Re: [PATCH bpf-next 3/3] selftests/bpf: add bpf_iter test with bpf_get_task_stack_trace()

[Yonghong Song <yhs@fb.com>]

On 6/23/20 12:08 AM, Song Liu wrote:
> The new test is similar to other bpf_iter tests.
> 
> Signed-off-by: Song Liu <songliubraving@fb.com>
> ---
>   .../selftests/bpf/prog_tests/bpf_iter.c       | 17 +++++++
>   .../selftests/bpf/progs/bpf_iter_task_stack.c | 50 +++++++++++++++++++
>   2 files changed, 67 insertions(+)
>   create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
> index 87c29dde1cf96..baa83328f810d 100644
> --- a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
> +++ b/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_test_kern1.skel.h"
>   #include "bpf_iter_test_kern2.skel.h"
> @@ -106,6 +107,20 @@ 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;
> @@ -392,6 +407,8 @@ 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("anon"))
> diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c b/tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c
> new file mode 100644
> index 0000000000000..4fc939e0fca77
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/bpf_iter_task_stack.c
> @@ -0,0 +1,50 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2020 Facebook */
> +/* "undefine" structs in vmlinux.h, because we "override" them below */
> +#define bpf_iter_meta bpf_iter_meta___not_used
> +#define bpf_iter__task bpf_iter__task___not_used
> +#include "vmlinux.h"
> +#undef bpf_iter_meta
> +#undef bpf_iter__task
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +struct bpf_iter_meta {
> +	struct seq_file *seq;
> +	__u64 session_id;
> +	__u64 seq_num;
> +} __attribute__((preserve_access_index));
> +
> +struct bpf_iter__task {
> +	struct bpf_iter_meta *meta;
> +	struct task_struct *task;
> +} __attribute__((preserve_access_index));
> +
> +#define MAX_STACK_TRACE_DEPTH   64
> +unsigned long entries[MAX_STACK_TRACE_DEPTH];
> +
> +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;
> +	unsigned int i, num_entries;
> +
> +	if (task == (void *)0)
> +		return 0;
> +
> +	num_entries = bpf_get_task_stack_trace(task, entries, MAX_STACK_TRACE_DEPTH);
> +
> +	BPF_SEQ_PRINTF(seq, "pid: %8u\n", task->pid);
> +
> +	for (i = 0; i < MAX_STACK_TRACE_DEPTH; i++) {
> +		if (num_entries > i)
> +			BPF_SEQ_PRINTF(seq, "[<0>] %pB\n", (void *)entries[i]);

We may have an issue on 32bit issue.
On 32bit system, the following is called in the kernel
+	return stack_trace_save_tsk(task, (unsigned long *)entries, size, 0);
it will pack addresses at 4 byte increment.
But in BPF program, the reading is in 8 byte increment.

If "entries" are handled in user space, user space can just using "long 
*" pointer and will be able to correctly retrieve the stack addresses.

Maybe add some comments to clarify this prog only works for 64bit system.

> +	}
> +
> +	BPF_SEQ_PRINTF(seq, "\n");
> +
> +	return 0;
> +}
>