samples/bpf: bpf_tail_call example for tracing

kprobe example that demonstrates how future seccomp programs may look like.
It attaches to seccomp_phase1() function and tail-calls other BPF programs
depending on syscall number.

Existing optimized classic BPF seccomp programs generated by Chrome look like:
if (sd.nr < 121) {
  if (sd.nr < 57) {
    if (sd.nr < 22) {
      if (sd.nr < 7) {
        if (sd.nr < 4) {
          if (sd.nr < 1) {
            check sys_read
          } else {
            if (sd.nr < 3) {
              check sys_write and sys_open
            } else {
              check sys_close
            }
          }
        } else {
      } else {
    } else {
  } else {
} else {
}

the future seccomp using native eBPF may look like:
  bpf_tail_call(&sd, &syscall_jmp_table, sd.nr);
which is simpler, faster and leaves more room for per-syscall checks.

Usage:
$ sudo ./tracex5
<...>-366   [001] d...     4.870033: : read(fd=1, buf=00007f6d5bebf000, size=771)
<...>-369   [003] d...     4.870066: : mmap
<...>-369   [003] d...     4.870077: : syscall=110 (one of get/set uid/pid/gid)
<...>-369   [003] d...     4.870089: : syscall=107 (one of get/set uid/pid/gid)
   sh-369   [000] d...     4.891740: : read(fd=0, buf=00000000023d1000, size=512)
   sh-369   [000] d...     4.891747: : write(fd=1, buf=00000000023d3000, size=512)
   sh-369   [000] d...     4.891747: : read(fd=1, buf=00000000023d3000, size=512)
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/Makefile

@@ -10,6 +10,7 @@ hostprogs-y += tracex1
 hostprogs-y += tracex2
 hostprogs-y += tracex3
 hostprogs-y += tracex4
+hostprogs-y += tracex5
 
 test_verifier-objs := test_verifier.o libbpf.o
 test_maps-objs := test_maps.o libbpf.o
@@ -20,6 +21,7 @@ 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
+tracex5-objs := bpf_load.o libbpf.o tracex5_user.o
 
 # Tell kbuild to always build the programs
 always := $(hostprogs-y)
@@ -29,6 +31,7 @@ always += tracex1_kern.o
 always += tracex2_kern.o
 always += tracex3_kern.o
 always += tracex4_kern.o
+always += tracex5_kern.o
 always += tcbpf1_kern.o
 
 HOSTCFLAGS += -I$(objtree)/usr/include
@@ -40,6 +43,7 @@ HOSTLOADLIBES_tracex1 += -lelf
 HOSTLOADLIBES_tracex2 += -lelf
 HOSTLOADLIBES_tracex3 += -lelf
 HOSTLOADLIBES_tracex4 += -lelf -lrt
+HOSTLOADLIBES_tracex5 += -lelf
 
 # point this to your LLVM backend with bpf support
 LLC=$(srctree)/tools/bpf/llvm/bld/Debug+Asserts/bin/llc

samples/bpf/bpf_helpers.h

@@ -21,6 +21,8 @@ static unsigned long long (*bpf_ktime_get_ns)(void) =
 	(void *) BPF_FUNC_ktime_get_ns;
 static int (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
 	(void *) BPF_FUNC_trace_printk;
+static void (*bpf_tail_call)(void *ctx, void *map, int index) =
+	(void *) BPF_FUNC_tail_call;
 
 /* llvm builtin functions that eBPF C program may use to
  * emit BPF_LD_ABS and BPF_LD_IND instructions

samples/bpf/bpf_load.c

@@ -16,6 +16,7 @@
 #include <sys/ioctl.h>
 #include <sys/mman.h>
 #include <poll.h>
+#include <ctype.h>
 #include "libbpf.h"
 #include "bpf_helpers.h"
 #include "bpf_load.h"
@@ -29,6 +30,19 @@ int map_fd[MAX_MAPS];
 int prog_fd[MAX_PROGS];
 int event_fd[MAX_PROGS];
 int prog_cnt;
+int prog_array_fd = -1;
+
+static int populate_prog_array(const char *event, int prog_fd)
+{
+	int ind = atoi(event), err;
+
+	err = bpf_update_elem(prog_array_fd, &ind, &prog_fd, BPF_ANY);
+	if (err < 0) {
+		printf("failed to store prog_fd in prog_array\n");
+		return -1;
+	}
+	return 0;
+}
 
 static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
 {
@@ -54,12 +68,40 @@ static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
 		return -1;
 	}
 
+	fd = bpf_prog_load(prog_type, prog, size, license, kern_version);
+	if (fd < 0) {
+		printf("bpf_prog_load() err=%d\n%s", errno, bpf_log_buf);
+		return -1;
+	}
+
+	prog_fd[prog_cnt++] = fd;
+
+	if (is_socket) {
+		event += 6;
+		if (*event != '/')
+			return 0;
+		event++;
+		if (!isdigit(*event)) {
+			printf("invalid prog number\n");
+			return -1;
+		}
+		return populate_prog_array(event, fd);
+	}
+
 	if (is_kprobe || is_kretprobe) {
 		if (is_kprobe)
 			event += 7;
 		else
 			event += 10;
 
