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

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

[David Ahern]

From: David Ahern <dsahern@gmail.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@zx2c4.com>
Signed-off-by: David Ahern <dsahern@gmail.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++;
-- 
2.11.0

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

[Stephen Hemminger]

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

> From: David Ahern <dsahern@gmail.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@zx2c4.com>
> Signed-off-by: David Ahern <dsahern@gmail.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.

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

[Stephen Hemminger]

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

> 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++;

Why not warn user that any prefix length (ie not 32) is ignored,
then do what you propose.

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

[David Ahern]

On 5/22/19 5:16 PM, Stephen Hemminger wrote:
> On Fri, 17 May 2019 10:59:13 -0700
> David Ahern <dsahern@kernel.org> wrote:
> 
>> 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++;
> 
> Why not warn user that any prefix length (ie not 32) is ignored,
> then do what you propose.
> 

My first version did that. I thought people would complain about a
stderr message. At least with failing the 'route get' scripts can be fixed.

After more thought even changing the prefix length for users presents a
limitation on future changes.