linux-kernel - [RFC PATCH 3/5] btf: Avoid weak external references

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Date: Mon, 22 Jan 2024 10:08:55 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: Ard Biesheuvel <ardb@...nel.org>, Kevin Loughlin <kevinloughlin@...gle.com>, 
	Tom Lendacky <thomas.lendacky@....com>, Dionna Glaze <dionnaglaze@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski <luto@...nel.org>, 
	Arnd Bergmann <arnd@...db.de>, Martin KaFai Lau <martin.lau@...ux.dev>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, linux-arch@...r.kernel.org, bpf@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: [RFC PATCH 3/5] 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pass, and superseded by the actual definitions in the subsequent ones.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 a39e050416c7..ef45331fb043 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 596471189176..a659fc7045bb 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5581,8 +5581,8 @@ 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)
-- 
2.43.0.429.g432eaa2c6b-goog