Subject: Re: [PATCH v3] openvswitch: enable NSH support

From: Jiri Benc <[email-masked]>
Date: Wed, 16 Aug 2017 16:03:22 +0200

On Wed, 16 Aug 2017 17:31:30 +0800, Yang, Yi wrote:
> On Wed, Aug 16, 2017 at 11:19:21AM +0200, Jiri Benc wrote:
> > > --- a/include/uapi/linux/openvswitch.h
> > > +++ b/include/uapi/linux/openvswitch.h
> > [...]
> > > +#define NSH_MD1_CONTEXT_SIZE 4
> > 
> > Please move this to nsh.h and use it there, too, instead of the open
> > coded 4.
> 
> ovs code is very ugly, it will convert array[4] in
> datapath/linux/compat/include/linux/openvswitch.h to other struct, I
> have to change context[4] to such format :-), we can use 4 here for
> Linux kernel.

Oh, right, this is uAPI and nsh.h is kernel internal. My suggestion was
nonsense, let's keep it as it was in your patch.

> > > +	case OVS_KEY_ATTR_NSH: {
> > > +		struct ovs_key_nsh nsh;
> > > +		struct ovs_key_nsh nsh_mask;
> > > +		size_t size = nla_len(a) / 2;
> > > +		struct nlattr attr[1 + DIV_ROUND_UP(sizeof(struct ovs_key_ipv6)
> > > +						    , sizeof(struct nlattr))];
> > > +		struct nlattr mask[1 + DIV_ROUND_UP(sizeof(struct ovs_key_ipv6)
> > > +						    , sizeof(struct nlattr))];
> > > +
> > > +		attr->nla_type = nla_type(a);
> > > +		mask->nla_type = attr->nla_type;
> > > +		attr->nla_len = NLA_HDRLEN + size;
> > > +		mask->nla_len = attr->nla_len;
> > > +		memcpy(attr + 1, (char *)(a + 1), size);
> > > +		memcpy(mask + 1, (char *)(a + 1) + size, size);
> > 
> > This is too hacky. Please find a better way to handle this.
> > 
> > One option is to create a struct with struct nlattr as the first member
> > followed by a char buffer. Still not nice but at least it's clear
> > what's the intent.
> 
> The issue is nested attributes only can use this way, nested attribute
> for SET_MASKED is very special, we have to handle it specially.

I'm not sure you understood what I meant. Let me explain in code:

	struct {
		struct nlattr attr;
		struct ovs_key_ipv6 data;
	} attr, mask;

	attr->attr.nla_type = nla_type(a);
	attr->attr.nl_len = NLA_HDRLEN + size;
	memcpy(&attr->data, a + 1, size);

It's much less hacky and doing the same thing.

I'm not sure we don't need verification of size not overflowing the
available buffer. Is it checked beforehand elsewhere?

I also spotted one more bug: the 'mask' variable is not used anywhere.
The second call of nsh_key_from_nlattr should use mask, not attr.

> > > +	key->nsh.path_hdr = nsh->path_hdr;
> > > +	switch (key->nsh.mdtype) {
> > > +	case NSH_M_TYPE1:
> > > +		if ((length << 2) != NSH_M_TYPE1_LEN)
> > 
> > Why length << 2?
> 
> len in NSH header is number of 4 octets, so need to multiply 4.

Should nsh_get_len take care of that, then?

Thanks,

 Jiri