{
+	const struct btf_type *t;
+	const char *var_name;
+	int i, n;
+
+	if (!btf)
+		return -ESRCH;
+
+	n = btf__get_nr_types(btf);
+	for (i = 1; i <= n; i++) {
+		t = btf__type_by_id(btf, i);
+
+		if (!btf_is_var(t))
+			continue;
+
+		var_name = btf__name_by_offset(btf, t->name_off);
+		if (strcmp(var_name, ext_name))
+			continue;
+
+		if (btf_var(t)->linkage != BTF_VAR_GLOBAL_EXTERN)
+			return -EINVAL;
+
+		return i;
+	}
+
+	return -ENOENT;
+}
+
+static enum extern_type find_extern_type(const struct btf *btf, int id,
+					 bool *is_signed)
+{
+	const struct btf_type *t;
+	const char *name;
+
+	t = skip_mods_and_typedefs(btf, id, NULL);
+	name = btf__name_by_offset(btf, t->name_off);
+
+	if (is_signed)
+		*is_signed = false;
+	switch (btf_kind(t)) {
+	case BTF_KIND_INT: {
+		int enc = btf_int_encoding(t);
+
+		if (enc & BTF_INT_BOOL)
+			return t->size == 1 ? EXT_BOOL : EXT_UNKNOWN;
+		if (is_signed)
+			*is_signed = enc & BTF_INT_SIGNED;
+		if (t->size == 1)
+			return EXT_CHAR;
+		if (t->size < 1 || t->size > 8 || (t->size & (t->size - 1)))
+			return EXT_UNKNOWN;
+		return EXT_INT;
+	}
+	case BTF_KIND_ENUM:
+		if (t->size != 4)
+			return EXT_UNKNOWN;
+		if (strcmp(name, "libbpf_tristate"))
+			return EXT_UNKNOWN;
+		return EXT_TRISTATE;
+	case BTF_KIND_ARRAY:
+		if (btf_array(t)->nelems == 0)
+			return EXT_UNKNOWN;
+		if (find_extern_type(btf, btf_array(t)->type, NULL) != EXT_CHAR)
+			return EXT_UNKNOWN;
+		return EXT_CHAR_ARR;
+	default:
+		return EXT_UNKNOWN;
+	}
+}
+
+static int cmp_externs(const void *_a, const void *_b)
+{
+	const struct extern_desc *a = _a;
+	const struct extern_desc *b = _b;
+
+	/* descending order by alignment requirements */
+	if (a->align != b->align)
+		return a->align > b->align ? -1 : 1;
+	/* ascending order by size, within same alignment class */
+	if (a->sz != b->sz)
+		return a->sz < b->sz ? -1 : 1;
+	/* resolve ties by name */
+	return strcmp(a->name, b->name);
+}
+
+static int bpf_object__collect_externs(struct bpf_object *obj)
+{
+	const struct btf_type *t;
+	struct extern_desc *ext;
+	int i, n, off, btf_id;
+	struct btf_type *sec;
+	const char *ext_name;
+	Elf_Scn *scn;
+	GElf_Shdr sh;
+
+	if (!obj->efile.symbols)
+		return 0;
+
+	scn = elf_getscn(obj->efile.elf, obj->efile.symbols_shndx);
+	if (!scn)
+		return -LIBBPF_ERRNO__FORMAT;
+	if (gelf_getshdr(scn, &sh) != &sh)
+		return -LIBBPF_ERRNO__FORMAT;
+	n = sh.sh_size / sh.sh_entsize;
+
+	pr_debug("looking for externs among %d symbols...\n", n);
+	for (i = 0; i < n; i++) {
+		GElf_Sym sym;
+
+		if (!gelf_getsym(obj->efile.symbols, i, &sym))
+			return -LIBBPF_ERRNO__FORMAT;
+		if (!sym_is_extern(&sym))
+			continue;
+		ext_name = elf_strptr(obj->efile.elf, obj->efile.strtabidx,
+				      sym.st_name);
+		if (!ext_name || !ext_name[0])
+			continue;
+
+		ext = obj->externs;
+		ext = reallocarray(ext, obj->nr_extern + 1, sizeof(*ext));
+		if (!ext)
+			return -ENOMEM;
+		obj->externs = ext;
+		ext = &ext[obj->nr_extern];
+		memset(ext, 0, sizeof(*ext));
+		obj->nr_extern++;
+
+		ext->btf_id = find_extern_btf_id(obj->btf, ext_name);
+		if (ext->btf_id <= 0) {
+			pr_warn("failed to find BTF for extern '%s': %d\n",
+				ext_name, ext->btf_id);
+			return ext->btf_id;
+		}
+		t = btf__type_by_id(obj->btf, ext->btf_id);
+		ext->name = btf__name_by_offset(obj->btf, t->name_off);
+		ext->sym_idx = i;
+		ext->is_weak = GELF_ST_BIND(sym.st_info) == STB_WEAK;
