Re: [PATCH iproute2] mptcp: avoid uninitialised errno usage

[David Ahern <dsahern@gmail.com>]

On 5/3/21 4:36 AM, Florian Westphal wrote:
> The function called *might* set errno based on errno value in NLMSG_ERROR
> message, but in case no such message exists errno is left alone.
> 
> This may cause ip to fail with
>     "can't subscribe to mptcp events: Success"
> 
> on kernels that support mptcp but lack event support (all kernels <= 5.11).
> 
> Set errno to a meaningful value.  This will then still exit with the
> more specific 'permission denied' or some such when called by process
> that lacks CAP_NET_ADMIN on 5.12 and later.
> 
> Fixes: ff619e4fd370 ("mptcp: add support for event monitoring")
> Signed-off-by: Florian Westphal <fw@strlen.de>
> ---
>  ip/ipmptcp.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/ip/ipmptcp.c b/ip/ipmptcp.c
> index 5f490f0026d9..504b5b2f5329 100644
> --- a/ip/ipmptcp.c
> +++ b/ip/ipmptcp.c
> @@ -491,6 +491,9 @@ out:
>  
>  static int mptcp_monitor(void)
>  {
> +	/* genl_add_mcast_grp may or may not set errno */
> +	errno = EOPNOTSUPP;
> +
>  	if (genl_add_mcast_grp(&genl_rth, genl_family, MPTCP_PM_EV_GRP_NAME) < 0) {
>  		perror("can't subscribe to mptcp events");
>  		return 1;
> 

Seems like this should be set in genl_add_mcast_grp vs its caller.