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 [off-list ref] wrote:

On 4/16/21 1:23 PM, Andrii Nakryiko wrote:

quoted

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@kernel.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.

quoted

+
  /* 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

[...]

quoted

@@ -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.

quoted

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

[...]