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

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Message-ID: <fefe8b7a-3264-333a-bab3-7e0dd82efed1@gmail.com>
Date:   Sun, 14 May 2017 16:14:16 -0600
From:   David Ahern <dsahern@...il.com>
To:     Johannes Berg <johannes@...solutions.net>,
        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 5/14/17 3:00 PM, Johannes Berg wrote:
> On Sat, 2017-05-13 at 19:29 +0200, Jan Moskyto Matejka wrote:
>>
>>> 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?)
>>
>> EMSGSIZE seems OK for me.
> 
> If we return an error here, and consequently allow for userspace
> changes to pick this up, perhaps we could also consider allowing to
> split the dump between nexthops, so that arbitrary such things can be
> returned.

Returning an error should not impact existing userspace; it should
already be checking for an error response in the message.

Splitting the dump between nexthops across multiple messages will have
repercussions on userspace. I think (at least for rtnetlink and links,
addresses, routes) userspace needs to provide a buffer large enough for
a complete object. If we limit the number of nexthops to something
reasonable (e.g., 256), then ipv4 for example will be ~ 3kB + lwt encap
size we are talking on the order of an 8kb maybe 16kB buffer. That is a
reasonable request for the API.