netdev - Re: [PATCH iproute2] devlink: Ignore unknown attributes

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Date:   Fri, 19 Jan 2018 15:40:31 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     David Ahern <dsahern@...il.com>
Cc:     Arkadi Sharshevsky <arkadis@...lanox.com>, netdev@...r.kernel.org,
        davem@...emloft.net, mlxsw@...lanox.com
Subject: Re: [PATCH iproute2] devlink: Ignore unknown attributes

On Fri, 19 Jan 2018 14:27:06 -0800
David Ahern <dsahern@...il.com> wrote:

> On 1/19/18 2:02 PM, Stephen Hemminger wrote:
> > On Thu, 18 Jan 2018 20:42:44 -0800
> > David Ahern <dsahern@...il.com> wrote:
> >   
> >> On 1/17/18 5:28 AM, Arkadi Sharshevsky wrote:  
> >>> In case of extending the UAPI old packages would break.
> >>>
> >>> Signed-off-by: Arkadi Sharshevsky <arkadis@...lanox.com>
> >>> ---
> >>>  devlink/devlink.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/devlink/devlink.c b/devlink/devlink.c
> >>> index 39cda06..c9d1838 100644
> >>> --- a/devlink/devlink.c
> >>> +++ b/devlink/devlink.c
> >>> @@ -343,7 +343,7 @@ static int attr_cb(const struct nlattr *attr, void *data)
> >>>  	int type;
> >>>  
> >>>  	if (mnl_attr_type_valid(attr, DEVLINK_ATTR_MAX) < 0)
> >>> -		return MNL_CB_ERROR;
> >>> +		return MNL_CB_OK;
> >>>  
> >>>  	type = mnl_attr_get_type(attr);
> >>>  	if (mnl_attr_validate(attr, devlink_policy[type]) < 0)
> >>>     
> >>
> >> What's the point of calling mnl_attr_type_valid if you disregard a
> >> failure? you might as well not call mnl_attr_type_valid at all.  
> > 
> > The way mnl handles attributes, you have to have a callback and it is up
> > to the callback to copy the values it wants.  The idea is that old code
> > running against a newer kernel will have a smaller array of attributes
> > it wants, and only copy those.
> >   
> 
> mnl_attr_type_valid calls mnl_attr_get_type which does attr->nla_type &
> NLA_TYPE_MASK. Since you are no longer acknowledging the return code of
> mnl_attr_type_valid, you don't care about its checks so you might as
> well not call it. I don't see anything in libmnl that checks that
> mnl_attr_type_valid is invoked on an attr, so hence my question -- given
> the change above why call it all?

 The part that matters is:

static int attr_cb(const struct nlattr *attr, void *data)
{
	const struct nlattr **tb = data;
	int type;

	if (mnl_attr_type_valid(attr, DEVLINK_ATTR_MAX) < 0)         << makes sure that type < DEVLINK_ATTR_MAX
		return MNL_CB_OK;

	type = mnl_attr_get_type(attr);
	if (mnl_attr_validate(attr, devlink_policy[type]) < 0)        << this part doesn't matter really
		return MNL_CB_ERROR;

	tb[type] = attr;					<< necessary so that tb[] is filled in.
	return MNL_CB_OK;
}