On Thu, Apr 22, 2021 at 4:00 PM Yonghong Song <yhs@xxxxxx> wrote:
>
>
>
> On 4/22/21 11:09 AM, Andrii Nakryiko wrote:
> > On Wed, Apr 21, 2021 at 10:43 PM Yonghong Song <yhs@xxxxxx> 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@xxxxxxxxxx>
> >>> ---
> >>> 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.
>
> That is fine. Conflict of __hidden is a low risk and we can deal with it
> later if needed.
>
> >
> >
> >>> +
> >>> /* 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?
>
> Yes, let us just deal with STV_DEFAULT and STV_HIDDEN. We already
> specialized STV_HIDDEN, so we should not treat STV_INTERNAL/PROTECTED
> as what they mean in ELF standard, so let us disable them for now.
Yep, will do
>
> >
> > 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;
> >>>
> >
> > [...]
> >
