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

[Elise Lennion <elise.lennion@gmail.com>]

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?

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);
 }

...

Thank you,

elise