netdev - Re: [PATCH] net: ipv6: Truncate single route when it doesn't fit into dump buffer.

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Date:   Sat, 13 May 2017 11:13:38 -0600
From:   David Ahern <dsahern@...il.com>
To:     Jan Moskyto Matejka <mq@....cz>
Cc:     David Miller <davem@...emloft.net>, mq@....cz,
        netdev@...r.kernel.org, roopa <roopa@...ulusnetworks.com>
Subject: Re: [PATCH] net: ipv6: Truncate single route when it doesn't fit into
 dump buffer.

On 5/13/17 4:54 AM, Jan Moskyto Matejka wrote:
>> I see 2 problems:
>> 1. the kernel is not telling the user the supplied buffer is too small
>> (ie., if a single route does not fit in the skb then it should fail and
>> return an error code to the user),
> 
> Definitely. I want just to note that this condition usually occurs
> somewhere during route dump. To know it before starting output, we would 
> have to walk the FIB once before dump to calculate max route len.

When adding a route to the skb, track whether it contains at least 1
route. If not, it means the next route in the dump is larger than the
given buffer. Detect this condition and error out of the dump -
returning an error to the user (-ENOSPC? or EMSGSIZE?)

> 
>> 2. multipath routes for IPv4 and IPv6 do not have a limit.
>>
>> Should the kernel put a limit on the number of nexthops? I recently put
>> a cap on MPLS route size as 4096 bytes, but I think this should be
>> revisited in terms of a limit on number of nexthops to create a
>> consistent limit even if struct sizes change. And, the limit on the
>> number of nexthops should be consistent across address families (same
>> limit for IPv4, IPv6, and MPLS).
>>
>> From discussions I have had, 32 nexthops for a single route is on the
>> laughably high side, but some people do crazy things. How about a limit
>> of 256 nexthops?
> 
> 256 should be OK even for a crazy developer of BIRD.
> 
> It would be nice to have if the returned error were somehow useful for
> the userspace -- to know what is happening, not only something like
> "impossible to add / append route".

Top of tree kernel has extended error reporting so a message can be
returned that says something to the effect of "route size is larger than
supplied buffer size".