+		ext->sz = btf__resolve_size(obj->btf, t->type);
+		if (ext->sz <= 0) {
+			pr_warn("failed to resolve size of extern '%s': %d\n",
+				ext_name, ext->sz);
+			return ext->sz;
+		}
+		ext->align = btf__align_of(obj->btf, t->type);
+		if (ext->align <= 0) {
+			pr_warn("failed to determine alignment of extern '%s': %d\n",
+				ext_name, ext->align);
+			return -EINVAL;
+		}
+		ext->type = find_extern_type(obj->btf, t->type,
+					     &ext->is_signed);
+		if (ext->type == EXT_UNKNOWN) {
+			pr_warn("extern '%s' type is unsupported\n", ext_name);
+			return -ENOTSUP;
+		}
+	}
+	pr_debug("collected %d externs total\n", obj->nr_extern);
+
+	if (!obj->nr_extern)
+		return 0;
+
+	/* sort externs by (alignment, size, name) and calculate their offsets
+	 * within a map */
+	qsort(obj->externs, obj->nr_extern, sizeof(*ext), cmp_externs);
+	off = 0;
+	for (i = 0; i < obj->nr_extern; i++) {
+		ext = &obj->externs[i];
+		ext->data_off = roundup(off, ext->align);
+		off = ext->data_off + ext->sz;
+		pr_debug("extern #%d: symbol %d, off %u, name %s\n",
+			 i, ext->sym_idx, ext->data_off, ext->name);
+	}
+
+	btf_id = btf__find_by_name(obj->btf, EXTERN_SEC);
+	if (btf_id <= 0) {
+		pr_warn("no BTF info found for '%s' datasec\n", EXTERN_SEC);
+		return -ESRCH;
+	}
+
+	sec = (struct btf_type *)btf__type_by_id(obj->btf, btf_id);
+	sec->size = off;
+	n = btf_vlen(sec);
+	for (i = 0; i < n; i++) {
+		struct btf_var_secinfo *vs = btf_var_secinfos(sec) + i;
+
+		t = btf__type_by_id(obj->btf, vs->type);
+		ext_name = btf__name_by_offset(obj->btf, t->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);
+			return -ESRCH;
+		}
+		vs->offset = ext->data_off;
+		btf_var(t)->linkage = BTF_VAR_GLOBAL_ALLOCATED;
+	}
+
+	return 0;
+}
+
 static struct bpf_program *
 bpf_object__find_prog_by_idx(struct bpf_object *obj, int idx)
 {
@@ -1796,6 +2333,8 @@ bpf_object__section_to_libbpf_map_type(const struct bpf_object *obj, int shndx)
 		return LIBBPF_MAP_BSS;
 	else if (shndx == obj->efile.rodata_shndx)
 		return LIBBPF_MAP_RODATA;
+	else if (shndx == obj->efile.symbols_shndx)
+		return LIBBPF_MAP_EXTERN;
 	else
 		return LIBBPF_MAP_UNSPEC;
 }
@@ -1840,6 +2379,30 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
 			insn_idx, insn->code);
 		return -LIBBPF_ERRNO__RELOC;
 	}
+
+	if (sym_is_extern(sym)) {
+		int sym_idx = GELF_R_SYM(rel->r_info);
+		int i, n = obj->nr_extern;
+		struct extern_desc *ext;
+
+		for (i = 0; i < n; i++) {
+			ext = &obj->externs[i];
+			if (ext->sym_idx == sym_idx)
+				break;
+		}
+		if (i >= n) {
+			pr_warn("extern relo failed to find extern for sym %d\n",
+				sym_idx);
+			return -LIBBPF_ERRNO__RELOC;
+		}
+		pr_debug("found extern #%d '%s' (sym %d, off %u) for insn %u\n",
+			 i, ext->name, ext->sym_idx, ext->data_off, insn_idx);
+		reloc_desc->type = RELO_EXTERN;
+		reloc_desc->insn_idx = insn_idx;
+		reloc_desc->sym_off = ext->data_off;
+		return 0;
+	}
+
 	if (!shdr_idx || shdr_idx >= SHN_LORESERVE) {
 		pr_warn("invalid relo for \'%s\' in special section 0x%x; forgot to initialize global var?..\n",
 			name, shdr_idx);
@@ -2301,11 +2864,12 @@ bpf_object__reuse_map(struct bpf_map *map)
 static int
 bpf_object__populate_internal_map(struct bpf_object *obj, struct bpf_map *map)
 {
+	enum libbpf_map_type map_type = map->libbpf_type;
 	char *cp, errmsg[STRERR_BUFSIZE];
 	int err, zero = 0;
 
