Subject: Re: [PATCH tip 4/9] samples: bpf: simple tracing example in eBPF assembler

From: Masami Hiramatsu <[email-masked]>
Date: Tue, 20 Jan 2015 20:57:38 +0900

(2015/01/16 13:16), Alexei Starovoitov wrote:
> 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

Hmm, this eBPF assembly macros are very interesting. Maybe I can
replace current kprobe's argument "fetching methods" with this.

Thank you,

> 
> Usage:
> $ sudo dropmon
> location 0xffffffff81695995 count 1
> location 0xffffffff816d0da9 count 2
> 
> location 0xffffffff81695995 count 2
> location 0xffffffff816d0da9 count 2
> 
> location 0xffffffff81695995 count 3
> location 0xffffffff816d0da9 count 2
> 
> $ addr2line -ape ./bld_x64/vmlinux 0xffffffff81695995 0xffffffff816d0da9
> 0xffffffff81695995: ./bld_x64/../net/ipv4/icmp.c:1038
> 0xffffffff816d0da9: ./bld_x64/../net/unix/af_unix.c:1231
> 
> Signed-off-by: Alexei Starovoitov <[email-masked]>
> ---
>  samples/bpf/Makefile  |    2 +
>  samples/bpf/dropmon.c |  129 +++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 131 insertions(+)
>  create mode 100644 samples/bpf/dropmon.c
> 
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index b5b3600dcdf5..789691374562 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -6,7 +6,9 @@ hostprogs-y := test_verifier test_maps
>  hostprogs-y += sock_example
>  hostprogs-y += sockex1
>  hostprogs-y += sockex2
> +hostprogs-y += dropmon
>  
> +dropmon-objs := dropmon.o libbpf.o
>  test_verifier-objs := test_verifier.o libbpf.o
>  test_maps-objs := test_maps.o libbpf.o
>  sock_example-objs := sock_example.o libbpf.o
> diff --git a/samples/bpf/dropmon.c b/samples/bpf/dropmon.c
> new file mode 100644
> index 000000000000..9a2cd3344d69
> --- /dev/null
> +++ b/samples/bpf/dropmon.c
> @@ -0,0 +1,129 @@
> +/* 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)
> +{
> +	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;
> +	}
> +
> +	/* the following eBPF program is equivalent to C:
> +	 * int filter(struct bpf_context *ctx)
> +	 * {
> +	 *   long loc = ctx->arg2;
> +	 *   long init_val = 1;
> +	 *   long *value;
> +	 *
> +	 *   value = bpf_map_lookup_elem(MAP_ID, &loc);
> +	 *   if (value) {
> +	 *      __sync_fetch_and_add(value, 1);
> +	 *   } else {
> +	 *      bpf_map_update_elem(MAP_ID, &loc, &init_val, BPF_ANY);
> +	 *   }
> +	 *   return 0;
> +	 * }
> +	 */
> +	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, map_fd),
> +		BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
> +		BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 4),
> +		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_MOV64_IMM(BPF_REG_0, 0), /* r0 = 0 */
> +		BPF_EXIT_INSN(),
> +		BPF_ST_MEM(BPF_DW, BPF_REG_10, -16, 1), /* *(u64 *)(fp - 16) = 1 */
> +		BPF_MOV64_IMM(BPF_REG_4, BPF_ANY),
> +		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, map_fd),
> +		BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_update_elem),
> +		BPF_MOV64_IMM(BPF_REG_0, 0), /* r0 = 0 */
> +		BPF_EXIT_INSN(),
> +	};
> +
> +	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%s", strerror(errno), bpf_log_buf);
> +		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)
> +{
> +	FILE *f;
> +
> +	/* start ping in the background to get some kfree_skb events */
> +	f = popen("ping -c5 localhost", "r");
> +	(void) f;
> +
> +	dropmon();
> +	return 0;
> +}
> 

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: [email-masked]