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From: Phil Sutter <phil@nwl.cc>
To: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: netfilter-devel@vger.kernel.org
Subject: Re: [libnftnl PATCH] src: Fix nftnl_assert() on data_len
Date: Sat, 15 Feb 2020 01:43:11 +0100
Message-ID: <20200215004311.GS20005@orbyte.nwl.cc> (raw)
In-Reply-To: <20200214174200.4xrvnlb72qebtvnb@salvia>

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

  reply index

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-14 17:24 Phil Sutter
2020-02-14 17:32 ` Pablo Neira Ayuso
2020-02-14 17:34   ` Phil Sutter
2020-02-14 17:42     ` Pablo Neira Ayuso
2020-02-15  0:43       ` Phil Sutter [this message]
2020-02-15 13:17         ` Pablo Neira Ayuso
2020-02-15 22:58           ` Phil Sutter
2020-02-18 13:42             ` Pablo Neira Ayuso
2020-02-18 18:18               ` Phil Sutter
2020-02-18 21:06                 ` Pablo Neira Ayuso
2020-02-18 23:02                   ` Phil Sutter
2020-02-19  9:32                     ` Pablo Neira Ayuso

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