linux-kernel - [PATCH v3 0/3] kbuild: Avoid weak external linkage where possible

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Message-ID: <20240415075837.2349766-5-ardb+git@google.com>
Date: Mon, 15 Apr 2024 09:58:38 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: Ard Biesheuvel <ardb@...nel.org>, Masahiro Yamada <masahiroy@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, linux-arch@...r.kernel.org, 
	linux-kbuild@...r.kernel.org, bpf@...r.kernel.org, 
	Andrii Nakryiko <andrii@...nel.org>, Jiri Olsa <olsajiri@...il.com>
Subject: [PATCH v3 0/3] kbuild: Avoid weak external linkage where possible

From: Ard Biesheuvel <ardb@...nel.org>

Weak external linkage is intended for cases where a symbol reference
can remain unsatisfied in the final link. Taking the address of such a
symbol should yield NULL if the reference was not satisfied.

Given that ordinary RIP or PC relative references cannot produce NULL,
some kind of indirection is always needed in such cases, and in position
independent code, this results in a GOT entry. In ordinary code, it is
arch specific but amounts to the same thing.

While unavoidable in some cases, weak references are currently also used
to declare symbols that are always defined in the final link, but not in
the first linker pass. This means we end up with worse codegen for no
good reason. So let's clean this up, by providing preliminary
definitions that are only used as a fallback.

Changes since v2:
- fix build issue in patch #3 reported by Jiri
- add Arnd's acks

Changes since v1:
- update second occurrence of BTF start/end markers
- drop NULL check of __start_BTF[] which is no longer meaningful
- avoid the preliminary BTF symbols if CONFIG_DEBUG_INFO_BTF is not set
- add Andrii's ack to patch #3
- patches #1 and #2 unchanged

Cc: Masahiro Yamada <masahiroy@...nel.org>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>
Cc: linux-arch@...r.kernel.org
Cc: linux-kbuild@...r.kernel.org
Cc: bpf@...r.kernel.org
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Jiri Olsa <olsajiri@...il.com>

Ard Biesheuvel (3):
  kallsyms: Avoid weak references for kallsyms symbols
  vmlinux: Avoid weak reference to notes section
  btf: Avoid weak external references

 include/asm-generic/vmlinux.lds.h | 28 ++++++++++++++++++
 kernel/bpf/btf.c                  |  7 +++--
 kernel/bpf/sysfs_btf.c            |  6 ++--
 kernel/kallsyms.c                 |  6 ----
 kernel/kallsyms_internal.h        | 30 ++++++++------------
 kernel/ksysfs.c                   |  4 +--
 lib/buildid.c                     |  4 +--
 7 files changed, 52 insertions(+), 33 deletions(-)

-- 
2.44.0.683.g7961c838ac-goog