[PATCH bpf-next] samples/bpf: Add hello world sample for newbies

[Tiezhu Yang]

The program is made in a way that everytime an execve syscall
is executed it prints Hello, BPF World!

This is inspired and based on the code example for the book
Linux Observability with BPF [1], load_bpf_file() has been
removed after commit ceb5dea56543 ("samples: bpf: Remove
bpf_load loader completely"), so the old version can not
work in the latest mainline kernel.

Since it is very simple and useful for newbies, I think it is
necessary to be upstreamed.

[1] https://github.com/bpftools/linux-observability-with-bpf/tree/master/code/chapter-2/hello_world

Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
 samples/bpf/Makefile     |  3 +++
 samples/bpf/hello_kern.c | 14 ++++++++++++++
 samples/bpf/hello_user.c | 42 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 59 insertions(+)
 create mode 100644 samples/bpf/hello_kern.c
 create mode 100644 samples/bpf/hello_user.c

diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 45ceca4..fd17cbd 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -55,6 +55,7 @@ tprogs-y += task_fd_query
 tprogs-y += xdp_sample_pkts
 tprogs-y += ibumad
 tprogs-y += hbm
+tprogs-y += hello
 
 # Libbpf dependencies
 LIBBPF = $(TOOLS_PATH)/lib/bpf/libbpf.a
@@ -113,6 +114,7 @@ task_fd_query-objs := task_fd_query_user.o $(TRACE_HELPERS)
 xdp_sample_pkts-objs := xdp_sample_pkts_user.o
 ibumad-objs := ibumad_user.o
 hbm-objs := hbm.o $(CGROUP_HELPERS)
+hello-objs := hello_user.o $(TRACE_HELPERS)
 
 # Tell kbuild to always build the programs
 always-y := $(tprogs-y)
@@ -174,6 +176,7 @@ always-y += ibumad_kern.o
 always-y += hbm_out_kern.o
 always-y += hbm_edt_kern.o
 always-y += xdpsock_kern.o
+always-y += hello_kern.o
 
 ifeq ($(ARCH), arm)
 # Strip all except -D__LINUX_ARM_ARCH__ option needed to handle linux
diff --git a/samples/bpf/hello_kern.c b/samples/bpf/hello_kern.c
new file mode 100644
index 0000000..b841029
--- /dev/null
+++ b/samples/bpf/hello_kern.c
@@ -0,0 +1,14 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+SEC("tracepoint/syscalls/sys_enter_execve")
+int trace_enter_execve(void *ctx)
+{
+	static const char msg[] = "Hello, BPF World!\n";
+
+	bpf_trace_printk(msg, sizeof(msg));
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
diff --git a/samples/bpf/hello_user.c b/samples/bpf/hello_user.c
new file mode 100644
index 0000000..9423bbb
--- /dev/null
+++ b/samples/bpf/hello_user.c
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <stdio.h>
+#include <bpf/libbpf.h>
+#include "trace_helpers.h"
+
+int main(int argc, char **argv)
+{
+	struct bpf_link *link = NULL;
+	struct bpf_program *prog;
+	struct bpf_object *obj;
+
+	obj = bpf_object__open_file("hello_kern.o", NULL);
+	if (libbpf_get_error(obj)) {
+		fprintf(stderr, "ERROR: opening BPF object file failed\n");
+		return 0;
+	}
+
+	if (bpf_object__load(obj)) {
+		fprintf(stderr, "ERROR: loading BPF object file failed\n");
+		goto cleanup;
+	}
+
+	prog = bpf_object__find_program_by_name(obj, "trace_enter_execve");
+	if (!prog) {
+		fprintf(stderr, "ERROR: finding a prog in obj file failed\n");
+		goto cleanup;
+	}
+
+	link = bpf_program__attach(prog);
+	if (libbpf_get_error(link)) {
+		fprintf(stderr, "ERROR: bpf_program__attach failed\n");
+		link = NULL;
+		goto cleanup;
+	}
+
+	read_trace_pipe();
+
+cleanup:
+	bpf_link__destroy(link);
+	bpf_object__close(obj);
+	return 0;
+}
-- 
2.1.0