netdev - Re: [PATCH v2 bpf-next 04/17] libbpf: mark BPF subprogs with hidden visibility as static for BPF verifier

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Date:   Thu, 22 Apr 2021 11:09:17 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v2 bpf-next 04/17] libbpf: mark BPF subprogs with hidden
 visibility as static for BPF verifier

On Wed, Apr 21, 2021 at 10:43 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 4/16/21 1:23 PM, Andrii Nakryiko wrote:
> > Define __hidden helper macro in bpf_helpers.h, which is a short-hand for
> > __attribute__((visibility("hidden"))). Add libbpf support to mark BPF
> > subprograms marked with __hidden as static in BTF information to enforce BPF
> > verifier's static function validation algorithm, which takes more information
> > (caller's context) into account during a subprogram validation.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> > ---
> >   tools/lib/bpf/bpf_helpers.h     |  8 ++++++
> >   tools/lib/bpf/btf.c             |  5 ----
> >   tools/lib/bpf/libbpf.c          | 45 ++++++++++++++++++++++++++++++++-
> >   tools/lib/bpf/libbpf_internal.h |  6 +++++
> >   4 files changed, 58 insertions(+), 6 deletions(-)
> >
> > diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
> > index 75c7581b304c..9720dc0b4605 100644
> > --- a/tools/lib/bpf/bpf_helpers.h
> > +++ b/tools/lib/bpf/bpf_helpers.h
> > @@ -47,6 +47,14 @@
> >   #define __weak __attribute__((weak))
> >   #endif
> >
> > +/*
> > + * Use __hidden attribute to mark a non-static BPF subprogram effectively
> > + * static for BPF verifier's verification algorithm purposes, allowing more
> > + * extensive and permissive BPF verification process, taking into account
> > + * subprogram's caller context.
> > + */
> > +#define __hidden __attribute__((visibility("hidden")))
>
> To prevent potential external __hidden macro definition conflict, how
> about
>
> #ifdef __hidden
> #undef __hidden
> #define __hidden __attribute__((visibility("hidden")))
> #endif
>

We do force #undef only with __always_inline because of the bad
definition in linux/stddef.h And we check #ifndef for __weak, because
__weak is defined in kernel headers. This is not really the case for
__hidden, the only definition is in
tools/lib/traceevent/event-parse-local.h, which I don't think we
should worry about in BPF context. So I wanted to keep it simple and
fix only if that really causes some real conflicts.

And keep in mind that in BPF code bpf_helpers.h is usually included as
one of the first few headers anyways.


> > +
> >   /* When utilizing vmlinux.h with BPF CO-RE, user BPF programs can't include
> >    * any system-level headers (such as stddef.h, linux/version.h, etc), and
> >    * commonly-used macros like NULL and KERNEL_VERSION aren't available through

[...]

> > @@ -698,6 +700,15 @@ bpf_object__add_programs(struct bpf_object *obj, Elf_Data *sec_data,
> >               if (err)
> >                       return err;
> >
> > +             /* if function is a global/weak symbol, but has hidden
> > +              * visibility (or any non-default one), mark its BTF FUNC as
> > +              * static to enable more permissive BPF verification mode with
> > +              * more outside context available to BPF verifier
> > +              */
> > +             if (GELF_ST_BIND(sym.st_info) != STB_LOCAL
> > +                 && GELF_ST_VISIBILITY(sym.st_other) != STV_DEFAULT)
>
> Maybe we should check GELF_ST_VISIBILITY(sym.st_other) == STV_HIDDEN
> instead?

It felt like only STV_DEFAULT should be "exported", semantically
speaking. Everything else would be treated as if it was static, except
that C rules require that function has to be global. Do you think
there is some danger to do it this way?

Currently static linker doesn't do anything special for STV_INTERNAL
and STV_PROTECTED, so we could just disable those. Do you prefer that?

I just felt that there is no risk of regression if we do this for
non-STV_DEFAULT generically.


>
> > +                     prog->mark_btf_static = true;
> > +
> >               nr_progs++;
> >               obj->nr_programs = nr_progs;
> >

[...]