Re: [PATCH bpf-next 1/4] bpf: enable task local storage for tracing programs

[Yonghong Song <yhs@fb.com>]

On 1/11/21 2:17 AM, KP Singh wrote:
> On Mon, Jan 11, 2021 at 7:27 AM Yonghong Song <yhs@fb.com> wrote:
>>
>>
>>
>> On 1/8/21 3:19 PM, Song Liu wrote:
>>> To access per-task data, BPF program typically creates a hash table with
>>> pid as the key. This is not ideal because:
>>>    1. The use need to estimate requires size of the hash table, with may be
>>>       inaccurate;
>>>    2. Big hash tables are slow;
>>>    3. To clean up the data properly during task terminations, the user need
>>>       to write code.
>>>
>>> Task local storage overcomes these issues and becomes a better option for
>>> these per-task data. Task local storage is only available to BPF_LSM. Now
>>> enable it for tracing programs.
>>>
>>> Reported-by: kernel test robot <lkp@intel.com>
>>> Signed-off-by: Song Liu <songliubraving@fb.com>
>>> ---
> 
> [...]
> 
>>>    struct cfs_rq;
>>>    struct fs_struct;
>>> @@ -1348,6 +1349,10 @@ struct task_struct {
>>>        /* Used by LSM modules for access restriction: */
>>>        void                            *security;
>>>    #endif
>>> +#ifdef CONFIG_BPF_SYSCALL
>>> +     /* Used by BPF task local storage */
>>> +     struct bpf_local_storage        *bpf_storage;
>>> +#endif
>>
>> I remembered there is a discussion where KP initially wanted to put
>> bpf_local_storage in task_struct, but later on changed to
>> use in lsm as his use case mostly for lsm. Did anybody
>> remember the details of the discussion? Just want to be
>> sure what is the concern people has with putting bpf_local_storage
>> in task_struct and whether the use case presented by
>> Song will justify it.
>>
> 
> If I recall correctly, the discussion was about inode local storage and
> it was decided to use the security blob since the use-case was only LSM
> programs. Since we now plan to use it in tracing,
> detangling the dependency from CONFIG_BPF_LSM
> sounds logical to me.

Sounds good. Thanks for explanation.

> 
> 
>>>
>>>    #ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>>>        unsigned long                   lowest_stack;
>>> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
>>> index d1249340fd6ba..ca995fdfa45e7 100644
>>> --- a/kernel/bpf/Makefile
>>> +++ b/kernel/bpf/Makefile
>>> @@ -8,9 +8,8 @@ CFLAGS_core.o += $(call cc-disable-warning, override-init) $(cflags-nogcse-yy)
>>>
>>>    obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o tnum.o bpf_iter.o map_iter.o task_iter.o prog_iter.o
>>>    obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o map_in_map.o
>>> -obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o
>>> +obj-$(CONFIG_BPF_SYSCALL) += local_storage.o queue_stack_maps.o ringbuf.o bpf_task_storage.o
>>>    obj-${CONFIG_BPF_LSM}         += bpf_inode_storage.o
>>> -obj-${CONFIG_BPF_LSM}          += bpf_task_storage.o
>>>    obj-$(CONFIG_BPF_SYSCALL) += disasm.o
>>>    obj-$(CONFIG_BPF_JIT) += trampoline.o
>>>    obj-$(CONFIG_BPF_SYSCALL) += btf.o
>> [...]