[PATCH RESEND 1/8] netlink: call get_fd_nl_family before decode nlmsghdr

[Dmitry V. Levin]

On Sun, Aug 06, 2017 at 07:52:17AM +0800, JingPiao Chen wrote:
> On Sun, Aug 06, 2017 at 02:10:16AM +0300, Dmitry V. Levin wrote:
> > On Tue, Aug 01, 2017 at 07:48:41AM +0800, JingPiao Chen wrote:
> > > Prepare for NETLINK_KOBJECT_UEVENT decode. The messages
> > > of NETLINK_KOBJECT_UEVENT do not contain nlmsghdr.
> > [...]
> > > (decode_nlmsghdr_with_payload): Skip family specific decoders
> > > for type < NLMSG_MIN_TYPE && type != NLMSG_DONE.
> > [...]
> > > @@ -182,7 +177,7 @@ decode_nlmsg_type(const uint16_t type, const unsigned int family)
> > >  	const struct xlat *xlat = netlink_types;
> > >  	const char *dflt = "NLMSG_???";
> > >  
> > > -	if (type != NLMSG_DONE && family < ARRAY_SIZE(nlmsg_types)) {
> > > +	if (type >= NLMSG_MIN_TYPE && family < ARRAY_SIZE(nlmsg_types)) {
> > 
> > Why?  How this is related to the rest of NETLINK_KOBJECT_UEVENT change?
> 
> Previous code when type < NLMSG_MIN_TYPE && type != NLMSG_DONE,
> family = -2 (NL_FAMILY_DEFAULT), family < ARRAY_SIZE(nlmsg_types) filter
> this case. Now get family ignore the nlmsg_type.
> When type < NLMSG_MIN_TYPE && type != NLMSG_DONE, family is not
> a negative. family < ARRAY_SIZE(nlmsg_types) can not filter this case.
> 
> Related commit:
> v4.17-43-g1b63425, v4.17-44-g8700030: These commit introduce get_fd_nl_family.
> And get family only when type >= NLMSG_MIN_TYPE.
> 
> v4.17-123-g54aed21: This commit changed get family when
> type >= NLMSG_MIN_TYPE || type == NLMSG_DONE.

Now that you change things to call get_fd_nl_family from decode_netlink
unconditionally, family is available unconditionally, too (although it
can be -1).  This is all clear enough but doesn't answer my question:

Why do you think it means that decode_nlmsg_type should not use
family-specific decoders for message type < NLMSG_MIN_TYPE?
Is decoding of these message types 100% family-agnostic?

It's so far from being obvious that if it's true, there must be a comment
explaining why it's true.


-- 
ldv
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