[iproute2,v2] libgenl: make genl_add_mcast_grp set errno on error

genl_add_mcast_grp doesn't set errno in all cases.

On kernels that support mptcp but lack event support (all kernels <= 5.11)
MPTCP_PM_EV_GRP_NAME won't be found and ip will exit with

    "can't subscribe to mptcp events: Success"

Set errno to a meaningful value (ENOENT) when the group name isn't found
and also cover other spots where it returns nonzero with errno unset.

Fixes: ff619e4fd370 ("mptcp: add support for event monitoring")
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 Change since v1: fix libgenl instead of setting errno in the caller.

 lib/libgenl.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

On 5/10/21 6:06 AM, Florian Westphal wrote:
> diff --git a/lib/libgenl.c b/lib/libgenl.c

> index 4c51d47af46b..8b22c06e7941 100644

> --- a/lib/libgenl.c

> +++ b/lib/libgenl.c

> @@ -84,6 +84,7 @@ static int genl_parse_grps(struct rtattr *attr, const char *name, unsigned int *

>  		}

>  	}

>  

> +	errno = ENOENT;

>  	return -1;

>  }

>  

> @@ -108,17 +109,22 @@ int genl_add_mcast_grp(struct rtnl_handle *grth, __u16 fnum, const char *group)

>  	ghdr = NLMSG_DATA(answer);

>  	len = answer->nlmsg_len;

>  

> -	if (answer->nlmsg_type != GENL_ID_CTRL)

> +	if (answer->nlmsg_type != GENL_ID_CTRL) {

> +		errno = EINVAL;

>  		goto err_free;

> +	}

>  

>  	len -= NLMSG_LENGTH(GENL_HDRLEN);

> -	if (len < 0)

> +	if (len < 0) {

> +		errno = EINVAL;

>  		goto err_free;

> +	}

>  

>  	attrs = (struct rtattr *) ((char *) ghdr + GENL_HDRLEN);

>  	parse_rtattr(tb, CTRL_ATTR_MAX, attrs, len);

>  

>  	if (tb[CTRL_ATTR_MCAST_GROUPS] == NULL) {

> +		errno = ENOENT;

>  		fprintf(stderr, "Missing mcast groups TLV\n");

>  		goto err_free;

>  	}

> 


I get compile errors on Ubuntu 20.04:


lib
    CC       libgenl.o
libgenl.c: In function ‘genl_parse_grps’:
libgenl.c:87:2: error: ‘errno’ undeclared (first use in this function)
   87 |  errno = ENOENT;
      |  ^~~~~
libgenl.c:12:1: note: ‘errno’ is defined in header ‘<errno.h>’; did you
forget to ‘#include <errno.h>’?
   11 | #include "libgenl.h"
  +++ |+#include <errno.h>
   12 |
libgenl.c:87:2: note: each undeclared identifier is reported only once
for each function it appears in
   87 |  errno = ENOENT;
      |  ^~~~~
libgenl.c:87:10: error: ‘ENOENT’ undeclared (first use in this function)
   87 |  errno = ENOENT;
      |          ^~~~~~
libgenl.c: In function ‘genl_add_mcast_grp’:
libgenl.c:113:3: error: ‘errno’ undeclared (first use in this function)
  113 |   errno = EINVAL;
      |   ^~~~~
libgenl.c:113:3: note: ‘errno’ is defined in header ‘<errno.h>’; did you
forget to ‘#include <errno.h>’?
libgenl.c:113:11: error: ‘EINVAL’ undeclared (first use in this function)
  113 |   errno = EINVAL;
      |           ^~~~~~
libgenl.c:127:11: error: ‘ENOENT’ undeclared (first use in this function)
  127 |   errno = ENOENT;