When writing to the ignore_routes_with_linkdown sysctl, if
proc_dointvec() fails to parse the input, it returns a negative error
code. The current implementation is overwriting that error for write
operations.

This results in a silent failure, it returns a successful write although
the configuration was not modified at all. When modifying the "all"
variant it can also modify the configuration of existing interfaces to
the wrong value.

Fix this by checking the return value of proc_dointvec() and returning
early on failure.

Fixes: 35103d11173b ("net: ipv6 sysctl option to ignore routes when nexthop link is down")
Signed-off-by: Fernando Fernandez Mancera <redacted>
---
 net/ipv6/addrconf.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index c901b444a995..70058d971205 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c

@@ -6671,6 +6671,8 @@ int addrconf_sysctl_ignore_routes_with_linkdown(const struct ctl_table *ctl,
 	lctl.data = &val;
 
 	ret = proc_dointvec(&lctl, write, buffer, lenp, ppos);
+	if (ret)
+		return ret;
 
 	if (write)
 		ret = addrconf_fixup_linkdown(ctl, valp, val);

-- 
2.54.0