[PATCH RFC v7 net-next 22/28] samples: bpf: example of tracing filters with eBPF

[Alexei Starovoitov]

simple packet drop monitor:
- in-kernel eBPF program attaches to kfree_skb() event and records number
  of packet drops at given location
- userspace iterates over the map every second and prints stats

Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxxxx>
---
 samples/bpf/Makefile  |   12 +++++
 samples/bpf/dropmon.c |  122 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)
 create mode 100644 samples/bpf/Makefile
 create mode 100644 samples/bpf/dropmon.c

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
new file mode 100644
index 000000000000..8dfbacaef9ba
--- /dev/null
+++ b/samples/bpf/Makefile
@@ -0,0 +1,12 @@
+# kbuild trick to avoid linker error. Can be omitted if a module is built.
+obj- := dummy.o
+
+# List of programs to build
+hostprogs-y := dropmon
+
+dropmon-objs := dropmon.o libbpf.o
+
+# Tell kbuild to always build the programs
+always := $(hostprogs-y)
+
+HOSTCFLAGS += -I$(objtree)/usr/include
diff --git a/samples/bpf/dropmon.c b/samples/bpf/dropmon.c
new file mode 100644
index 000000000000..3b921e2f4c8f
--- /dev/null
+++ b/samples/bpf/dropmon.c
@@ -0,0 +1,122 @@
+/* simple packet drop monitor:
+ * - in-kernel eBPF program attaches to kfree_skb() event and records number
+ *   of packet drops at given location
+ * - userspace iterates over the map every second and prints stats
+ */
+#include <stdio.h>
+#include <unistd.h>
+#include <linux/bpf.h>
+#include <errno.h>
+#include <linux/unistd.h>
+#include <string.h>
+#include <linux/filter.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <stdbool.h>
+#include "libbpf.h"
+
+#define TRACEPOINT "/sys/kernel/debug/tracing/events/skb/kfree_skb/"
+
+static int write_to_file(const char *file, const char *str, bool keep_open)
+{
+	int fd, err;
+
+	fd = open(file, O_WRONLY);
+	err = write(fd, str, strlen(str));
+	(void) err;
+
+	if (keep_open) {
+		return fd;
+	} else {
+		close(fd);
+		return -1;
+	}
+}
+
+static int dropmon(void)
+{
+	/* the following eBPF program is equivalent to C:
+	 * void filter(struct bpf_context *ctx)
+	 * {
+	 *   long loc = ctx->arg2;
+	 *   long init_val = 1;
+	 *   void *value;
+	 *
+	 *   value = bpf_map_lookup_elem(MAP_ID, &loc);
+	 *   if (value) {
+	 *      (*(long *) value) += 1;
+	 *   } else {
+	 *      bpf_map_update_elem(MAP_ID, &loc, &init_val);
+	 *   }
+	 * }
+	 */
+	static struct bpf_insn prog[] = {
+		BPF_LDX_MEM(BPF_DW, BPF_REG_2, BPF_REG_1, 8), /* r2 = *(u64 *)(r1 + 8) */
+		BPF_STX_MEM(BPF_DW, BPF_REG_10, BPF_REG_2, -8), /* *(u64 *)(fp - 8) = r2 */
+		BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+		BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), /* r2 = fp - 8 */
+		BPF_LD_MAP_FD(BPF_REG_1, 0),
+		BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+		BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 3),
+		BPF_MOV64_IMM(BPF_REG_1, 1), /* r1 = 1 */
+		BPF_RAW_INSN(BPF_STX | BPF_XADD | BPF_DW, BPF_REG_0, BPF_REG_1, 0, 0), /* xadd r0 += r1 */
+		BPF_EXIT_INSN(),
+		BPF_ST_MEM(BPF_DW, BPF_REG_10, -16, 1), /* *(u64 *)(fp - 16) = 1 */
+		BPF_MOV64_REG(BPF_REG_3, BPF_REG_10),
+		BPF_ALU64_IMM(BPF_ADD, BPF_REG_3, -16), /* r3 = fp - 16 */
+		BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+		BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8), /* r2 = fp - 8 */
+		BPF_LD_MAP_FD(BPF_REG_1, 0),
+		BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_update_elem),
+		BPF_EXIT_INSN(),
+	};
+
+	long long key, next_key, value = 0;
+	int prog_fd, map_fd, i;
+	char fmt[32];
+
+	map_fd = bpf_create_map(BPF_MAP_TYPE_HASH, sizeof(key), sizeof(value), 1024);
+	if (map_fd < 0) {
+		printf("failed to create map '%s'\n", strerror(errno));
+		goto cleanup;
+	}
+
+	prog[4].imm = map_fd;
+	prog[16].imm = map_fd;
+
+	prog_fd = bpf_prog_load(BPF_PROG_TYPE_TRACING_FILTER, prog,
+				sizeof(prog), "GPL");
+	if (prog_fd < 0) {
+		printf("failed to load prog '%s'\n", strerror(errno));
+		return -1;
+	}
+
+	sprintf(fmt, "bpf_%d", prog_fd);
+
+	write_to_file(TRACEPOINT "filter", fmt, true);
+
+	for (i = 0; i < 10; i++) {
+		key = 0;
+		while (bpf_get_next_key(map_fd, &key, &next_key) == 0) {
+			bpf_lookup_elem(map_fd, &next_key, &value);
+			printf("location 0x%llx count %lld\n", next_key, value);
+			key = next_key;
+		}
+		if (key)
+			printf("\n");
+		sleep(1);
+	}
+
+cleanup:
+	/* maps, programs, tracepoint filters will auto cleanup on process exit */
+
+	return 0;
+}
+
+int main(void)
+{
+	dropmon();
+	return 0;
+}
-- 
1.7.9.5

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