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From: Pablo Neira Ayuso <pablo@netfilter.org>
To: Phil Sutter <phil@nwl.cc>, netfilter-devel@vger.kernel.org
Subject: Re: [libnftnl PATCH] src: Fix nftnl_assert() on data_len
Date: Tue, 18 Feb 2020 14:42:27 +0100
Message-ID: <20200218134227.yndixbtxjzq3jznk@salvia> (raw)
In-Reply-To: <20200215225855.GU20005@orbyte.nwl.cc>

Hi Phil,

On Sat, Feb 15, 2020 at 11:58:55PM +0100, Phil Sutter wrote:
> 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?

Indeed, good point.

I don't think nftnl_flowtable_set_data() is good for these two device
array.

I just sent a patch, I forgot to finish the _set_array() and
_get_array() helpers for the flowtable, the definition in the header
file prooves this.

Can we introduce these new interfaces? Then, update nftables to use it.
Then, at some point, set *data_len = 0 for these array datatypes. Yes,
it's a bit longer term, but better fix this interface. But setting all
these data_len to zero when in most cases it is going to be thereafter
properly set to the datatype length is...

Would this work for you? I know it is not so short term.

  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
2020-02-15 13:17         ` Pablo Neira Ayuso
2020-02-15 22:58           ` Phil Sutter
2020-02-18 13:42             ` Pablo Neira Ayuso [this message]
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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