samples/bpf: Add simple non-portable kprobe filter example

tracex1_kern.c - C program compiled into BPF.

It attaches to kprobe:netif_receive_skb()

When skb->dev->name == "lo", it prints sample debug message into
trace_pipe via bpf_trace_printk() helper function.

tracex1_user.c - corresponding user space component that:
  - loads BPF program via bpf() syscall
  - opens kprobes:netif_receive_skb event via perf_event_open()
    syscall
  - attaches the program to event via ioctl(event_fd,
    PERF_EVENT_IOC_SET_BPF, prog_fd);
  - prints from trace_pipe

Note, this BPF program is non-portable. It must be recompiled
with current kernel headers. kprobe is not a stable ABI and
BPF+kprobe scripts may no longer be meaningful when kernel
internals change.

No matter in what way the kernel changes, neither the kprobe,
nor the BPF program can ever crash or corrupt the kernel,
assuming the kprobes, perf and BPF subsystem has no bugs.

The verifier will detect that the program is using
bpf_trace_printk() and the kernel will print 'this is a DEBUG
kernel' warning banner, which means that bpf_trace_printk()
should be used for debugging of the BPF program only.

Usage:
$ sudo tracex1
            ping-19826 [000] d.s2 63103.382648: : skb ffff880466b1ca00 len 84
            ping-19826 [000] d.s2 63103.382684: : skb ffff880466b1d300 len 84

            ping-19826 [000] d.s2 63104.382533: : skb ffff880466b1ca00 len 84
            ping-19826 [000] d.s2 63104.382594: : skb ffff880466b1d300 len 84

Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1427312966-8434-7-git-send-email-ast@plumgrid.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/samples/bpf/libbpf.c b/samples/bpf/libbpf.c
index 46d50b7..7e1efa7 100644
--- a/samples/bpf/libbpf.c
+++ b/samples/bpf/libbpf.c
@@ -81,7 +81,7 @@ char bpf_log_buf[LOG_BUF_SIZE];
 
 int bpf_prog_load(enum bpf_prog_type prog_type,
 		  const struct bpf_insn *insns, int prog_len,
-		  const char *license)
+		  const char *license, int kern_version)
 {
 	union bpf_attr attr = {
 		.prog_type = prog_type,
@@ -93,6 +93,11 @@ int bpf_prog_load(enum bpf_prog_type prog_type,
 		.log_level = 1,
 	};
 
+	/* assign one field outside of struct init to make sure any
+	 * padding is zero initialized
+	 */
+	attr.kern_version = kern_version;
+
 	bpf_log_buf[0] = 0;
 
 	return syscall(__NR_bpf, BPF_PROG_LOAD, &attr, sizeof(attr));
@@ -121,3 +126,10 @@ int open_raw_sock(const char *name)
 
 	return sock;
 }
+
+int perf_event_open(struct perf_event_attr *attr, int pid, int cpu,
+		    int group_fd, unsigned long flags)
+{
+	return syscall(__NR_perf_event_open, attr, pid, cpu,
+		       group_fd, flags);
+}