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Message-ID: <03A78005-BDCD-4A90-BD27-724DA6056D9B@fb.com>
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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