Re: [PATCH iproute2-next] iplink: add support for reporting multiple XDP programs

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Fri, 13 Jul 2018 13:59:41 -0700, Stephen Hemminger wrote:
> On Fri, 13 Jul 2018 13:43:59 -0700
> Jakub Kicinski <jakub.kicinski@netronome.com> wrote:
> 
> >  
> > +static void xdp_dump_prog_one(FILE *fp, struct rtattr *tb[IFLA_XDP_MAX + 1],
> > +			      __u32 attr, bool link, bool details, char *pfx)
> > +{
> > +	__u32 prog_id;
> > +
> > +	if (!tb[attr])
> > +		return;
> > +
> > +	prog_id = rta_getattr_u32(tb[attr]);
> > +	if (!details) {
> > +		if (prog_id && !link && attr == IFLA_XDP_PROG_ID)
> > +			fprintf(fp, "/id:%u", prog_id);
> > +		return;
> > +	}
> > +
> > +	if (prog_id) {
> > +		fprintf(fp, "%s    prog/xdp%s ", _SL_, pfx);
> > +		bpf_dump_prog_info(fp, prog_id);
> > +	}  
> 
> Maybe const char *pfx.

Will do!  Looking again at this code, I think I will also do this:

diff --git a/ip/iplink_xdp.c b/ip/iplink_xdp.c
index 0328bc01a981..68629834cc00 100644
--- a/ip/iplink_xdp.c
+++ b/ip/iplink_xdp.c
@@ -121,6 +121,12 @@ static void xdp_dump_json(struct rtattr *tb[IFLA_XDP_MAX + 1])
        xdp_dump_json_one(tb, IFLA_XDP_SKB_PROG_ID, XDP_ATTACHED_SKB);
        xdp_dump_json_one(tb, IFLA_XDP_DRV_PROG_ID, XDP_ATTACHED_DRV);
        xdp_dump_json_one(tb, IFLA_XDP_HW_PROG_ID, XDP_ATTACHED_HW);
+       /* Older kernel - use IFLA_XDP_PROG_ID */
+       if (tb[IFLA_XDP_PROG_ID] &&
+           !(tb[IFLA_XDP_ATTACHED_SKB] ||
+             tb[IFLA_XDP_ATTACHED_DRV] ||
+             tb[IFLA_XDP_ATTACHED_HW]))
+               xdp_dump_json_one(tb, IFLA_XDP_PROG_ID, mode);
        close_json_array(PRINT_JSON, NULL);
 
        close_json_object();

So that on older kernels we will still be able to depend on the
contents of the "attached" array, even if kernel does not know to
report program per-mode, yet.

> I prefer to not use "printf(fp," and use print_string(PRINT_FP, NULL, "%s", ...)
> because otherwise you end up mixing strings and json format output in the
> same result.
> 
> You should be able to do
> 	tc -j ... 
> and always get valid JSON output.
> 
> One quick way to test json validation is to pipe it into python:
> 	tc -j ... | python -mjson.tool

Note that XDP has separate print functions for plain text and JSON, and
the flow gets separated early on:

	mode = rta_getattr_u8(tb[IFLA_XDP_ATTACHED]);
	if (mode == XDP_ATTACHED_NONE)
		return;
	else if (is_json_context())
		return details ? (void)0 : xdp_dump_json(tb);

	... non-JSON handling follows...

The use of fprintfs is therefore okay.  Do you have a preference for
using the wrapper, even if fprintf is safe?  It's brevity vs
consistency, I guess.  We'd need a separate patch for that, 'cause I'm
not touching all the fprintfs in the file, anyway.