bpf tools: Introduce 'bpf' library and add bpf feature check

This is the first patch of libbpf. The goal of libbpf is to create a
standard way for accessing eBPF object files. This patch creates
'Makefile' and 'Build' for it, allows 'make' to build libbpf.a and
libbpf.so, 'make install' to put them into proper directories.
Most part of Makefile is borrowed from traceevent.

Before building, it checks the existence of libelf in Makefile, and deny
to build if not found. Instead of throwing an error if libelf not found,
the error raises in a phony target "elfdep". This design is to ensure
'make clean' still workable even if libelf is not found.

Because libbpf requires 'kern_version' field set for 'union bpf_attr'
(bpfdep" is used for that dependency), Kernel BPF API is also checked
by intruducing a new feature check 'bpf' into tools/build/feature,
which checks the existence and version of linux/bpf.h. When building
libbpf, it searches that file from include/uapi/linux in kernel source
tree (controlled by FEATURE_CHECK_CFLAGS-bpf). Since it searches kernel
source tree it reside, installing of newest kernel headers is not
required, except we are trying to port these files to an old kernel.

To avoid checking that file when perf building, the newly introduced
'bpf' feature check doesn't added into FEATURE_TESTS and
FEATURE_DISPLAY by default in tools/build/Makefile.feature, but added
into libbpf's specific.

Signed-off-by: Wang Nan <[e-mail]>
Acked-by: Alexei Starovoitov <[e-mail]>
Cc: Brendan Gregg <[e-mail]>
Cc: Daniel Borkmann <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: He Kuang <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Kaixu Xia <[e-mail]>
Cc: Masami Hiramatsu <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Zefan Li <[e-mail]>
Bcc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/build/feature/Makefile

···
 	test-compile-32.bin		\
 	test-compile-x32.bin		\
 	test-zlib.bin			\
-	test-lzma.bin
+	test-lzma.bin			\
+	test-bpf.bin
 
 CC := $(CROSS_COMPILE)gcc -MD
 PKG_CONFIG := $(CROSS_COMPILE)pkg-config
···
 
 test-lzma.bin:
 	$(BUILD) -llzma
+
+test-bpf.bin:
+	$(BUILD)
 
 -include *.d
 

tools/build/feature/test-bpf.c

···
+#include <linux/bpf.h>
+
+int main(void)
+{
+	union bpf_attr attr;
+
+	attr.prog_type = BPF_PROG_TYPE_KPROBE;
+	attr.insn_cnt = 0;
+	attr.insns = 0;
+	attr.license = 0;
+	attr.log_buf = 0;
+	attr.log_size = 0;
+	attr.log_level = 0;
+	attr.kern_version = 0;
+
+	attr = attr;
+	return 0;
+}

