Re: [PATCH nft v2] datatype: Display pre-defined inet_service values in host byte order

[Pablo Neira Ayuso <pablo@netfilter.org>]

On Thu, Dec 08, 2016 at 03:16:06PM -0200, Elise Lennion wrote:
> On Wed, Dec 07, 2016 at 08:25:10PM +0100, Pablo Neira Ayuso wrote:
> > Hi Elise,
> > 
> > On Wed, Dec 07, 2016 at 05:03:31PM -0200, Elise Lennion wrote:
> > > nft describe displays, to the user, which values are available for a selector,
> > > then the values should be in host byte order.
> > > 
> > > Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
> > > Fixes: ccc5da470e76 ("datatype: Replace getnameinfo() by internal lookup table")
> > > Signed-off-by: Elise Lennion <elise.lennion@gmail.com>
> > > ---
> > > 
> > >  v2: Created a function to convert different types and number of bytes
> > > 
> > >  include/datatype.h |  3 ++-
> > >  src/datatype.c     | 27 ++++++++++++++++++++++++---
> > >  src/expression.c   |  3 ++-
> > >  3 files changed, 28 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/include/datatype.h b/include/datatype.h
> > > index d4fe817..a7db1df 100644
> > > --- a/include/datatype.h
> > > +++ b/include/datatype.h
> > > @@ -191,7 +191,8 @@ extern struct error_record *symbolic_constant_parse(const struct expr *sym,
> > >  extern void symbolic_constant_print(const struct symbol_table *tbl,
> > >  				    const struct expr *expr, bool quotes);
> > >  extern void symbol_table_print(const struct symbol_table *tbl,
> > > -			       const struct datatype *dtype);
> > > +			       const struct datatype *dtype,
> > > +			       enum byteorder byteorder);
> > >  
> > >  extern struct symbol_table *rt_symbol_table_init(const char *filename);
> > >  extern void rt_symbol_table_free(struct symbol_table *tbl);
> > > diff --git a/src/datatype.c b/src/datatype.c
> > > index b5d73bc..c884171 100644
> > > --- a/src/datatype.c
> > > +++ b/src/datatype.c
> > > @@ -180,15 +180,36 @@ void symbolic_constant_print(const struct symbol_table *tbl,
> > >  		printf("%s", s->identifier);
> > >  }
> > >  
> > > +static void big_to_host_byteorder(void *value, const unsigned int nbytes)
> > > +{
> > > +	unsigned char *p = value;
> > > +	unsigned int i;
> > > +
> > > +	for (i = 0; i < nbytes / 2; i++) {
> > > +		p[i] ^= p[nbytes - 1 - i];
> > > +		p[nbytes - 1 - i] ^= p[i];
> > > +		p[i] ^= p[nbytes - 1 - i];
> > > +	}
> > 
> > Sorry, probably I should have specified a bit more.
> > 
> > I suggest you use libgmp for this, that allows any arbitrary word
> > size. See mpz_init() to initialize the variable, then
> > mpz_switch_byteorder() to change byteorder. Then, export it via
> > mpz_export_data() back to fixed standard datatypes.
> > 
> > libgmp is well documented, please have a look at it.
> > 
> > Thanks.
> 
> Hello Pablo,
> 
> Should I write a function that switches byteorder of different types,
> for possible general use, or just use libgmp inside symbol_table_print?

A function that you can place under src/gmputil.c would be good, so we
can reuse it later if we have similar needs.

> In symbol_table_print the converted values have a specific type, so,
> something similar to the following is enough, right?
> 
> ...
> 
>  if (byteorder == BYTEORDER_BIG_ENDIAN) {
>  	mpz_init_set_ui(v, value);
>  	mpz_switch_byteorder(v, nbytes);
>  	mpz_export_data(&value, v, BYTEORDER_HOST_ENDIAN, nbytes);
>  	mpz_clear(v);
>  }

Yes, I think that code is on the right track. Thanks.