netdev - tools/bpf regression causing samples/bpf/ to hang

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Date:   Tue, 11 Sep 2018 13:11:29 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Netdev <netdev@...r.kernel.org>, yhs@...com
Cc:     ast@...nel.org, Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: tools/bpf regression causing samples/bpf/ to hang

Hi Yonghong, I tried to run the XDP samples from the bpf-next tip
today, and was hit by a regression.

Commit f7010770fbac ("tools/bpf: move bpf/lib netlink related
functions into a new file") adds a while(1) around the recv call in
bpf_set_link_xdp_fd making that call getting stuck in an infinite
loop.

I simply removed the loop, and that solved my problem (patch below).

However, I don't know if removing the loop would break bpftool for
you. If not, I can submit the patch as a proper one for bpf-next.

Thanks!
Björn

From: =?UTF-8?q?Bj=C3=B6rn=20T=C3=B6pel?= <bjorn.topel@...el.com>
Date: Tue, 11 Sep 2018 12:35:44 +0200
Subject: [PATCH] tools/bpf: remove loop around netlink recv

Commit f7010770fbac ("tools/bpf: move bpf/lib netlink related
functions into a new file") moved the bpf_set_link_xdp_fd and split it
up into multiple functions. The added receive function
bpf_netlink_recv added a loop around the recv syscall leading to
multiple recv calls. This resulted in all XDP samples in the
samples/bpf/ to stop working, since they were stuck in a blocking
recv.

This commits removes the while (1)-statement.

Fixes: f7010770fbac ("tools/bpf: move bpf/lib netlink related
functions into a new file")
Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
---
 tools/lib/bpf/netlink.c | 64 ++++++++++++++++++++---------------------
 1 file changed, 31 insertions(+), 33 deletions(-)

diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
index 469e068dd0c5..0eae1fbf46c6 100644
--- a/tools/lib/bpf/netlink.c
+++ b/tools/lib/bpf/netlink.c
@@ -70,41 +70,39 @@ static int bpf_netlink_recv(int sock, __u32 nl_pid, int seq,
     char buf[4096];
     int len, ret;

-    while (1) {
-        len = recv(sock, buf, sizeof(buf), 0);
-        if (len < 0) {
-            ret = -errno;
+    len = recv(sock, buf, sizeof(buf), 0);
+    if (len < 0) {
+        ret = -errno;
+        goto done;
+    }
+
+    for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, len);
+         nh = NLMSG_NEXT(nh, len)) {
+        if (nh->nlmsg_pid != nl_pid) {
+            ret = -LIBBPF_ERRNO__WRNGPID;
             goto done;
         }
-
-        for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, len);
-             nh = NLMSG_NEXT(nh, len)) {
-            if (nh->nlmsg_pid != nl_pid) {
-                ret = -LIBBPF_ERRNO__WRNGPID;
-                goto done;
-            }
-            if (nh->nlmsg_seq != seq) {
-                ret = -LIBBPF_ERRNO__INVSEQ;
-                goto done;
-            }
-            switch (nh->nlmsg_type) {
-            case NLMSG_ERROR:
-                err = (struct nlmsgerr *)NLMSG_DATA(nh);
-                if (!err->error)
-                    continue;
-                ret = err->error;
-                nla_dump_errormsg(nh);
-                goto done;
-            case NLMSG_DONE:
-                return 0;
-            default:
-                break;
-            }
-            if (_fn) {
-                ret = _fn(nh, fn, cookie);
-                if (ret)
-                    return ret;
-            }
+        if (nh->nlmsg_seq != seq) {
+            ret = -LIBBPF_ERRNO__INVSEQ;
+            goto done;
+        }
+        switch (nh->nlmsg_type) {
+        case NLMSG_ERROR:
+            err = (struct nlmsgerr *)NLMSG_DATA(nh);
+            if (!err->error)
+                continue;
+            ret = err->error;
+            nla_dump_errormsg(nh);
+            goto done;
+        case NLMSG_DONE:
+            return 0;
+        default:
+            break;
+        }
+        if (_fn) {
+            ret = _fn(nh, fn, cookie);
+            if (ret)
+                return ret;
         }
     }
     ret = 0;
-- 
2.17.1