netdev - Re: [PATCH v2 linux-trace 4/8] samples: bpf: simple tracing example in C

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Date:	Wed, 28 Jan 2015 13:25:57 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	linux-api@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 linux-trace 4/8] samples: bpf: simple tracing example
 in C

Em Wed, Jan 28, 2015 at 01:24:15PM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Jan 27, 2015 at 08:06:09PM -0800, Alexei Starovoitov escreveu:
> > diff --git a/samples/bpf/tracex1_kern.c b/samples/bpf/tracex1_kern.c
> > new file mode 100644
> > index 000000000000..7849ceb4bce6
> > --- /dev/null
> > +++ b/samples/bpf/tracex1_kern.c
> > @@ -0,0 +1,28 @@
> > +#include <linux/skbuff.h>
> > +#include <linux/netdevice.h>
> > +#include <uapi/linux/bpf.h>
> > +#include <trace/bpf_trace.h>
> > +#include "bpf_helpers.h"
> > +
> > +SEC("events/net/netif_receive_skb")
> > +int bpf_prog1(struct bpf_context *ctx)
> > +{
> > +	/*
> > +	 * attaches to /sys/kernel/debug/tracing/events/net/netif_receive_skb
> > +	 * prints events for loobpack device only
> > +	 */
> > +	char devname[] = "lo";
> > +	struct net_device *dev;
> > +	struct sk_buff *skb = 0;
> > +
> > +	skb = (struct sk_buff *) ctx->arg1;
> > +	dev = bpf_fetch_ptr(&skb->dev);
> > +	if (bpf_memcmp(dev->name, devname, 2) == 0)
> 
> I'm only starting to look at all this, so bear with me... But why do we
> need to have it as "bpf_memcmp"? Can't we simply use it as "memcmp" and
> have it use the right function?
> 
> Less typing, perhaps we would need to have a:
> 
> #define memcmp bpf_memcmp(s1, s2, n) bpf_memcmp(s1, s2, n)

Argh, like this:

#define memcmp(s1, s2, n) bpf_memcmp(s1, s2, n)
 
> in bpf_helpers.h to have it work?
> 
> - Arnaldo
> 
> > +		/* print event using default tracepoint format */
> > +		return 1;
> > +
> > +	/* drop event */
> > +	return 0;
> > +}
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