[TCP]: Problem bug with sysctl_tcp_congestion_control function

From: "Sam Jansen" <sjansen@google.com>

sysctl_tcp_congestion_control seems to have a bug that prevents it
from actually calling the tcp_set_default_congestion_control
function. This is not so apparent because it does not return an error
and generally the /proc interface is used to configure the default TCP
congestion control algorithm.  This is present in 2.6.18 onwards and
probably earlier, though I have not inspected 2.6.15--2.6.17.

sysctl_tcp_congestion_control calls sysctl_string and expects a successful
return code of 0. In such a case it actually sets the congestion control
algorithm with tcp_set_default_congestion_control. Otherwise, it returns the
value returned by sysctl_string. This was correct in 2.6.14, as sysctl_string
returned 0 on success. However, sysctl_string was updated to return 1 on
success around about 2.6.15 and sysctl_tcp_congestion_control was not updated.
Even though sysctl_tcp_congestion_control returns 1, do_sysctl_strategy
converts this return code to '0', so the caller never notices the error.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/sysctl_net_ipv4.c

@@ -191,7 +191,7 @@ static int sysctl_tcp_congestion_control(ctl_table *table, int __user *name,
 
 	tcp_get_default_congestion_control(val);
 	ret = sysctl_string(&tbl, name, nlen, oldval, oldlenp, newval, newlen);
-	if (ret == 0 && newval && newlen)
+	if (ret == 1 && newval && newlen)
 		ret = tcp_set_default_congestion_control(val);
 	return ret;
 }