linux-kernel - Re: [PATCH bpf-next v1 4/9] bpf: Introduce sleepable tracepoints

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Date:   Thu, 3 Mar 2022 11:37:21 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Joe Burton <jevburton.kernel@...il.com>,
        Tejun Heo <tj@...nel.org>, joshdon@...gle.com, sdf@...gle.com,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v1 4/9] bpf: Introduce sleepable tracepoints

On Wed, Mar 2, 2022 at 11:41 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Fri, Feb 25, 2022 at 03:43:34PM -0800, Hao Luo wrote:
> > diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
> > index e7c2276be33e..c73c7ab3680e 100644
> > --- a/include/linux/tracepoint-defs.h
> > +++ b/include/linux/tracepoint-defs.h
> > @@ -51,6 +51,7 @@ struct bpf_raw_event_map {
> >       void                    *bpf_func;
> >       u32                     num_args;
> >       u32                     writable_size;
> > +     u32                     sleepable;
>
> It increases the size for all tracepoints.
> See BPF_RAW_TP in include/asm-generic/vmlinux.lds.h
> Please switch writeable_size and sleepable to u16.

No problem.

> >
> > -static const struct bpf_func_proto *
> > -syscall_prog_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
> > +/* Syscall helpers that are also allowed in sleepable tracing prog. */
> > +const struct bpf_func_proto *
> > +tracing_prog_syscall_func_proto(enum bpf_func_id func_id,
> > +                             const struct bpf_prog *prog)
> >  {
> >       switch (func_id) {
> >       case BPF_FUNC_sys_bpf:
> >               return &bpf_sys_bpf_proto;
> > -     case BPF_FUNC_btf_find_by_name_kind:
> > -             return &bpf_btf_find_by_name_kind_proto;
> >       case BPF_FUNC_sys_close:
> >               return &bpf_sys_close_proto;
> > -     case BPF_FUNC_kallsyms_lookup_name:
> > -             return &bpf_kallsyms_lookup_name_proto;
> >       case BPF_FUNC_mkdir:
> >               return &bpf_mkdir_proto;
> >       case BPF_FUNC_rmdir:
> >               return &bpf_rmdir_proto;
> >       case BPF_FUNC_unlink:
> >               return &bpf_unlink_proto;
> > +     default:
> > +             return NULL;
> > +     }
> > +}
>
> If I read this correctly the goal is to disallow find_by_name_kind
> and lookup_name from sleepable tps. Why? What's the harm?

A couple of thoughts, please correct me if they don't make sense. I
may think too much.

1. The very first reason is, I don't know the use case of them in
tracing. So I think I can leave them right now and add them later if
the maintainers want them.

2. A related question is, do we actually want all syscall helpers to
be in sleepable tracing? Some helpers may cause re-entering the
tracepoints. For a hypothetical example, if we call mkdir while
tracing some tracepoints in vfs_mkdir. Do we have protection for this?
Another potential problem is about lookup_name in particular,
sleepable_tracing could be triggered by any user, will lookup_name
leak kernel addresses to users in some way? The filesystem helpers
have some basic perm checks, I would think it's relatively safer.