tools/lib/bpf/.gitignore

···
+libbpf_version.h
+FEATURE-DUMP

tools/lib/bpf/Build

···
+libbpf-y := libbpf.o

tools/lib/bpf/Makefile

···
+# Most of this file is copied from tools/lib/traceevent/Makefile
+
+BPF_VERSION = 0
+BPF_PATCHLEVEL = 0
+BPF_EXTRAVERSION = 1
+
+MAKEFLAGS += --no-print-directory
+
+
+# Makefiles suck: This macro sets a default value of $(2) for the
+# variable named by $(1), unless the variable has been set by
+# environment or command line. This is necessary for CC and AR
+# because make sets default values, so the simpler ?= approach
+# won't work as expected.
+define allow-override
+  $(if $(or $(findstring environment,$(origin $(1))),\
+            $(findstring command line,$(origin $(1)))),,\
+    $(eval $(1) = $(2)))
+endef
+
+# Allow setting CC and AR, or setting CROSS_COMPILE as a prefix.
+$(call allow-override,CC,$(CROSS_COMPILE)gcc)
+$(call allow-override,AR,$(CROSS_COMPILE)ar)
+
+INSTALL = install
+
+# Use DESTDIR for installing into a different root directory.
+# This is useful for building a package. The program will be
+# installed in this directory as if it was the root directory.
+# Then the build tool can move it later.
+DESTDIR ?=
+DESTDIR_SQ = '$(subst ','\'',$(DESTDIR))'
+
+LP64 := $(shell echo __LP64__ | ${CC} ${CFLAGS} -E -x c - | tail -n 1)
+ifeq ($(LP64), 1)
+  libdir_relative = lib64
+else
+  libdir_relative = lib
+endif
+
+prefix ?= /usr/local
+libdir = $(prefix)/$(libdir_relative)
+man_dir = $(prefix)/share/man
+man_dir_SQ = '$(subst ','\'',$(man_dir))'
+
+export man_dir man_dir_SQ INSTALL
+export DESTDIR DESTDIR_SQ
+
+include ../../scripts/Makefile.include
+
+# copy a bit from Linux kbuild
+
+ifeq ("$(origin V)", "command line")
+  VERBOSE = $(V)
+endif
+ifndef VERBOSE
+  VERBOSE = 0
+endif
+
+ifeq ($(srctree),)
+srctree := $(patsubst %/,%,$(dir $(shell pwd)))
+srctree := $(patsubst %/,%,$(dir $(srctree)))
+srctree := $(patsubst %/,%,$(dir $(srctree)))
+#$(info Determined 'srctree' to be $(srctree))
+endif
+
+FEATURE_DISPLAY = libelf libelf-getphdrnum libelf-mmap bpf
+FEATURE_TESTS = libelf bpf
+
+INCLUDES = -I. -I$(srctree)/tools/include -I$(srctree)/arch/$(ARCH)/include/uapi -I$(srctree)/include/uapi
+FEATURE_CHECK_CFLAGS-bpf = $(INCLUDES)
+
+include $(srctree)/tools/build/Makefile.feature
+
+export prefix libdir src obj
+
+# Shell quotes
+libdir_SQ = $(subst ','\'',$(libdir))
+libdir_relative_SQ = $(subst ','\'',$(libdir_relative))
+plugin_dir_SQ = $(subst ','\'',$(plugin_dir))
+
+LIB_FILE = libbpf.a libbpf.so
+
+VERSION		= $(BPF_VERSION)
+PATCHLEVEL	= $(BPF_PATCHLEVEL)
+EXTRAVERSION	= $(BPF_EXTRAVERSION)
+
+OBJ		= $@
+N		=
+
+LIBBPF_VERSION = $(BPF_VERSION).$(BPF_PATCHLEVEL).$(BPF_EXTRAVERSION)
+
+# Set compile option CFLAGS
+ifdef EXTRA_CFLAGS
+  CFLAGS := $(EXTRA_CFLAGS)
+else
+  CFLAGS := -g -Wall
+endif
+
+ifeq ($(feature-libelf-mmap), 1)
+  override CFLAGS += -DHAVE_LIBELF_MMAP_SUPPORT
+endif
+
+ifeq ($(feature-libelf-getphdrnum), 1)
+  override CFLAGS += -DHAVE_ELF_GETPHDRNUM_SUPPORT
+endif
+
+# Append required CFLAGS
+override CFLAGS += $(EXTRA_WARNINGS)
+override CFLAGS += -Werror -Wall
+override CFLAGS += -fPIC
+override CFLAGS += $(INCLUDES)
+
+ifeq ($(VERBOSE),1)
+  Q =
+else
+  Q = @
+endif
+
+# Disable command line variables (CFLAGS) overide from top
+# level Makefile (perf), otherwise build Makefile will get
+# the same command line setup.
+MAKEOVERRIDES=
+
+export srctree OUTPUT CC LD CFLAGS V
+build := -f $(srctree)/tools/build/Makefile.build dir=. obj
+
+BPF_IN    := $(OUTPUT)libbpf-in.o
+LIB_FILE := $(addprefix $(OUTPUT),$(LIB_FILE))
+
+CMD_TARGETS = $(LIB_FILE)
+
+TARGETS = $(CMD_TARGETS)
+
+all: $(VERSION_FILES) all_cmd
+
+all_cmd: $(CMD_TARGETS)
+
+$(BPF_IN): force elfdep bpfdep
+	$(Q)$(MAKE) $(build)=libbpf
+
+$(OUTPUT)libbpf.so: $(BPF_IN)
+	$(QUIET_LINK)$(CC) --shared $^ -o $@
+
+$(OUTPUT)libbpf.a: $(BPF_IN)
+	$(QUIET_LINK)$(RM) $@; $(AR) rcs $@ $^
+
+define update_dir
+  (echo $1 > $@.tmp;				\
+   if [ -r $@ ] && cmp -s $@ $@.tmp; then	\
+     rm -f $@.tmp;				\
+   else						\
+     echo '  UPDATE                 $@';	\
+     mv -f $@.tmp $@;				\
+   fi);
+endef
+
+define do_install
+	if [ ! -d '$(DESTDIR_SQ)$2' ]; then		\
+		$(INSTALL) -d -m 755 '$(DESTDIR_SQ)$2';	\
+	fi;						\
+	$(INSTALL) $1 '$(DESTDIR_SQ)$2'
+endef
+
+install_lib: all_cmd
+	$(call QUIET_INSTALL, $(LIB_FILE)) \
+		$(call do_install,$(LIB_FILE),$(libdir_SQ))
+
+install: install_lib
+
+### Cleaning rules
+
+config-clean:
+	$(call QUIET_CLEAN, config)
+	$(Q)$(MAKE) -C $(srctree)/tools/build/feature/ clean >/dev/null
+
+clean:
+	$(call QUIET_CLEAN, libbpf) $(RM) *.o *~ $(TARGETS) *.a *.so $(VERSION_FILES) .*.d \
+		$(RM) LIBBPF-CFLAGS
+	$(call QUIET_CLEAN, core-gen) $(RM) $(OUTPUT)FEATURE-DUMP
+
+
+
+PHONY += force elfdep bpfdep
+force:
+
+elfdep:
+	@if [ "$(feature-libelf)" != "1" ]; then echo "No libelf found"; exit -1 ; fi
+
+bpfdep:
+	@if [ "$(feature-bpf)" != "1" ]; then echo "BPF API too old"; exit -1 ; fi
+
+# Declare the contents of the .PHONY variable as phony.  We keep that
+# information in a variable so we can use it in if_changed and friends.
+.PHONY: $(PHONY)

tools/lib/bpf/libbpf.c

···
+/*
+ * Common eBPF ELF object loading operations.
+ *
+ * Copyright (C) 2013-2015 Alexei Starovoitov <[e-mail]>
+ * Copyright (C) 2015 Wang Nan <[e-mail]>
+ * Copyright (C) 2015 Huawei Inc.
+ */
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <asm/unistd.h>
+#include <linux/bpf.h>
+
+#include "libbpf.h"

tools/lib/bpf/libbpf.h

···
+/*
+ * Common eBPF ELF object loading operations.
+ *
+ * Copyright (C) 2013-2015 Alexei Starovoitov <[e-mail]>
+ * Copyright (C) 2015 Wang Nan <[e-mail]>
+ * Copyright (C) 2015 Huawei Inc.
+ */
+#ifndef __BPF_LIBBPF_H
+#define __BPF_LIBBPF_H
+
+#endif