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

[Greg Rose]

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?

- Greg