[ovs-dev] [PATCH 2/2] lib/netlink: Use correct netlink max message size

[Ben Pfaff]

On Wed, Nov 01, 2017 at 10:33:25AM -0700, Greg Rose wrote:
> On 11/01/2017 09:38 AM, Ben Pfaff wrote:
> >On Wed, Nov 01, 2017 at 08:28:48AM -0700, Greg Rose wrote:
> >> On 10/31/2017 12:53 PM, Ben Pfaff wrote:
> >>> On Fri, Sep 22, 2017 at 07:44:53AM -0700, Greg Rose wrote:
> >>>> The maximum message size for recent Linux kernels is 32Kb and in older
> >>>> kernels it is 16KB.
> >>>>
> >>>> See http://www.spinics.net/lists/netdev/msg431592.html
> >>>>
> >>>> Adjust the size checked and update a comment.
> >>>>
> >>>> Signed-off-by: Greg Rose <gvrose8192 at gmail.com>
> >>>
> >>> ...
> >>>
> >>>> diff --git a/lib/netlink.c b/lib/netlink.c
> >>>> index de3ebcd..04310ff 100644
> >>>> --- a/lib/netlink.c
> >>>> +++ b/lib/netlink.c
> >>>> @@ -570,7 +570,7 @@ nl_msg_next(struct ofpbuf *buffer, struct ofpbuf *msg)
> >>>>   bool
> >>>>   nl_attr_oversized(size_t payload_size)
> >>>>   {
> >>>> -    return payload_size > UINT16_MAX - NLA_HDRLEN;
> >>>> +    return payload_size > INT16_MAX - NLA_HDRLEN;
> >>>>   }
> >>>
> >>> Thanks for the patch!
> >>>
> >>> I am confused by a difference between the commit message and the code.
> >>> Before this patch, nl_attr_oversized() considered an attribute of about
> >>> 64 kB to be oversize; after this patch, about 32 kB.  Shouldn't the new
> >>> value be about 16 kB?
> >>>
> >>> Thanks,
> >>>
> >>> Ben.
> >>>
> >>
> >> IIRC this is in user space so we prepare for whatever the maximum size we might
> >> get from a kernel, which is 32KB.
> >>
> >> We could have just left it at 64KB but we don't ever need that much.
> >
> >This response implies that nl_attr_oversized() has something to do with
> >Netlink attributes for messages that userspace receives from the kernel.
> >This is not the case.  OVS userspace is prepared to handle any length
> >attribute in Netlink messages that it receives.  It won't apply
> >nl_attr_oversized() to received attributes, it will just go ahead and
> >process them.
> >
> >On the contrary, userspace uses nl_attr_oversized() to limit the maximum
> >size of a Netlink attribute it *sends to* the kernel.  If the kernel
> >only supports a maximum 16 kB or 32 kB attribute, according to its
> >version, then the function needs to apply this limit.  The current code
> >assumes that the kernel can process anything that can actually be
> >formulated into Netlink.
> >
> >What actual limit does Linux apply to Netlink messages, for sending and
> >receiving?  I am beginning to believe that this function is a red
> >herring and does not need to change.
> >
> So you're right, I didn't recall correctly... that's what I get for quick answers.
> 
> In net/netlink/af_netlink.c the core netlink handler will attempt  to copy the
> maximum length of the message and if it is not able to then it will set the
> MSG_TRUNC flag and attempt to process whatever it can.  The netlink
> skb's are processed as datagrams.
> 
> Generally the large size messages come from the kernel, not to the kernel
> so it is not much of a concern for any applications I'm aware of.
> 
> Does OVS send large messages to the kernel?

One example is when OVS sends a large number of actions as part of
OVS_FLOW_CMD_NEW or OVS_FLOW_CMD_SET or OVS_PACKET_CMD_EXECUTE.  OVS has
a fallback for that, but it needs to know when to use it, that is, it
needs to know how large is too large.