[PATCH 3/3] bpftool: Allow to link libbpf dynamically

[Jiri Olsa <jolsa@kernel.org>]

Currently we support only static linking with kernel's libbpf
(tools/lib/bpf). This patch adds LIBBPF_DYNAMIC compile variable
that triggers libbpf detection and bpf dynamic linking:

  $ make -C tools/bpf/bpftool make LIBBPF_DYNAMIC=1

If libbpf is not installed, build (with LIBBPF_DYNAMIC=1) stops with:

  $ make -C tools/bpf/bpftool LIBBPF_DYNAMIC=1
    Auto-detecting system features:
    ...                        libbfd: [ on  ]
    ...        disassembler-four-args: [ on  ]
    ...                          zlib: [ on  ]
    ...                        libbpf: [ OFF ]

  Makefile:102: *** Error: libbpf-devel is missing, please install it.  Stop.

Adding specific bpftool's libbpf check for libbpf_netlink_open (LIBBPF_0.0.6)
which is the latest we need for bpftool at the moment.

Adding LIBBPF_DIR compile variable to allow linking with
libbpf installed into specific directory:

  $ make -C tools/lib/bpf/ prefix=/tmp/libbpf/ install_lib install_headers
  $ make -C tools/bpf/bpftool/ LIBBPF_DYNAMIC=1 LIBBPF_DIR=/tmp/libbpf/

It might be needed to clean build tree first because features
framework does not detect the change properly:

  $ make -C tools/build/feature clean
  $ make -C tools/bpf/bpftool/ clean

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 tools/bpf/bpftool/Makefile        | 40 ++++++++++++++++++++++++++++++-
 tools/build/feature/test-libbpf.c |  9 +++++++
 2 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
index 39bc6f0f4f0b..2b6ed08cb31e 100644
--- a/tools/bpf/bpftool/Makefile
+++ b/tools/bpf/bpftool/Makefile
@@ -1,6 +1,15 @@
 # SPDX-License-Identifier: GPL-2.0-only
+# LIBBPF_DYNAMIC to enable libbpf dynamic linking.
+
 include ../../scripts/Makefile.include
 include ../../scripts/utilities.mak
+include ../../scripts/Makefile.arch
+
+ifeq ($(LP64), 1)
+  libdir_relative = lib64
+else
+  libdir_relative = lib
+endif
 
 ifeq ($(srctree),)
 srctree := $(patsubst %/,%,$(dir $(CURDIR)))
@@ -55,7 +64,7 @@ ifneq ($(EXTRA_LDFLAGS),)
 LDFLAGS += $(EXTRA_LDFLAGS)
 endif
 
-LIBS = $(LIBBPF) -lelf -lz
+LIBS = -lelf -lz
 
 INSTALL ?= install
 RM ?= rm -f
@@ -64,6 +73,23 @@ FEATURE_USER = .bpftool
 FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib
 FEATURE_DISPLAY = libbfd disassembler-four-args zlib
 
+ifdef LIBBPF_DYNAMIC
+  # Add libbpf check with the flags to ensure bpftool
+  # specific version is detected.
+  FEATURE_CHECK_CFLAGS-libbpf := -DBPFTOOL
+  FEATURE_TESTS   += libbpf
+  FEATURE_DISPLAY += libbpf
+
+  # for linking with debug library run:
+  # make LIBBPF_DYNAMIC=1 LIBBPF_DIR=/opt/libbpf
+  ifdef LIBBPF_DIR
+    LIBBPF_CFLAGS  := -I$(LIBBPF_DIR)/include
+    LIBBPF_LDFLAGS := -L$(LIBBPF_DIR)/$(libdir_relative)
+    FEATURE_CHECK_CFLAGS-libbpf  := $(LIBBPF_CFLAGS)
+    FEATURE_CHECK_LDFLAGS-libbpf := $(LIBBPF_LDFLAGS)
+  endif
+endif
+
 check_feat := 1
 NON_CHECK_FEAT_TARGETS := clean uninstall doc doc-clean doc-install doc-uninstall
 ifdef MAKECMDGOALS
@@ -88,6 +114,18 @@ ifeq ($(feature-reallocarray), 0)
 CFLAGS += -DCOMPAT_NEED_REALLOCARRAY
 endif
 
+ifdef LIBBPF_DYNAMIC
+  ifeq ($(feature-libbpf), 1)
+    LIBS    += -lbpf
+    CFLAGS  += $(LIBBPF_CFLAGS)
+    LDFLAGS += $(LIBBPF_LDFLAGS)
+  else
+    dummy := $(error Error: No libbpf devel library found, please install libbpf-devel)
+  endif
+else
+  LIBS += $(LIBBPF)
+endif
+
 include $(wildcard $(OUTPUT)*.d)
 
 all: $(OUTPUT)bpftool
diff --git a/tools/build/feature/test-libbpf.c b/tools/build/feature/test-libbpf.c
index a508756cf4cc..93566d105a64 100644
--- a/tools/build/feature/test-libbpf.c
+++ b/tools/build/feature/test-libbpf.c
@@ -3,5 +3,14 @@
 
 int main(void)
 {
+#ifdef BPFTOOL
+	/*
+	 * libbpf_netlink_open (LIBBPF_0.0.6) is the latest
+	 * we need for bpftool at the moment
+	 */
+	libbpf_netlink_open(NULL);
+	return 0;
+#else
 	return bpf_object__open("test") ? 0 : -1;
+#endif
 }
-- 
2.21.0