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Date:   Wed, 24 Jun 2020 20:37:07 +0000
From:   Song Liu <songliubraving@...com>
To:     Yonghong Song <yhs@...com>
CC:     bpf <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "kpsingh@...omium.org" <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next 3/3] selftests/bpf: add bpf_iter test with
 bpf_get_task_stack_trace()



> On Jun 23, 2020, at 3:27 PM, Yonghong Song <yhs@...com> wrote:
> 
> 
> 
> On 6/23/20 3:07 PM, Song Liu wrote:
>>> On Jun 23, 2020, at 11:57 AM, Yonghong Song <yhs@...com> wrote:
>>> 
>>> 
>>> 
>>> 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@...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.
>> Can we avoid potential issues by requiring size % 8 == 0? Or maybe round down
>> size to closest multiple of 8?
> 
> This is what I mean:
>  for bpf program: "long" means u64, so we allocate 64 * 8 buffer size
>                   and pass it to the helper
>  in the helper, the address will be increased along sizeof(long), which
>                 is 4 for 32bit system.
>          So address is recorded at buf, buf + 4, buf + 8, buf + 12, ...
>  After the helper returns, the bpf program tries to retrieve
>          the address at buf, buf + 8, buf + 16.
> 
> The helper itself is okay. But BPF_SEQ_PRINTF above is wrong.
> Is this interpretation correct?

Thanks for the clarification. I guess the best solution is to fix this 
once in the kernel, so BPF programs don't have to worry about it. 

Song

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