linux-kernel - [PATCH 3/3] btf: Avoid weak external references

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Message-ID: <20240329093356.276289-8-ardb+git@google.com>
Date: Fri, 29 Mar 2024 10:34:00 +0100
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
Subject: [PATCH 3/3] btf: Avoid weak external references

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

If the BTF code is enabled in the build configuration, the start/stop
BTF markers are guaranteed to exist in the final link but not during the
first linker pass.

Avoid GOT based relocations to these markers in the final executable by
providing preliminary definitions that will be used by the first linker
pass, and superseded by the actual definitions in the subsequent ones.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
 include/asm-generic/vmlinux.lds.h | 2 ++
 kernel/bpf/btf.c                  | 4 ++--
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e8449be62058..141bddb511ee 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -456,6 +456,8 @@
  * independent code.
  */
 #define PRELIMINARY_SYMBOL_DEFINITIONS					\
+	PROVIDE(__start_BTF = .);					\
+	PROVIDE(__stop_BTF = .);					\
 	PROVIDE(kallsyms_addresses = .);				\
 	PROVIDE(kallsyms_offsets = .);					\
 	PROVIDE(kallsyms_names = .);					\
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 90c4a32d89ff..46a56bf067a8 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5642,8 +5642,8 @@ static struct btf *btf_parse(const union bpf_attr *attr, bpfptr_t uattr, u32 uat
 	return ERR_PTR(err);
 }
 
-extern char __weak __start_BTF[];
-extern char __weak __stop_BTF[];
+extern char __start_BTF[];
+extern char __stop_BTF[];
 extern struct btf *btf_vmlinux;
 
 #define BPF_MAP_TYPE(_id, _ops)
-- 
2.44.0.478.gd926399ef9-goog