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

[Phil Sutter <phil@nwl.cc>]

Hi Pablo,

On Sat, Feb 15, 2020 at 02:17:13PM +0100, Pablo Neira Ayuso wrote:
> On Sat, Feb 15, 2020 at 01:43:11AM +0100, Phil Sutter wrote:
> > 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.
> 
> Thanks for explaining, Phil. If the problem is just
> NFTNL_CHAIN_DEVICES and NFTNL_FLOWTABLE_DEVICES, probably this is just
> fine? So zero data-length is reversed for arrays and update
> nftnl_assert() to skip data_len == 0, ie.
> 
> > | #define nftnl_assert(val, attr, expr) \
> > |  ((!val || data_len == 0 || expr) ? \
> > |  (void)0 : __nftnl_assert_fail(attr, __FILE__, __LINE__))

Your proposed patch would allow to call e.g.:

| nftnl_chain_get_u32(c, NFTNL_CHAIN_DEVICES)

This would return (uint32_t)*(&c->dev_array[0]), I highly doubt we
should allow this. Unless I miss something, it is certainly a
programming error if someone calls any of the nftnl_chain_get_{u,s}*
getters on NFTNL_CHAIN_DEVICES attribute. So aborting with error message
in nftnl_assert() is not only OK but actually helpful, no?

Cheers, Phil