-	/* Nothing to do here since kernel already zero-initializes .bss map. */
-	if (map->libbpf_type == LIBBPF_MAP_BSS)
+	/* kernel already zero-initializes .bss map. */
+	if (map_type == LIBBPF_MAP_BSS)
 		return 0;
 
 	err = bpf_map_update_elem(map->fd, &zero, map->mmaped, 0);
@@ -2317,8 +2881,8 @@ bpf_object__populate_internal_map(struct bpf_object *obj, struct bpf_map *map)
 		return err;
 	}
 
-	/* Freeze .rodata map as read-only from syscall side. */
-	if (map->libbpf_type == LIBBPF_MAP_RODATA) {
+	/* Freeze .rodata and .extern map as read-only from syscall side. */
+	if (map_type == LIBBPF_MAP_RODATA || map_type == LIBBPF_MAP_EXTERN) {
 		err = bpf_map_freeze(map->fd);
 		if (err) {
 			err = -errno;
@@ -3567,9 +4131,6 @@ bpf_program__reloc_text(struct bpf_program *prog, struct bpf_object *obj,
 	size_t new_cnt;
 	int err;
 
-	if (relo->type != RELO_CALL)
-		return -LIBBPF_ERRNO__RELOC;
-
 	if (prog->idx == obj->efile.text_shndx) {
 		pr_warn("relo in .text insn %d into off %d (insn #%d)\n",
 			relo->insn_idx, relo->sym_off, relo->sym_off / 8);
@@ -3631,27 +4192,37 @@ bpf_program__relocate(struct bpf_program *prog, struct bpf_object *obj)
 
 	for (i = 0; i < prog->nr_reloc; i++) {
 		struct reloc_desc *relo = &prog->reloc_desc[i];
+		struct bpf_insn *insn = &prog->insns[relo->insn_idx];
 
-		if (relo->type == RELO_LD64 || relo->type == RELO_DATA) {
-			struct bpf_insn *insn = &prog->insns[relo->insn_idx];
-
-			if (relo->insn_idx + 1 >= (int)prog->insns_cnt) {
-				pr_warn("relocation out of range: '%s'\n",
-					prog->section_name);
-				return -LIBBPF_ERRNO__RELOC;
-			}
+		if (relo->insn_idx + 1 >= (int)prog->insns_cnt) {
+			pr_warn("relocation out of range: '%s'\n",
+				prog->section_name);
+			return -LIBBPF_ERRNO__RELOC;
+		}
 
