libbpf: Support libbpf-provided extern variables

Add support for extern variables, provided to BPF program by libbpf. Currently
the following extern variables are supported:
  - LINUX_KERNEL_VERSION; version of a kernel in which BPF program is
    executing, follows KERNEL_VERSION() macro convention, can be 4- and 8-byte
    long;
  - CONFIG_xxx values; a set of values of actual kernel config. Tristate,
    boolean, strings, and integer values are supported.

Set of possible values is determined by declared type of extern variable.
Supported types of variables are:
- Tristate values. Are represented as `enum libbpf_tristate`. Accepted values
  are **strictly** 'y', 'n', or 'm', which are represented as TRI_YES, TRI_NO,
  or TRI_MODULE, respectively.
- Boolean values. Are represented as bool (_Bool) types. Accepted values are
  'y' and 'n' only, turning into true/false values, respectively.
- Single-character values. Can be used both as a substritute for
  bool/tristate, or as a small-range integer:
  - 'y'/'n'/'m' are represented as is, as characters 'y', 'n', or 'm';
  - integers in a range [-128, 127] or [0, 255] (depending on signedness of
    char in target architecture) are recognized and represented with
    respective values of char type.
- Strings. String values are declared as fixed-length char arrays. String of
  up to that length will be accepted and put in first N bytes of char array,
  with the rest of bytes zeroed out. If config string value is longer than
  space alloted, it will be truncated and warning message emitted. Char array
  is always zero terminated. String literals in config have to be enclosed in
  double quotes, just like C-style string literals.
- Integers. 8-, 16-, 32-, and 64-bit integers are supported, both signed and
  unsigned variants. Libbpf enforces parsed config value to be in the
  supported range of corresponding integer type. Integers values in config can
  be:
  - decimal integers, with optional + and - signs;
  - hexadecimal integers, prefixed with 0x or 0X;
  - octal integers, starting with 0.

Config file itself is searched in /boot/config-$(uname -r) location with
fallback to /proc/config.gz, unless config path is specified explicitly
through bpf_object_open_opts' kernel_config_path option. Both gzipped and
plain text formats are supported. Libbpf adds explicit dependency on zlib
because of this, but this shouldn't be a problem, given libelf already depends
on zlib.

All detected extern variables, are put into a separate .extern internal map.
It, similarly to .rodata map, is marked as read-only from BPF program side, as
well as is frozen on load. This allows BPF verifier to track extern values as
constants and perform enhanced branch prediction and dead code elimination.
This can be relied upon for doing kernel version/feature detection and using
potentially unsupported field relocations or BPF helpers in a CO-RE-based BPF
program, while still having a single version of BPF program running on old and
new kernels. Selftests are validating this explicitly for unexisting BPF
helper.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191214014710.3449601-3-andriin@fb.com

include/uapi/linux/btf.h

@@ -142,7 +142,8 @@ struct btf_param {
 
 enum {
 	BTF_VAR_STATIC = 0,
-	BTF_VAR_GLOBAL_ALLOCATED,
+	BTF_VAR_GLOBAL_ALLOCATED = 1,
+	BTF_VAR_GLOBAL_EXTERN = 2,
 };
 
 /* BTF_KIND_VAR is followed by a single "struct btf_var" to describe

tools/include/uapi/linux/btf.h

@@ -22,9 +22,9 @@ struct btf_header {
 };
 
 /* Max # of type identifier */
-#define BTF_MAX_TYPE	0x0000ffff
+#define BTF_MAX_TYPE	0x000fffff
 /* Max offset into the string section */
-#define BTF_MAX_NAME_OFFSET	0x0000ffff
+#define BTF_MAX_NAME_OFFSET	0x00ffffff
 /* Max # of struct/union/enum members or func args */
 #define BTF_MAX_VLEN	0xffff
 
@@ -142,7 +142,8 @@ struct btf_param {
 
 enum {
 	BTF_VAR_STATIC = 0,
-	BTF_VAR_GLOBAL_ALLOCATED,
+	BTF_VAR_GLOBAL_ALLOCATED = 1,
+	BTF_VAR_GLOBAL_EXTERN = 2,
 };
 
 /* BTF_KIND_VAR is followed by a single "struct btf_var" to describe

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
 

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

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;
 }