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

[Song Liu <songliubraving@fb.com>]

> 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? 

Thanks,
Song