Re: [libnftnl PATCH] src: Fix nftnl_assert() on data_len

[Phil Sutter <phil@nwl.cc>]

Hi Pablo,

On Fri, Feb 14, 2020 at 06:42:00PM +0100, Pablo Neira Ayuso wrote:
> On Fri, Feb 14, 2020 at 06:34:50PM +0100, Phil Sutter wrote:
> > On Fri, Feb 14, 2020 at 06:32:47PM +0100, Pablo Neira Ayuso wrote:
> > > On Fri, Feb 14, 2020 at 06:24:17PM +0100, Phil Sutter wrote:
> > > > Typical idiom for *_get_u*() getters is to call *_get_data() and make
> > > > sure data_len matches what each of them is returning. Yet they shouldn't
> > > > trust *_get_data() to write into passed pointer to data_len since for
> > > > chains and NFTNL_CHAIN_DEVICES attribute, it does not. Make sure these
> > > > assert() calls trigger in those cases.
> > > 
> > > The intention to catch for unset attributes through the assertion,
> > > right?
> > 
> > No, this is about making sure that no wrong getter is called, e.g.
> > nftnl_chain_get_u64() with e.g. NFTNL_CHAIN_HOOKNUM attribute which is
> > only 32bits.
> 
> I think it will also catch the case I'm asking. If attribute is unset,
> then nftnl_chain_get_data() returns NULL and the assertion checks
> data_len, which has not been properly initialized.

With nftnl_assert() being (shortened):

| #define nftnl_assert(val, attr, expr) \
|  ((!val || expr) ? \
|  (void)0 : __nftnl_assert_fail(attr, __FILE__, __LINE__))

Check for 'expr' (which is passed as 'data_len == sizeof(<something>)')
will only happen if 'val' is not NULL. Callers then return like so:

| return val ? *val : 0;

This means that if you pass an unset attribute to the getter, it will
simply return 0.

Cheers, Phil