the logic of the example is similar to tracex2, but syscall 'write' statistics
is capturead from kprobe placed at sys_write function instead of through
syscall instrumentation.
Also tracex4_kern.c has a different way of doing log2 in C.
Note, unlike tracepoint and syscall programs, kprobe programs receive
'struct pt_regs' as an input. It's responsibility of the program author
or higher level dynamic tracing tool to match registers to function arguments.
Since pt_regs are architecture dependent, programs are also arch dependent,
unlike tracepoint/syscalls programs which are universal.

Usage:
$ sudo tracex4
writing bpf-6 -> /sys/kernel/debug/tracing/events/kprobes/sys_write/filter
2216443+0 records in
2216442+0 records out
1134818304 bytes (1.1 GB) copied, 2.00746 s, 565 MB/s

           kprobe sys_write() stats
     byte_size       : count     distribution
       1 -> 1        : 0        |                                      |
       2 -> 3        : 0        |                                      |
       4 -> 7        : 0        |                                      |
       8 -> 15       : 0        |                                      |
      16 -> 31       : 0        |                                      |
      32 -> 63       : 0        |                                      |
      64 -> 127      : 1        |                                      |
     128 -> 255      : 0        |                                      |
     256 -> 511      : 0        |                                      |
     512 -> 1023     : 2214734  |************************************* |

Signed-off-by: Alexei Starovoitov <a...@plumgrid.com>
---
 samples/bpf/Makefile       |    4 +++
 samples/bpf/bpf_load.c     |    3 ++
 samples/bpf/tracex4_kern.c |   36 +++++++++++++++++++
 samples/bpf/tracex4_user.c |   83 ++++++++++++++++++++++++++++++++++++++++++++
 4 files changed, 126 insertions(+)
 create mode 100644 samples/bpf/tracex4_kern.c
 create mode 100644 samples/bpf/tracex4_user.c

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index da0efd8032ab..22c7a38f3f95 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -10,6 +10,7 @@ hostprogs-y += dropmon
 hostprogs-y += tracex1
 hostprogs-y += tracex2
 hostprogs-y += tracex3
+hostprogs-y += tracex4
 
 dropmon-objs := dropmon.o libbpf.o
 test_verifier-objs := test_verifier.o libbpf.o
@@ -20,6 +21,7 @@ sockex2-objs := bpf_load.o libbpf.o sockex2_user.o
 tracex1-objs := bpf_load.o libbpf.o tracex1_user.o
 tracex2-objs := bpf_load.o libbpf.o tracex2_user.o
 tracex3-objs := bpf_load.o libbpf.o tracex3_user.o
+tracex4-objs := bpf_load.o libbpf.o tracex4_user.o
 
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)
@@ -28,6 +30,7 @@ always += sockex2_kern.o
 always += tracex1_kern.o
 always += tracex2_kern.o
 always += tracex3_kern.o
+always += tracex4_kern.o
 
 HOSTCFLAGS += -I$(objtree)/usr/include
 
@@ -37,6 +40,7 @@ HOSTLOADLIBES_sockex2 += -lelf
 HOSTLOADLIBES_tracex1 += -lelf
 HOSTLOADLIBES_tracex2 += -lelf
 HOSTLOADLIBES_tracex3 += -lelf
+HOSTLOADLIBES_tracex4 += -lelf
 
 # point this to your LLVM backend with bpf support
 LLC=$(srctree)/tools/bpf/llvm/bld/Debug+Asserts/bin/llc
diff --git a/samples/bpf/bpf_load.c b/samples/bpf/bpf_load.c
index 788ac51c1024..d8c5176f0564 100644
--- a/samples/bpf/bpf_load.c
+++ b/samples/bpf/bpf_load.c
@@ -25,6 +25,7 @@ int prog_cnt;
 static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
 {
        bool is_socket = strncmp(event, "socket", 6) == 0;
+       bool is_kprobe = strncmp(event, "events/kprobes/", 15) == 0;
        enum bpf_prog_type prog_type;
        char path[256] = DEBUGFS;
        char fmt[32];
@@ -32,6 +33,8 @@ static int load_and_attach(const char *event, struct bpf_insn 
*prog, int size)
 
