[PATCH libnetfilter_conntrack] examples: check return value of nfct_nlmsg_build()

[PATCH libnetfilter_conntrack] examples: check return value of nfct_nlmsg_build()

[Eyal Birger]

nfct_nlmsg_build() may fail for different reasons, for example if
insufficient parameters exist in the ct object. The resulting nlh would
not contain any of the ct attributes.

Some conntrack operations would still operate in such case, for example
an IPCTNL_MSG_CT_DELETE message would just delete all existing conntrack
entries.

While the example as it is does supply correct parameters, it's safer
as reference to validate the return value.

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
---
 examples/nfct-mnl-create.c    | 6 +++++-
 examples/nfct-mnl-del.c       | 6 +++++-
 examples/nfct-mnl-get.c       | 6 +++++-
 examples/nfct-mnl-set-label.c | 7 ++++++-
 4 files changed, 21 insertions(+), 4 deletions(-)

diff --git a/examples/nfct-mnl-create.c b/examples/nfct-mnl-create.c
index 64387a7..7fd224d 100644
--- a/examples/nfct-mnl-create.c
+++ b/examples/nfct-mnl-create.c
@@ -60,7 +60,11 @@ int main(void)
 	nfct_set_attr_u8(ct, ATTR_TCP_STATE, TCP_CONNTRACK_SYN_SENT);
 	nfct_set_attr_u32(ct, ATTR_TIMEOUT, 100);
 
-	nfct_nlmsg_build(nlh, ct);
+	ret = nfct_nlmsg_build(nlh, ct);
+	if (ret == -1) {
+		perror("nfct_nlmsg_build");
+		exit(EXIT_FAILURE);
+	}
 
 	ret = mnl_socket_sendto(nl, nlh, nlh->nlmsg_len);
 	if (ret == -1) {
diff --git a/examples/nfct-mnl-del.c b/examples/nfct-mnl-del.c
index 91ad9e4..806d9f8 100644
--- a/examples/nfct-mnl-del.c
+++ b/examples/nfct-mnl-del.c
@@ -55,7 +55,11 @@ int main(void)
 	nfct_set_attr_u16(ct, ATTR_PORT_SRC, htons(20));
 	nfct_set_attr_u16(ct, ATTR_PORT_DST, htons(10));
 
-	nfct_nlmsg_build(nlh, ct);
+	ret = nfct_nlmsg_build(nlh, ct);
+	if (ret == -1) {
+		perror("nfct_nlmsg_build");
+		exit(EXIT_FAILURE);
+	}
 
 	ret = mnl_socket_sendto(nl, nlh, nlh->nlmsg_len);
 	if (ret == -1) {
diff --git a/examples/nfct-mnl-get.c b/examples/nfct-mnl-get.c
index 4858acf..5be3331 100644
--- a/examples/nfct-mnl-get.c
+++ b/examples/nfct-mnl-get.c
@@ -74,7 +74,11 @@ int main(void)
 	nfct_set_attr_u16(ct, ATTR_PORT_SRC, htons(20));
 	nfct_set_attr_u16(ct, ATTR_PORT_DST, htons(10));
 
-	nfct_nlmsg_build(nlh, ct);
+	ret = nfct_nlmsg_build(nlh, ct);
+	if (ret == -1) {
+		perror("nfct_nlmsg_build");
+		exit(EXIT_FAILURE);
+	}
 
 	ret = mnl_socket_sendto(nl, nlh, nlh->nlmsg_len);
 	if (ret == -1) {
diff --git a/examples/nfct-mnl-set-label.c b/examples/nfct-mnl-set-label.c
index c52b267..50bebb0 100644
--- a/examples/nfct-mnl-set-label.c
+++ b/examples/nfct-mnl-set-label.c
@@ -19,6 +19,7 @@ static void set_label(struct nf_conntrack *ct, struct callback_args *cbargs)
 	char buf[MNL_SOCKET_BUFFER_SIZE];
 	struct nlmsghdr *nlh;
 	struct nfgenmsg *nfh;
+	int ret;
 
 	if (b) {
 		if (bit < 0)
@@ -55,7 +56,11 @@ static void set_label(struct nf_conntrack *ct, struct callback_args *cbargs)
 	nfh->version = NFNETLINK_V0;
 	nfh->res_id = 0;
 
-	nfct_nlmsg_build(nlh, ct);
+	ret = nfct_nlmsg_build(nlh, ct);
+	if (ret == -1) {
+		perror("nfct_nlmsg_build");
+		exit(EXIT_FAILURE);
+	}
 
 	if (mnl_socket_sendto(cbargs->nl, nlh, nlh->nlmsg_len) < 0)
 		perror("mnl_socket_sendto");
-- 
2.25.1

Re: [PATCH libnetfilter_conntrack] examples: check return value of nfct_nlmsg_build()

[Pablo Neira Ayuso]

On Fri, Jan 01, 2021 at 11:02:26AM +0200, Eyal Birger wrote:
> nfct_nlmsg_build() may fail for different reasons, for example if
> insufficient parameters exist in the ct object. The resulting nlh would
> not contain any of the ct attributes.
> 
> Some conntrack operations would still operate in such case, for example
> an IPCTNL_MSG_CT_DELETE message would just delete all existing conntrack
> entries.
> 
> While the example as it is does supply correct parameters, it's safer
> as reference to validate the return value.

Applied, thanks.