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.
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;
[...]
