selftests/bpf: Add bpf_iter test with bpf_get_task_stack()

The new test is similar to other bpf_iter tests. It dumps all
/proc/<pid>/stack to a seq_file. Here is some example output:

pid:     2873 num_entries:        3
[<0>] worker_thread+0xc6/0x380
[<0>] kthread+0x135/0x150
[<0>] ret_from_fork+0x22/0x30

pid:     2874 num_entries:        9
[<0>] __bpf_get_stack+0x15e/0x250
[<0>] bpf_prog_22a400774977bb30_dump_task_stack+0x4a/0xb3c
[<0>] bpf_iter_run_prog+0x81/0x170
[<0>] __task_seq_show+0x58/0x80
[<0>] bpf_seq_read+0x1c3/0x3b0
[<0>] vfs_read+0x9e/0x170
[<0>] ksys_read+0xa7/0xe0
[<0>] do_syscall_64+0x4c/0xa0
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Note: bpf_iter test as-is doesn't print the contents of the seq_file. To
see the example above, it is necessary to add printf() to do_dummy_read.
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200630062846.664389-5-songliubraving@fb.com

tools/testing/selftests/bpf/prog_tests/bpf_iter.c

@@ -5,6 +5,7 @@
 #include "bpf_iter_netlink.skel.h"
 #include "bpf_iter_bpf_map.skel.h"
 #include "bpf_iter_task.skel.h"
+#include "bpf_iter_task_stack.skel.h"
 #include "bpf_iter_task_file.skel.h"
 #include "bpf_iter_tcp4.skel.h"
 #include "bpf_iter_tcp6.skel.h"
@@ -110,6 +111,20 @@ static void test_task(void)
 	bpf_iter_task__destroy(skel);
 }
 
+static void test_task_stack(void)
+{
+	struct bpf_iter_task_stack *skel;
+
+	skel = bpf_iter_task_stack__open_and_load();
+	if (CHECK(!skel, "bpf_iter_task_stack__open_and_load",
+		  "skeleton open_and_load failed\n"))
+		return;
+
+	do_dummy_read(skel->progs.dump_task_stack);
+
+	bpf_iter_task_stack__destroy(skel);
+}
+
 static void test_task_file(void)
 {
 	struct bpf_iter_task_file *skel;
@@ -452,6 +467,8 @@ void test_bpf_iter(void)
 		test_bpf_map();
 	if (test__start_subtest("task"))
 		test_task();
+	if (test__start_subtest("task_stack"))
+		test_task_stack();
 	if (test__start_subtest("task_file"))
 		test_task_file();
 	if (test__start_subtest("tcp4"))

tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include "bpf_iter.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+#define MAX_STACK_TRACE_DEPTH   64
+unsigned long entries[MAX_STACK_TRACE_DEPTH];
+#define SIZE_OF_ULONG (sizeof(unsigned long))
+
+SEC("iter/task")
+int dump_task_stack(struct bpf_iter__task *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct task_struct *task = ctx->task;
+	long i, retlen;
+
+	if (task == (void *)0)
+		return 0;
+
+	retlen = bpf_get_task_stack(task, entries,
+				    MAX_STACK_TRACE_DEPTH * SIZE_OF_ULONG, 0);
+	if (retlen < 0)
+		return 0;
+
+	BPF_SEQ_PRINTF(seq, "pid: %8u num_entries: %8u\n", task->pid,
+		       retlen / SIZE_OF_ULONG);
+	for (i = 0; i < MAX_STACK_TRACE_DEPTH; i++) {
+		if (retlen > i * SIZE_OF_ULONG)
+			BPF_SEQ_PRINTF(seq, "[<0>] %pB\n", (void *)entries[i]);
+	}
+	BPF_SEQ_PRINTF(seq, "\n");
+
+	return 0;
+}