Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20220225234339.2386398-1-haoluo@google.com> <20220225234339.2386398-5-haoluo@google.com>
 <20220302194141.c4gvqz5v4mmmbwsv@ast-mbp.dhcp.thefacebook.com>
In-Reply-To: <20220302194141.c4gvqz5v4mmmbwsv@ast-mbp.dhcp.thefacebook.com>
From:   Hao Luo <haoluo@google.com>
Date:   Thu, 3 Mar 2022 11:37:21 -0800
Message-ID: <CA+khW7iRP8b69usnAy_j4hrYE-U0hC4Rv65K5m4wuP5ArnWsEQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v1 4/9] bpf: Introduce sleepable tracepoints
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        KP Singh <kpsingh@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Joe Burton <jevburton.kernel@gmail.com>,
        Tejun Heo <tj@kernel.org>, joshdon@google.com, sdf@google.com,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Wed, Mar 2, 2022 at 11:41 AM Alexei Starovoitov
<alexei.starovoitov@gmail.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.