Re: [nft PATCH 2/2] monitor: Make JSON output respect output_fp

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

On Thu, Nov 16, 2017 at 02:54:02PM +0100, Phil Sutter wrote:
> On Thu, Nov 16, 2017 at 02:38:24PM +0100, Pablo Neira Ayuso wrote:
> > On Thu, Nov 16, 2017 at 09:06:29AM +0100, Phil Sutter wrote:
> > > This introduces a rather nasty macro to call nftnl_*_fprintf() only if
> > > output_fp is valid. On the other hand, it allows to pull the common
> > > parts (format argument, event conversion) into a single place.
> > > 
> > > Signed-off-by: Phil Sutter <phil@nwl.cc>
> > > ---
> > >  src/netlink.c | 38 ++++++++++++++++++--------------------
> > >  1 file changed, 18 insertions(+), 20 deletions(-)
> > > 
> > > diff --git a/src/netlink.c b/src/netlink.c
> > > index 845eeeffd7387..81b92ac1e2d7c 100644
> > > --- a/src/netlink.c
> > > +++ b/src/netlink.c
> > > @@ -40,6 +40,12 @@
> > >  #include <iface.h>
> > >  
> > >  #define nft_mon_print(monh, ...) nft_print(monh->ctx->octx, __VA_ARGS__)
> > > +#define nftnl_mon_print(monh, type, obj, event)				\
> > > +	if (monh->ctx->octx->output_fp) {				\
> > > +		nftnl_##type##_fprintf(monh->ctx->octx->output_fp,	\
> > > +				       obj, monh->format,		\
> > > +				       netlink_msg2nftnl_of(event));	\
> > > +	}
> > 
> > Wait.
> > 
> > Can't we just change nftnl_*_fprintf to do nothing if output_fp is
> > NULL.
> > 
> > That should be safe.
> 
> Looks like you just caught me trying to avoid changing libnftnl. :D
> 
> Yet I still consider the macro valuable since it avoids the nasty
> pointer deref chain.
> 
> I'll prepare a patch to libnftnl and then provide v2 of this patch with
> simplified nftnl_mon_print() macro. ACK?

I wonder if we can just avoid such change by making sure that
monh->ctx->octx->output_fp is always set.

May that work too?