From:   =?UTF-8?q?Pali=20Roh=C3=A1r?= <pali@kernel.org>
To:     linux-man@vger.kernel.org,
        Alejandro Colomar <alx.manpages@gmail.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>
Subject: [PATCH] sock_diag.7: Fix recvmsg() usage in the example
Date:   Sat, 16 Jan 2021 19:45:54 +0100
Message-Id: <20210116184554.30730-1-pali@kernel.org>
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Precedence: bulk

The msg_name field for recvmsg() call points to a caller-allocated buffer
nladdr that is used to return the source address of the (netlink) socket.

As recvmsg() does not read this buffer and fills it for a caller, do not
initialize it and instead check its value in the example.

Signed-off-by: Pali Rohár <pali@kernel.org>
---
 man7/sock_diag.7 | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/man7/sock_diag.7 b/man7/sock_diag.7
index 91ccf3ac5..191e6ac49 100644
--- a/man7/sock_diag.7
+++ b/man7/sock_diag.7
@@ -753,9 +753,7 @@ static int
 receive_responses(int fd)
 {
     long buf[8192 / sizeof(long)];
-    struct sockaddr_nl nladdr = {
-        .nl_family = AF_NETLINK
-    };
+    struct sockaddr_nl nladdr;
     struct iovec iov = {
         .iov_base = buf,
         .iov_len = sizeof(buf)
@@ -782,6 +780,11 @@ receive_responses(int fd)
         if (ret == 0)
             return 0;
 
+        if (nladdr.nl_family != AF_NETLINK) {
+            fputs("!AF_NETLINK\en", stderr);
+            return \-1;
+        }
+
         const struct nlmsghdr *h = (struct nlmsghdr *) buf;
 
         if (!NLMSG_OK(h, ret)) {
-- 
2.20.1