[PATCHv6 iproute2-next 1/5] iproute2: add check_libbpf() and get_libbpf_version()

[Hangbin Liu <haliu@redhat.com>]

This patch aim to add basic checking functions for later iproute2
libbpf support.

First we add check_libbpf() in configure to see if we have bpf library
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.

Another variable LIBBPF_FORCE is used 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.

When dynamically linking against libbpf, we can't be sure that the
version we discovered at compile time is actually the one we are
using at runtime. This can lead to hard-to-debug errors. So we add
a new file lib/bpf_glue.c and a helper function get_libbpf_version()
to get correct libbpf version at runtime.

Signed-off-by: Hangbin Liu <haliu@redhat.com>
---

v6:
1) Add a new helper get_libbpf_version() to get runtime libbpf version
  based on Toke's xdp-tools patch. The libbpf version will be printed
  when exec ip -V or tc -V.

v5:
1) Fix LIBBPF_DIR type 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          | 113 +++++++++++++++++++++++++++++++++++++++++++++
 include/bpf_util.h |   3 ++
 ip/ip.c            |  10 +++-
 lib/Makefile       |   2 +-
 lib/bpf_glue.c     |  63 +++++++++++++++++++++++++
 tc/tc.c            |  10 +++-
 6 files changed, 196 insertions(+), 5 deletions(-)
 create mode 100644 lib/bpf_glue.c

diff --git a/configure b/configure
index 307912aa..2c363d3b 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 +245,111 @@ 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 >> $CONFIG
+        echo "CFLAGS += -DLIBBPF_VERSION=\\\"$LIBBPF_VERSION\\\"" >> $CONFIG
+        echo 'LDLIBS += ' $LIBBPF_LDLIBS >> $CONFIG
+
+        if [ -z "$LIBBPF_DIR" ]; then
+            echo "CFLAGS += -DLIBBPF_DYNAMIC" >> $CONFIG
+        fi
+    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 +495,9 @@ check_setns
 echo -n "SELinux support: "
 check_selinux
 
+echo -n "libbpf support: "
+check_libbpf
+
 echo -n "ELF support: "
 check_elf
 
diff --git a/include/bpf_util.h b/include/bpf_util.h
index 63db07ca..dee5bb02 100644
--- a/include/bpf_util.h
+++ b/include/bpf_util.h
@@ -300,4 +300,7 @@ static inline int bpf_recv_map_fds(const char *path, int *fds,
 	return -1;
 }
 #endif /* HAVE_ELF */
+
+const char *get_libbpf_version(void);
+
 #endif /* __BPF_UTIL__ */
diff --git a/ip/ip.c b/ip/ip.c
index 5e31957f..466dbb52 100644
--- a/ip/ip.c
+++ b/ip/ip.c
@@ -24,6 +24,7 @@
 #include "namespace.h"
 #include "color.h"
 #include "rt_names.h"
+#include "bpf_util.h"
 
 int preferred_family = AF_UNSPEC;
 int human_readable;
@@ -147,8 +148,9 @@ static int batch(const char *name)
 
 int main(int argc, char **argv)
 {
-	char *basename;
+	const char *libbpf_version;
 	char *batch_file = NULL;
+	char *basename;
 	int color = 0;
 
 	/* to run vrf exec without root, capabilities might be set, drop them
@@ -229,7 +231,11 @@ int main(int argc, char **argv)
 			++timestamp;
 			++timestamp_short;
 		} else if (matches(opt, "-Version") == 0) {
-			printf("ip utility, iproute2-%s\n", version);
+			printf("ip utility, iproute2-%s", version);
+			libbpf_version = get_libbpf_version();
+			if (libbpf_version)
+				printf(", libbpf %s", libbpf_version);
+			printf("\n");
 			exit(0);
 		} else if (matches(opt, "-force") == 0) {
 			++force;
diff --git a/lib/Makefile b/lib/Makefile
index 13f4ee15..a02775a5 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -5,7 +5,7 @@ CFLAGS += -fPIC
 
