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

[KP Singh <kpsingh@kernel.org>]

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.


> >
> >   #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
> [...]