[PATCHv5 iproute2-next 1/5] configure: add check_libbpf() for later libbpf support

[Hangbin Liu <haliu@redhat.com>]

This patch adds a check to see if we have libbpf support. By default the
system libbpf will be used, but static linking against a custom libbpf
version can be achieved by passing libbpf DESTDIR to variable LIBBPF_DIR for
configure.

Add another variable LIBBPF_FORCE to control whether to build iproute2
with libbpf. If set to on, then force to build with libbpf and exit if
not available. If set to off, then force to not build with libbpf.

Signed-off-by: Hangbin Liu <haliu@redhat.com>
---
v5:
1) Fix LIBBPF_DIR typo and description, use libbpf DESTDIR as LIBBPF_DIR
   dest.

v4:
1) Remove duplicate LIBBPF_CFLAGS
2) Remove un-needed -L since using static libbpf.a
3) Fix == not supported in dash
4) Extend LIBBPF_FORCE to support on/off, when set to on, stop building when
   there is no libbpf support. If set to off, discard libbpf check.
5) Print libbpf version after checking

v3:
Check function bpf_program__section_name() separately and only use it
on higher libbpf version.

v2:
No update
---
 configure | 108 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 108 insertions(+)

diff --git a/configure b/configure
index 307912aa..290b4c86 100755
--- a/configure
+++ b/configure
@@ -2,6 +2,11 @@
 # SPDX-License-Identifier: GPL-2.0
 # This is not an autoconf generated configure
 #
+# Influential LIBBPF environment variables:
+#   LIBBPF_FORCE={on,off}   on: require link against libbpf;
+#                           off: disable libbpf probing
+#   LIBBPF_DIR              Path to libbpf DESTDIR to use
+
 INCLUDE=${1:-"$PWD/include"}
 
 # Output file which is input to Makefile
@@ -240,6 +245,106 @@ check_elf()
     fi
 }
 
+have_libbpf_basic()
+{
+    cat >$TMPDIR/libbpf_test.c <<EOF
+#include <bpf/libbpf.h>
+int main(int argc, char **argv) {
+    bpf_program__set_autoload(NULL, false);
+    bpf_map__ifindex(NULL);
+    bpf_map__set_pin_path(NULL, NULL);
+    bpf_object__open_file(NULL, NULL);
+    return 0;
+}
+EOF
+
+    $CC -o $TMPDIR/libbpf_test $TMPDIR/libbpf_test.c $LIBBPF_CFLAGS $LIBBPF_LDLIBS >/dev/null 2>&1
+    local ret=$?
+
+    rm -f $TMPDIR/libbpf_test.c $TMPDIR/libbpf_test
+    return $ret
+}
+
+have_libbpf_sec_name()
+{
+    cat >$TMPDIR/libbpf_sec_test.c <<EOF
+#include <bpf/libbpf.h>
+int main(int argc, char **argv) {
+    void *ptr;
+    bpf_program__section_name(NULL);
+    return 0;
+}
+EOF
+
+    $CC -o $TMPDIR/libbpf_sec_test $TMPDIR/libbpf_sec_test.c $LIBBPF_CFLAGS $LIBBPF_LDLIBS >/dev/null 2>&1
+    local ret=$?
+
+    rm -f $TMPDIR/libbpf_sec_test.c $TMPDIR/libbpf_sec_test
+    return $ret
+}
+
+check_force_libbpf_on()
+{
+    # if set LIBBPF_FORCE=on but no libbpf support, just exist the config
+    # process to make sure we don't build without libbpf.
+    if [ "$LIBBPF_FORCE" = on ]; then
+        echo "	LIBBPF_FORCE=on set, but couldn't find a usable libbpf"
+        exit 1
+    fi
+}
+
+check_libbpf()
+{
+    # if set LIBBPF_FORCE=off, disable libbpf entirely
+    if [ "$LIBBPF_FORCE" = off ]; then
+        echo "no"
+        return
+    fi
+
+    if ! ${PKG_CONFIG} libbpf --exists && [ -z "$LIBBPF_DIR" ] ; then
+        echo "no"
+        check_force_libbpf_on
+        return
+    fi
+
+    if [ $(uname -m) = x86_64 ]; then
+        local LIBBPF_LIBDIR="${LIBBPF_DIR}/usr/lib64"
+    else
+        local LIBBPF_LIBDIR="${LIBBPF_DIR}/usr/lib"
+    fi
+
+    if [ -n "$LIBBPF_DIR" ]; then
+        LIBBPF_CFLAGS="-I${LIBBPF_DIR}/usr/include"
+        LIBBPF_LDLIBS="${LIBBPF_LIBDIR}/libbpf.a -lz -lelf"
+        LIBBPF_VERSION=$(PKG_CONFIG_LIBDIR=${LIBBPF_LIBDIR}/pkgconfig ${PKG_CONFIG} libbpf --modversion)
+    else
+        LIBBPF_CFLAGS=$(${PKG_CONFIG} libbpf --cflags)
+        LIBBPF_LDLIBS=$(${PKG_CONFIG} libbpf --libs)
+        LIBBPF_VERSION=$(${PKG_CONFIG} libbpf --modversion)
+    fi
+
+    if ! have_libbpf_basic; then
+        echo "no"
+        echo "	libbpf version $LIBBPF_VERSION is too low, please update it to at least 0.1.0"
+        check_force_libbpf_on
+        return
+    else
+        echo "HAVE_LIBBPF:=y" >>$CONFIG
+        echo 'CFLAGS += -DHAVE_LIBBPF ' $LIBBPF_CFLAGS >> $CONFIG
+        echo 'LDLIBS += ' $LIBBPF_LDLIBS >>$CONFIG
+    fi
+
+    # bpf_program__title() is deprecated since libbpf 0.2.0, use
+    # bpf_program__section_name() instead if we support
+    if have_libbpf_sec_name; then
+        echo "HAVE_LIBBPF_SECTION_NAME:=y" >>$CONFIG
+        echo 'CFLAGS += -DHAVE_LIBBPF_SECTION_NAME ' >> $CONFIG
+    fi
+
+    echo "yes"
+    echo "	libbpf version $LIBBPF_VERSION"
+}
+
 check_selinux()
 # SELinux is a compile time option in the ss utility
 {
@@ -385,6 +490,9 @@ check_setns
 echo -n "SELinux support: "
 check_selinux
 
+echo -n "libbpf support: "
+check_libbpf
+
 echo -n "ELF support: "
 check_elf
 
-- 
2.25.4