netdev - Re: [PATCH iproute2] ip route: Set rtm_dst_len to 32 for all ip route get requests

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Date:   Fri, 17 May 2019 11:06:58 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Ahern <dsahern@...nel.org>
Cc:     netdev@...r.kernel.org, Jason@...c4.com,
        David Ahern <dsahern@...il.com>
Subject: Re: [PATCH iproute2] ip route: Set rtm_dst_len to 32 for all ip
 route get requests

On Fri, 17 May 2019 10:59:13 -0700
David Ahern <dsahern@...nel.org> wrote:

> From: David Ahern <dsahern@...il.com>
> 
> Jason reported that ip route get with a prefix length is now
> failing:
>     $ 192.168.50.0/24
>     RTNETLINK answers: Invalid argument
> 
> iproute2 now uses strict mode and strict mode in the kernel
> requires rtm_dst_len to be 32. Non-strict mode ignores the
> prefix length, so this allows ip to work without affecting
> existing users who add a prefix length to the request.
> 
> Fixes: aea41afcfd6d6 ("ip bridge: Set NETLINK_GET_STRICT_CHK on socket")
> Reported-by: Jason A. Donenfeld <Jason@...c4.com>
> Signed-off-by: David Ahern <dsahern@...il.com>
> ---
>  ip/iproute.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/ip/iproute.c b/ip/iproute.c
> index 2b3dcc5dbd53..d980b86ffd42 100644
> --- a/ip/iproute.c
> +++ b/ip/iproute.c
> @@ -2035,7 +2035,11 @@ static int iproute_get(int argc, char **argv)
>  			if (addr.bytelen)
>  				addattr_l(&req.n, sizeof(req),
>  					  RTA_DST, &addr.data, addr.bytelen);
> -			req.r.rtm_dst_len = addr.bitlen;
> +			/* kernel ignores prefix length on 'route get'
> +			 * requests; to allow ip to work with strict mode
> +			 * but not break existing users, just set to 32
> +			 */
> +			req.r.rtm_dst_len = 32;
>  			address_found = true;
>  		}
>  		argc--; argv++;

I don't like silently ignoring things. It was wrong before and it
is trapped now.

Probably better to error out in iproute2 if any prefix is given.