Re: [PATCH net-next v2 3/3] netlink: add validation of NLA_F_NESTED flag

[Thomas Haller]

On Tue, 2019-07-23 at 11:09 +0200, Michal Kubecek wrote:
> On Tue, Jul 23, 2019 at 10:57:54AM +0200, Thomas Haller wrote:
> > Does this flag and strict validation really provide any value?
> > Commonly a netlink message is a plain TLV blob, and the meaning
> > depends entirely on the policy.
> > 
> > What I mean is that for example
> > 
> >   NLA_PUT_U32 (msg, ATTR_IFINDEX, (uint32_t) ifindex)
> >   NLA_PUT_STRING (msg, ATTR_IFNAME, "net")
> > 
> > results in a 4 bytes payload that does not encode whether the data
> > is
> > a number or a string.
> > 
> > Why is it valuable in this case to encode additional type
> > information
> > inside the message, when it's commonly not done and also not
> > necessary?
> 
> One big advantage of having nested attributes explicitly marked is
> that
> it allows parsers not aware of the semantics to recognize nested
> attributes and parse their inner structure.
> 
> This is very important e.g. for debugging purposes as without the
> flag,
> wireshark can only recurse into nested attributes if it understands
> the
> protocol and knows they are nested, otherwise it displays them only
> as
> an opaque blob (which is what happens for most netlink based
> protocols).
> Another example is mnl_nlmsg_fprintf() function from libmnl which is
> also a valuable debugging aid but without NLA_F_NESTED flags it
> cannot
> show message structure properly.

Hi,

I don't question the use of the flag. I question whether it's necessary
for kernel to strictly require the sending side to aid debuggability.

"e.g. for debugging purposes" makes it sound like it would be important
for something else. I wonder what else.


Anyway. What you elaborate makes sense!! Thanks


My main point was to raise awareness that this is a problem for libnl3.


best,
Thomas

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