genl-family-get test fails with "Invalid Argument" on kernel 6.8

genl-family-get test fails with "Invalid Argument" on kernel 6.8

[Divyanshu Rathore]

Dear Team,

As part of our system testing for the libmnl component version 1.0.5, we are using the genl-family-get.c test file from the upstream repository:

libmnl/tree/examples/genl/genl-family-get.c

In our test setup, we execute this binary without any arguments. We have observed that the test behaves differently depending on the kernel version:

On systems running kernel 5.15, the binary executes successfully.

On systems running kernel 6.8, it consistently fails with the following error: "Invalid Argument"

We added debug logs and got the below output:

Test failing due to:

AssertionError: Regex didn't match: 'name=' not found in '[DBG] main: program start\n[DBG] main: building request: nlmsg_type=16, nlmsg_flags=0x5, nlmsg_seq=1756046191\n[DBG] main: dumping all families\n[DBG] main: opened netlink socket\n[DBG] main: socket bound, portid=9125\n[DBG] main: sent request to kernel\n[DBG] main: received 48 bytes from kernel\n[DBG] main: mnl_cb_run returned -1\n[DBG] main: program finished'

We ensured that the same libmnl version 1.0.5 is used across all systems, and we verified this issue on multiple hardware platforms. We also added debug prints and verified that the failure occurs in the call to mnl_socket_sendto().

From our investigation, We checked in multiple hardwares and added debug prints, from that it is concluded that, the kernel version upgrade is causing the issue
Can you please check this issue and provide the fix for it.

Let us know if additional details are needed.

Best Regards,
Divyanshu Rathore

[PATCH libmnl] examples: genl: fix wrong attribute size

[Florian Westphal]

This example no longer works on more recent kernels:

genl-family-get
error: Invalid argument

dmesg says:
netlink: 'genl-family-get': attribute type 1 has an invalid length.

Fix this and also zero out the reserved field in the genl header,
while not validated yet for dumps this could change.

Reported-by: Divyanshu Rathore <Divyanshu.Rathore@bmwtechworks.in>
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 I will push this out later today.

 examples/genl/genl-family-get.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/examples/genl/genl-family-get.c b/examples/genl/genl-family-get.c
index ba8de122094c..0c2006755e30 100644
--- a/examples/genl/genl-family-get.c
+++ b/examples/genl/genl-family-get.c
@@ -199,8 +199,9 @@ int main(int argc, char *argv[])
 	genl = mnl_nlmsg_put_extra_header(nlh, sizeof(struct genlmsghdr));
 	genl->cmd = CTRL_CMD_GETFAMILY;
 	genl->version = 1;
+	genl->reserved = 0;
 
-	mnl_attr_put_u32(nlh, CTRL_ATTR_FAMILY_ID, GENL_ID_CTRL);
+	mnl_attr_put_u16(nlh, CTRL_ATTR_FAMILY_ID, GENL_ID_CTRL);
 	if (argc >= 2)
 		mnl_attr_put_strz(nlh, CTRL_ATTR_FAMILY_NAME, argv[1]);
 	else
-- 
2.51.0

Re: [PATCH libmnl] examples: genl: fix wrong attribute size

[Pablo Neira Ayuso]

On Mon, Sep 15, 2025 at 02:40:30PM +0200, Florian Westphal wrote:
> This example no longer works on more recent kernels:
> 
> genl-family-get
> error: Invalid argument
> 
> dmesg says:
> netlink: 'genl-family-get': attribute type 1 has an invalid length.
> 
> Fix this and also zero out the reserved field in the genl header,
> while not validated yet for dumps this could change.
> 
> Reported-by: Divyanshu Rathore <Divyanshu.Rathore@bmwtechworks.in>
> Signed-off-by: Florian Westphal <fw@strlen.de>

Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>

> ---
>  I will push this out later today.
> 
>  examples/genl/genl-family-get.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/examples/genl/genl-family-get.c b/examples/genl/genl-family-get.c
> index ba8de122094c..0c2006755e30 100644
> --- a/examples/genl/genl-family-get.c
> +++ b/examples/genl/genl-family-get.c
> @@ -199,8 +199,9 @@ int main(int argc, char *argv[])
>  	genl = mnl_nlmsg_put_extra_header(nlh, sizeof(struct genlmsghdr));
>  	genl->cmd = CTRL_CMD_GETFAMILY;
>  	genl->version = 1;
> +	genl->reserved = 0;
>  
> -	mnl_attr_put_u32(nlh, CTRL_ATTR_FAMILY_ID, GENL_ID_CTRL);
> +	mnl_attr_put_u16(nlh, CTRL_ATTR_FAMILY_ID, GENL_ID_CTRL);
>  	if (argc >= 2)
>  		mnl_attr_put_strz(nlh, CTRL_ATTR_FAMILY_NAME, argv[1]);
>  	else
> -- 
> 2.51.0
> 
>