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

From: Arnaldo Carvalho de Melo <[email-masked]>
Date: Wed, 28 Jan 2015 13:24:15 -0300

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)

in bpf_helpers.h to have it work?

- Arnaldo

> +		/* print event using default tracepoint format */
> +		return 1;
> +
> +	/* drop event */
> +	return 0;
> +}