 UTILOBJ = utils.o rt_names.o ll_map.o ll_types.o ll_proto.o ll_addr.o \
 	inet_proto.o namespace.o json_writer.o json_print.o \
-	names.o color.o bpf.o exec.o fs.o cg_map.o
+	names.o color.o bpf.o bpf_glue.o exec.o fs.o cg_map.o
 
 NLOBJ=libgenl.o libnetlink.o mnl_utils.o
 
diff --git a/lib/bpf_glue.c b/lib/bpf_glue.c
new file mode 100644
index 00000000..67c41c22
--- /dev/null
+++ b/lib/bpf_glue.c
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * bpf_glue.c:	BPF code to call both legacy and libbpf code
+ * Authors:	Hangbin Liu <haliu@redhat.com>
+ *
+ */
+#include "bpf_util.h"
+
+#ifdef HAVE_LIBBPF
+static const char *_libbpf_compile_version = LIBBPF_VERSION;
+static char _libbpf_version[10] = {};
+
+const char *get_libbpf_version(void)
+{
+	/* Start by copying compile-time version into buffer so we have a
+	 * fallback value in case we are dynamically linked, or can't find a
+	 * version in /proc/self/maps below.
+	 */
+	strncpy(_libbpf_version, _libbpf_compile_version,
+		sizeof(_libbpf_version)-1);
+#ifdef LIBBPF_DYNAMIC
+	char buf[PATH_MAX], *s;
+	bool found = false;
+	FILE *fp;
+
+	/* When dynamically linking against libbpf, we can't be sure that the
+	 * version we discovered at compile time is actually the one we are
+	 * using at runtime. This can lead to hard-to-debug errors, so we try to
+	 * discover the correct version at runtime.
+	 *
+	 * The simple solution to this would be if libbpf itself exported a
+	 * version in its API. But since it doesn't, we work around this by
+	 * parsing the mappings of the binary at runtime, looking for the full
+	 * filename of libbpf.so and using that.
+	 */
+	fp = fopen("/proc/self/maps", "r");
+	if (fp == NULL)
+		goto out;
+
+	while ((s = fgets(buf, sizeof(buf), fp)) != NULL) {
+		if ((s = strstr(buf, "libbpf.so.")) != NULL) {
+			strncpy(_libbpf_version, s+10, sizeof(_libbpf_version)-1);
+			strtok(_libbpf_version, "\n");
+			found = true;
+			break;
+		}
+	}
+
+	fclose(fp);
+out:
+	if (!found)
+		fprintf(stderr, "Couldn't find runtime libbpf version - falling back to compile-time value!\n");
+#endif /* LIBBPF_DYNAMIC */
+
+	_libbpf_version[sizeof(_libbpf_version)-1] = '\0';
+	return _libbpf_version;
+}
+#else
+const char *get_libbpf_version(void)
+{
+	return NULL;
+}
+#endif /* HAVE_LIBBPF */
diff --git a/tc/tc.c b/tc/tc.c
index af9b21da..7557b977 100644
--- a/tc/tc.c
+++ b/tc/tc.c
@@ -30,6 +30,7 @@
 #include "tc_common.h"
 #include "namespace.h"
 #include "rt_names.h"
+#include "bpf_util.h"
 
 int show_stats;
 int show_details;
@@ -259,8 +260,9 @@ static int batch(const char *name)
 
 int main(int argc, char **argv)
 {
-	int ret;
+	const char *libbpf_version;
 	char *batch_file = NULL;
+	int ret;
 
 	while (argc > 1) {
 		if (argv[1][0] != '-')
@@ -277,7 +279,11 @@ int main(int argc, char **argv)
 		} else if (matches(argv[1], "-graph") == 0) {
 			show_graph = 1;
 		} else if (matches(argv[1], "-Version") == 0) {
-			printf("tc utility, iproute2-%s\n", version);
+			printf("tc utility, iproute2-%s", version);
+			libbpf_version = get_libbpf_version();
+			if (libbpf_version)
+				printf(", libbpf %s", libbpf_version);
+			printf("\n");
 			return 0;
 		} else if (matches(argv[1], "-iec") == 0) {
 			++use_iec;
-- 
2.25.4