examples/c: Add USDT example

Add a new example to demonstrate the usage of libbpf USDT
functionality.

Closes: libbpf#80
Signed-off-by: Hengqi Chen <chenhengqi@outlook.com>

README.md

@@ -115,6 +115,28 @@ $ sudo cat /sys/kernel/debug/tracing/trace_pipe
            <...>-461101 [018] d... 505434.345367: bpf_trace_printk: UPROBE EXIT: return = 5
 ```
 
+## USDT
+
+`usdt` is an example of dealing with USDT probe. It attaches USDT BPF programs to
+the [libc:setjmp](https://www.gnu.org/software/libc/manual/html_node/Non_002dlocal-Goto-Probes.html) probe, which is triggered by calling `setjmp` in user-space program once per second and logs USDT arguments using `bpf_printk()` macro:
+
+```shell
+$ sudo ./usdt
+libbpf: loading object 'usdt_bpf' from buffer
+...
+Successfully started!
+...........
+```
+
+You can see `usdt` demo output in `/sys/kernel/debug/tracing/trace_pipe`:
+```shell
+$ sudo cat /sys/kernel/debug/tracing/trace_pipe
+            usdt-1919077 [005] d..21 537310.886092: bpf_trace_printk: USDT auto attach to libc:setjmp: arg1 = 55d03d6a42a0, arg2 = 0, arg3 = 55d03d65e54e
+            usdt-1919077 [005] d..21 537310.886105: bpf_trace_printk: USDT manual attach to libc:setjmp: arg1 = 55d03d6a42a0, arg2 = 0, arg3 = 55d03d65e54e
+            usdt-1919077 [005] d..21 537311.886214: bpf_trace_printk: USDT auto attach to libc:setjmp: arg1 = 55d03d6a42a0, arg2 = 0, arg3 = 55d03d65e54e
+            usdt-1919077 [005] d..21 537311.886227: bpf_trace_printk: USDT manual attach to libc:setjmp: arg1 = 55d03d6a42a0, arg2 = 0, arg3 = 55d03d65e54e
+```
+
 # Fentry
 
 `fentry` is an example that uses fentry and fexit BPF programs for tracing. It

examples/c/.gitignore

@@ -1,7 +1,9 @@
 /.output
 /bootstrap
 /minimal
+/minimal_legacy
 /uprobe
 /kprobe
 /fentry
 /profile
+/usdt

examples/c/Makefile

@@ -18,7 +18,7 @@ INCLUDES := -I$(OUTPUT) -I../../libbpf/include/uapi -I$(dir $(VMLINUX))
 CFLAGS := -g -Wall
 ALL_LDFLAGS := $(LDFLAGS) $(EXTRA_LDFLAGS)
 
-APPS = minimal minimal_legacy bootstrap uprobe kprobe fentry
+APPS = minimal minimal_legacy bootstrap uprobe kprobe fentry usdt
 
 CARGO ?= $(shell which cargo)
 ifeq ($(strip $(CARGO)),)
@@ -131,4 +131,3 @@ $(APPS): %: $(OUTPUT)/%.o $(LIBBPF_OBJ) | $(OUTPUT)
 
 # keep intermediate (.skel.h, .bpf.o, etc) targets
 .SECONDARY:
-

examples/c/usdt.bpf.c

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+/* Copyright (c) 2022 Hengqi Chen */
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/usdt.bpf.h>
+
+pid_t my_pid;
+
+SEC("usdt/libc.so.6:libc:setjmp")
+int BPF_USDT(usdt_auto_attach, void *arg1, int arg2, void *arg3)
+{
+	pid_t pid = bpf_get_current_pid_tgid() >> 32;
+
+	if (pid != my_pid)
+		return 0;
+
+	bpf_printk("USDT auto attach to libc:setjmp: arg1 = %lx, arg2 = %d, arg3 = %lx", arg1, arg2, arg3);
+	return 0;
+}
+
+SEC("usdt")
+int BPF_USDT(usdt_manual_attach, void *arg1, int arg2, void *arg3)
+{
+	bpf_printk("USDT manual attach to libc:setjmp: arg1 = %lx, arg2 = %d, arg3 = %lx", arg1, arg2, arg3);
+	return 0;
+}
+
+char LICENSE[] SEC("license") = "Dual BSD/GPL";

examples/c/usdt.c

@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+/* Copyright (c) 2022 Hengqi Chen */
+#include <signal.h>
+#include <unistd.h>
+#include <setjmp.h>
+#include <linux/limits.h>
+#include "usdt.skel.h"
+
+static volatile sig_atomic_t exiting;
+static jmp_buf env;
+
+static void sig_int(int signo)
+{
+	exiting = 1;
+}
+
+static int libbpf_print_fn(enum libbpf_print_level level, const char *format, va_list args)
+{
+	return vfprintf(stderr, format, args);
+}
+
+static void usdt_trigger() {
+	setjmp(env);
+}
+
+int main(int argc, char **argv)
+{
+	struct usdt_bpf *skel;
+	int err;
+
+	libbpf_set_strict_mode(LIBBPF_STRICT_ALL);
+	libbpf_set_print(libbpf_print_fn);
+
+	skel = usdt_bpf__open();
+	if (!skel) {
+		fprintf(stderr, "Failed to open BPF skeleton\n");
+		return 1;
+	}
+
+	skel->bss->my_pid = getpid();
+
+	err = usdt_bpf__load(skel);
+	if (!skel) {
+		fprintf(stderr, "Failed to load BPF skeleton\n");
+		return 1;
+	}
+
+	/*
+	 * Manually attach to libc.so we find.
+	 * We specify pid here, so we don't have to do pid filtering in BPF program.
+	 */
+	skel->links.usdt_manual_attach = bpf_program__attach_usdt(skel->progs.usdt_manual_attach, getpid(),
+								  "libc.so.6", "libc", "setjmp", NULL);
+	if (!skel->links.usdt_manual_attach) {
+		err = errno;
+		fprintf(stderr, "Failed to attach BPF program `usdt_manual_attach`\n");
+		goto cleanup;
+	}
+
+	/*
+	 * Auto attach by libbpf, libbpf should be able to find libc.so in your system.
+	 * By default, auto attach does NOT specify pid, so we do pid filtering in BPF program
+	 */
+	err = usdt_bpf__attach(skel);
+	if (err) {
+		fprintf(stderr, "Failed to attach BPF skeleton\n");
+		goto cleanup;
+	}
+
+	if (signal(SIGINT, sig_int) == SIG_ERR) {
+		err = errno;
+		fprintf(stderr, "can't set signal handler: %s\n", strerror(errno));
+		goto cleanup;
+	}
+
+	printf("Successfully started! Please run `sudo cat /sys/kernel/debug/tracing/trace_pipe` "
+	       "to see output of the BPF programs.\n");
+
+	while (!exiting) {
+		/* trigger our BPF programs */
+		usdt_trigger();
+		fprintf(stderr, ".");
+		sleep(1);
+	}
+
+cleanup:
+	usdt_bpf__destroy(skel);
+	return -err;
+}