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 3:07 PM, Song Liu wrote:
> 
> 
>> On Jun 23, 2020, at 11:57 AM, Yonghong Song <yhs@fb.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@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.
> 
> 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,
> Song
>