From: Yonghong Song <yhs@fb.com>
Date: Tue, 11 Sep 2018 14:09:11 -0700
Subject: tools/bpf: fix a netlink recv issue
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Patch-mainline: v4.20-rc1
Git-commit: 9d0b3c1f1451d1b9a33de3c70ae3d50ccd77db1a
References: bsc#1109837

Commit f7010770fbac ("tools/bpf: move bpf/lib netlink related
functions into a new file") introduced a while loop for the
netlink recv path. This while loop is needed since the
buffer in recv syscall may not be enough to hold all the
information and in such cases multiple recv calls are needed.

There is a bug introduced by the above commit as
the while loop may block on recv syscall if there is no
more messages are expected. The netlink message header
flag NLM_F_MULTI is used to indicate that more messages
are expected and this patch fixed the bug by doing
further recv syscall only if multipart message is expected.

The patch added another fix regarding to message length of 0.
When netlink recv returns message length of 0, there will be
no more messages for returning data so the while loop
can end.

Fixes: f7010770fbac ("tools/bpf: move bpf/lib netlink related functions into a new file")
Reported-by: Björn Töpel <bjorn.topel@intel.com>
Tested-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Thomas Bogendoerfer <tbogendoerfer@suse.de>
---
 tools/lib/bpf/netlink.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -65,18 +65,23 @@ static int bpf_netlink_recv(int sock, __
 			    __dump_nlmsg_t _fn, dump_nlmsg_t fn,
 			    void *cookie)
 {
+	bool multipart = true;
 	struct nlmsgerr *err;
 	struct nlmsghdr *nh;
 	char buf[4096];
 	int len, ret;
 
-	while (1) {
+	while (multipart) {
+		multipart = false;
 		len = recv(sock, buf, sizeof(buf), 0);
 		if (len < 0) {
 			ret = -errno;
 			goto done;
 		}
 
+		if (len == 0)
+			break;
+
 		for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, len);
 		     nh = NLMSG_NEXT(nh, len)) {
 			if (nh->nlmsg_pid != nl_pid) {
@@ -87,6 +92,8 @@ static int bpf_netlink_recv(int sock, __
 				ret = -LIBBPF_ERRNO__INVSEQ;
 				goto done;
 			}
+			if (nh->nlmsg_flags & NLM_F_MULTI)
+				multipart = true;
 			switch (nh->nlmsg_type) {
 			case NLMSG_ERROR:
 				err = (struct nlmsgerr *)NLMSG_DATA(nh);