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>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

diff --git a/lib/libgenl.c b/lib/libgenl.c
index 4c51d47af46b09a37577c13b762a0054c2e53606..fca07f9fe768affe50c8d07bce63602b64d7e995 100644 (file)
--- a/lib/libgenl.c
+++ b/lib/libgenl.c
@@ -3,6 +3,7 @@
  * libgenl.c   GENL library
  */
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
@@ -84,6 +85,7 @@ static int genl_parse_grps(struct rtattr *attr, const char *name, unsigned int *
                }
        }
 
+       errno = ENOENT;
        return -1;
 }
 
@@ -108,17 +110,22 @@ 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;
        }