-			if (relo->type != RELO_DATA) {
-				insn[0].src_reg = BPF_PSEUDO_MAP_FD;
-			} else {
-				insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
-				insn[1].imm = insn[0].imm + relo->sym_off;
-			}
+		switch (relo->type) {
+		case RELO_LD64:
+			insn[0].src_reg = BPF_PSEUDO_MAP_FD;
+			insn[0].imm = obj->maps[relo->map_idx].fd;
+			break;
+		case RELO_DATA:
+			insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
+			insn[1].imm = insn[0].imm + relo->sym_off;
 			insn[0].imm = obj->maps[relo->map_idx].fd;
-		} else if (relo->type == RELO_CALL) {
+			break;
+		case RELO_EXTERN:
+			insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
+			insn[0].imm = obj->maps[obj->extern_map_idx].fd;
+			insn[1].imm = relo->sym_off;
+			break;
+		case RELO_CALL:
 			err = bpf_program__reloc_text(prog, obj, relo);
 			if (err)
 				return err;
+			break;
+		default:
+			pr_warn("relo #%d: bad relo type %d\n", i, relo->type);
+			return -EINVAL;
 		}
 	}
 
@@ -3931,11 +4502,10 @@ static struct bpf_object *
 __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 		   const struct bpf_object_open_opts *opts)
 {
+	const char *obj_name, *kconfig_path;
 	struct bpf_program *prog;
 	struct bpf_object *obj;
-	const char *obj_name;
 	char tmp_name[64];
-	__u32 attach_prog_fd;
 	int err;
 
 	if (elf_version(EV_CURRENT) == EV_NONE) {
@@ -3964,11 +4534,18 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 		return obj;
 
 	obj->relaxed_core_relocs = OPTS_GET(opts, relaxed_core_relocs, false);
-	attach_prog_fd = OPTS_GET(opts, attach_prog_fd, 0);
+	kconfig_path = OPTS_GET(opts, kconfig_path, NULL);
+	if (kconfig_path) {
+		obj->kconfig_path = strdup(kconfig_path);
+		if (!obj->kconfig_path)
+			return ERR_PTR(-ENOMEM);
+	}
 
 	err = bpf_object__elf_init(obj);
 	err = err ? : bpf_object__check_endianness(obj);
 	err = err ? : bpf_object__elf_collect(obj);
+	err = err ? : bpf_object__collect_externs(obj);
+	err = err ? : bpf_object__finalize_btf(obj);
 	err = err ? : bpf_object__init_maps(obj, opts);
 	err = err ? : bpf_object__init_prog_names(obj);
 	err = err ? : bpf_object__collect_reloc(obj);
@@ -3991,7 +4568,7 @@ __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
 		bpf_program__set_type(prog, prog_type);
 		bpf_program__set_expected_attach_type(prog, attach_type);
 		if (prog_type == BPF_PROG_TYPE_TRACING)
-			prog->attach_prog_fd = attach_prog_fd;
+			prog->attach_prog_fd = OPTS_GET(opts, attach_prog_fd, 0);
 	}
 
 	return obj;
@@ -4102,6 +4679,61 @@ static int bpf_object__sanitize_maps(struct bpf_object *obj)
 	return 0;
 }
 
+static int bpf_object__resolve_externs(struct bpf_object *obj,
+				       const char *config_path)
+{
+	bool need_config = false;
+	struct extern_desc *ext;
+	int err, i;
+	void *data;
+
+	if (obj->nr_extern == 0)
+		return 0;
+
+	data = obj->maps[obj->extern_map_idx].mmaped;
+
+	for (i = 0; i < obj->nr_extern; i++) {
+		ext = &obj->externs[i];
+
+		if (strcmp(ext->name, "LINUX_KERNEL_VERSION") == 0) {
+			void *ext_val = data + ext->data_off;
+			__u3