        if (is_socket)
                prog_type = BPF_PROG_TYPE_SOCKET_FILTER;
+       else if (is_kprobe)
+               prog_type = BPF_PROG_TYPE_KPROBE_FILTER;
        else
                prog_type = BPF_PROG_TYPE_TRACING_FILTER;
 
diff --git a/samples/bpf/tracex4_kern.c b/samples/bpf/tracex4_kern.c
new file mode 100644
index 000000000000..9646f9e43417
--- /dev/null
+++ b/samples/bpf/tracex4_kern.c
@@ -0,0 +1,36 @@
+#include <linux/skbuff.h>
+#include <linux/netdevice.h>
+#include <uapi/linux/bpf.h>
+#include <trace/bpf_trace.h>
+#include "bpf_helpers.h"
+
+static unsigned int log2l(unsigned long long n)
+{
+#define S(k) if (n >= (1ull << k)) { i += k; n >>= k; }
+       int i = -(n == 0);
+       S(32); S(16); S(8); S(4); S(2); S(1);
+       return i;
+#undef S
+}
+
+struct bpf_map_def SEC("maps") my_hist_map = {
+       .type = BPF_MAP_TYPE_ARRAY,
+       .key_size = sizeof(u32),
+       .value_size = sizeof(long),
+       .max_entries = 64,
+};
+
+SEC("events/kprobes/sys_write")
+int bpf_prog4(struct pt_regs *regs)
+{
+       long write_size = regs->dx; /* $rdx contains 3rd argument to a function 
*/
+       long init_val = 1;
+       void *value;
+       u32 index = log2l(write_size);
+
+       value = bpf_map_lookup_elem(&my_hist_map, &index);
+       if (value)
+               __sync_fetch_and_add((long *)value, 1);
+       return 0;
+}
+char _license[] SEC("license") = "GPL";
diff --git a/samples/bpf/tracex4_user.c b/samples/bpf/tracex4_user.c
new file mode 100644
index 000000000000..47dde2791f9e
--- /dev/null
+++ b/samples/bpf/tracex4_user.c
@@ -0,0 +1,83 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <linux/bpf.h>
+#include "libbpf.h"
+#include "bpf_load.h"
+
+#define MAX_INDEX      64
+#define MAX_STARS      38
+
+static void stars(char *str, long val, long max, int width)
+{
+       int i;
+
+       for (i = 0; i < (width * val / max) - 1 && i < width - 1; i++)
+               str[i] = '*';
+       if (val > max)
+               str[i - 1] = '+';
+       str[i] = '\0';
+}
+
+static void print_hist(int fd)
+{
+       int key;
+       long value;
+       long data[MAX_INDEX] = {};
+       char starstr[MAX_STARS];
+       int i;
+       int max_ind = -1;
+       long max_value = 0;
+
+       for (key = 0; key < MAX_INDEX; key++) {
+               bpf_lookup_elem(fd, &key, &value);
+               data[key] = value;
+               if (value && key > max_ind)
+                       max_ind = key;
+               if (value > max_value)
+                       max_value = value;
+       }
+
+       printf("\n           kprobe sys_write() stats\n");
+       printf("     byte_size       : count     distribution\n");
+       for (i = 1; i <= max_ind + 1; i++) {
+               stars(starstr, data[i - 1], max_value, MAX_STARS);
+               printf("%8ld -> %-8ld : %-8ld |%-*s|\n",
+                      (1l << i) >> 1, (1l << i) - 1, data[i - 1],
+                      MAX_STARS, starstr);
+       }
+}
+static void int_exit(int sig)
+{
+       print_hist(map_fd[0]);
+       exit(0);
+}
+
+int main(int ac, char **argv)
+{
+       char filename[256];
+       FILE *f;
+       int i;
+
+       snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
+
+       signal(SIGINT, int_exit);
+
+       i = system("echo 'p:sys_write sys_write' > 
/sys/kernel/debug/tracing/kprobe_events");
+       (void) i;
+       
+       /* start 'dd' in the background to have plenty of 'write' syscalls */
+       f = popen("dd if=/dev/zero of=/dev/null", "r");
+       (void) f;
+
+       if (load_bpf_file(filename)) {
+               printf("%s", bpf_log_buf);
+               return 1;
+       }
+
+       sleep(2);
+       kill(0, SIGINT); /* send Ctrl-C to self and to 'dd' */
+
+       return 0;
+}
-- 
1.7.9.5

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