+		if (*event == 0) {
+			printf("event name cannot be empty\n");
+			return -1;
+		}
+
+		if (isdigit(*event))
+			return populate_prog_array(event, fd);
+
 		snprintf(buf, sizeof(buf),
 			 "echo '%c:%s %s' >> /sys/kernel/debug/tracing/kprobe_events",
 			 is_kprobe ? 'p' : 'r', event, event);
@@ -71,18 +113,6 @@ static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
 		}
 	}
 
-	fd = bpf_prog_load(prog_type, prog, size, license, kern_version);
-
-	if (fd < 0) {
-		printf("bpf_prog_load() err=%d\n%s", errno, bpf_log_buf);
-		return -1;
-	}
-
-	prog_fd[prog_cnt++] = fd;
-
-	if (is_socket)
-		return 0;
-
 	strcpy(buf, DEBUGFS);
 	strcat(buf, "events/kprobes/");
 	strcat(buf, event);
@@ -130,6 +160,9 @@ static int load_maps(struct bpf_map_def *maps, int len)
 					   maps[i].max_entries);
 		if (map_fd[i] < 0)
 			return 1;
+
+		if (maps[i].type == BPF_MAP_TYPE_PROG_ARRAY)
+			prog_array_fd = map_fd[i];
 	}
 	return 0;
 }

samples/bpf/tracex5_kern.c

+/* Copyright (c) 2015 PLUMgrid, http://plumgrid.com
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of version 2 of the GNU General Public
+ * License as published by the Free Software Foundation.
+ */
+#include <linux/ptrace.h>
+#include <linux/version.h>
+#include <uapi/linux/bpf.h>
+#include <uapi/linux/seccomp.h>
+#include "bpf_helpers.h"
+
+#define PROG(F) SEC("kprobe/"__stringify(F)) int bpf_func_##F
+
+struct bpf_map_def SEC("maps") progs = {
+	.type = BPF_MAP_TYPE_PROG_ARRAY,
+	.key_size = sizeof(u32),
+	.value_size = sizeof(u32),
+	.max_entries = 1024,
+};
+
+SEC("kprobe/seccomp_phase1")
+int bpf_prog1(struct pt_regs *ctx)
+{
+	struct seccomp_data sd = {};
+
+	bpf_probe_read(&sd, sizeof(sd), (void *)ctx->di);
+
+	/* dispatch into next BPF program depending on syscall number */
+	bpf_tail_call(ctx, &progs, sd.nr);
+
+	/* fall through -> unknown syscall */
+	if (sd.nr >= __NR_getuid && sd.nr <= __NR_getsid) {
+		char fmt[] = "syscall=%d (one of get/set uid/pid/gid)\n";
+		bpf_trace_printk(fmt, sizeof(fmt), sd.nr);
+	}
+	return 0;
+}
+
+/* we jump here when syscall number == __NR_write */
+PROG(__NR_write)(struct pt_regs *ctx)
+{
+	struct seccomp_data sd = {};
+
+	bpf_probe_read(&sd, sizeof(sd), (void *)ctx->di);
+	if (sd.args[2] == 512) {
+		char fmt[] = "write(fd=%d, buf=%p, size=%d)\n";
+		bpf_trace_printk(fmt, sizeof(fmt),
+				 sd.args[0], sd.args[1], sd.args[2]);
+	}
+	return 0;
+}
+
+PROG(__NR_read)(struct pt_regs *ctx)
+{
+	struct seccomp_data sd = {};
+
+	bpf_probe_read(&sd, sizeof(sd), (void *)ctx->di);
+	if (sd.args[2] > 128 && sd.args[2] <= 1024) {
+		char fmt[] = "read(fd=%d, buf=%p, size=%d)\n";
+		bpf_trace_printk(fmt, sizeof(fmt),
+				 sd.args[0], sd.args[1], sd.args[2]);
+	}
+	return 0;
+}
+
+PROG(__NR_mmap)(struct pt_regs *ctx)
+{
+	char fmt[] = "mmap\n";
+	bpf_trace_printk(fmt, sizeof(fmt));
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
+u32 _version SEC("version") = LINUX_VERSION_CODE;

samples/bpf/tracex5_user.c

+#include <stdio.h>
+#include <linux/bpf.h>
+#include <unistd.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <sys/prctl.h>
+#include "libbpf.h"
+#include "bpf_load.h"
+
+/* install fake seccomp program to enable seccomp code path inside the kernel,
+ * so that our kprobe attached to seccomp_phase1() can be triggered
+ */
+static void install_accept_all_seccomp(void)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
+		.filter = filter,
+	};
+	if (prctl(PR_SET_SECCOMP, 2, &prog))
+		perror("prctl");
+}
+
+int main(int ac, char **argv)
+{
+	FILE *f;
+	char filename[256];
+
+	snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
+
+	if (load_bpf_file(filename)) {
+		printf("%s", bpf_log_buf);
+		return 1;
+	}
+
+	install_accept_all_seccomp();
+
+	f = popen("dd if=/dev/zero of=/dev/null count=5", "r");
+	(void) f;
+
+	read_trace_pipe();
+
+	return 0;
+}