On Wed, Nov 27, 2019 at 01:38:55PM +0000, Quentin Monnet wrote:

2019-11-27 10:48 UTC+0100 ~ Jiri Olsa [off-list ref]

quoted

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

quoted

@@ -55,7 +64,7 @@ ifneq ($(EXTRA_LDFLAGS),)
 LDFLAGS += $(EXTRA_LDFLAGS)
 endif
 
-LIBS = $(LIBBPF) -lelf -lz
+LIBS = -lelf -lz

Hi Jiri,

This change seems to be breaking the build with the static library for
me. I know you add back $(LIBBPF) later in the Makefile, see at the end
of this email...

quoted

 
 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.

Nit: We do not check for a specific bpftool version, we check for a
recent enough libbpf version?

hi,
we check for a version that has the latest exported function
that bpftool needs, which is currently libbpf_netlink_open

please check the '#ifdef BPFTOOL' in feature/test-libbpf.c

it's like that because there's currently no support to check
for particular library version in the build/features framework

I'll make that comment more clear

quoted

+  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)

libbpf-devel sounds like a RH/Fedora package name, but other
distributions might have different names (Debian/Ubuntu would go by
libbpf-dev I suppose, although I don't believe such package exists at
the moment). Maybe use a more generic message?

sure, actually in perf we use both package names like:

  Error: No libbpf devel library found, please install libbpf-devel or libbpf-dev.

or we can go with generic message:

  Error: No libbpf devel library found, please install.

quoted

+  endif
+else
+  LIBS += $(LIBBPF)

... I believe the order of the libraries is relevant, and it seems the
static libbpf should be passed before the dynamic libs. Here I could fix
the build with the static library on my setup by prepending the library
path instead, like this:

	LIBS := $(LIBBPF) $(LIBS)

could you please paste the build error? I don't see any on Fedora,
anyway I can make the change you're proposing

On the plus side, all build attempts from
tools/testing/selftests/bpf/test_bpftool_build.sh pass successfully on
my setup with dynamic linking from your branch.

cool, had no idea there was such test ;-)

thanks,
jirka