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

On Wed, Jan 28, 2015 at 8:25 AM, Arnaldo Carvalho de Melo
[off-list ref] wrote:

Em Wed, Jan 28, 2015 at 01:24:15PM -0300, Arnaldo Carvalho de Melo escreveu:

quoted

Em Tue, Jan 27, 2015 at 08:06:09PM -0800, Alexei Starovoitov escreveu:

quoted

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)

quoted

in bpf_helpers.h to have it work?

yes, that will work just fine.
Since it's an example I made it explicit that bpf_memcmp()
has memcmp() semantics, but little bit different:
int bpf_memcmp(void *unsafe_ptr, void *safe_ptr, int size)
meaning that one of the pointers can point anywhere and
the function will be doing probe_kernel_read() underneath
similar to bpf_fetch_*() helpers.

If it was plain memcmp() it would give a wrong impression
that vanilla memcmp() can be used.
In general the programs cannot use any library functions
outside of helpers defined in uapi/linux/bpf.h

bpf_fetch_*() helpers are also explicit in examples.
If one need to do a lot of pointer walking, then macro like
#define D(P) ((typeof(P))bpf_fetch_ptr(&P))
would be easier to use: p = D(D(skb->dev)->ifalias)
multiple pointer derefs would look more natural...