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Message-ID: <4644728a-4d0d-a5a7-e008-d8c3d7289397@fb.com>
Date: Wed, 24 Jun 2020 22:29:36 -0700
From: Yonghong Song <yhs@...com>
To: Song Liu <songliubraving@...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 6/24/20 1:37 PM, Song Liu wrote:
>
>
>> 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.
The kernel could make each entry 8 bytes. This will cause less potential
entries for 32bit, probably fine. Another option is BPF program declares
an extern variable CONFIG_64BIT and it is 'y', that means 64 bit.
Otherwise it is 32bit. libbpf should set CONFIG_64BIT correctly.
I guess storing each address as 64bit probably a better and less
confusion choice.
>
> Song
>
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