linux-kernel - [PATCH 6.1&6.6 3/3] btf: Avoid weak external references

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Message-ID: <20241206085810.112341-4-chenhuacai@loongson.cn>
Date: Fri,  6 Dec 2024 16:58:10 +0800
From: Huacai Chen <chenhuacai@...ngson.cn>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sasha Levin <sashal@...nel.org>,
	Huacai Chen <chenhuacai@...nel.org>
Cc: Xuerui Wang <kernel@...0n.name>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Nicolas Schier <nicolas@...sle.eu>,
	stable@...r.kernel.org,
	linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	loongarch@...ts.linux.dev,
	Ard Biesheuvel <ardb@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Arnd Bergmann <arnd@...db.de>,
	Jiri Olsa <jolsa@...nel.org>,
	Huacai Chen <chenhuacai@...ngson.cn>
Subject: [PATCH 6.1&6.6 3/3] btf: Avoid weak external references

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

[ Upstream commit fc5eb4a84e4c063e75a6a6e92308e9533c0f19b5 ]

If the BTF code is enabled in the build configuration, the start/stop
BTF markers are guaranteed to exist. Only when CONFIG_DEBUG_INFO_BTF=n,
the references in btf_parse_vmlinux() will remain unsatisfied, relying
on the weak linkage of the external references to avoid breaking the
build.

Avoid GOT based relocations to these markers in the final executable by
dropping the weak attribute and instead, make btf_parse_vmlinux() return
ERR_PTR(-ENOENT) directly if CONFIG_DEBUG_INFO_BTF is not enabled to
begin with.  The compiler will drop any subsequent references to
__start_BTF and __stop_BTF in that case, allowing the link to succeed.

Note that Clang will notice that taking the address of __start_BTF can
no longer yield NULL, so testing for that condition becomes unnecessary.

Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Andrii Nakryiko <andrii@...nel.org>
Acked-by: Arnd Bergmann <arnd@...db.de>
Acked-by: Jiri Olsa <jolsa@...nel.org>
Link: https://lore.kernel.org/bpf/20240415162041.2491523-8-ardb+git@google.com
Signed-off-by: Huacai Chen <chenhuacai@...ngson.cn>
---
 kernel/bpf/btf.c       | 7 +++++--
 kernel/bpf/sysfs_btf.c | 6 +++---
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index c8828016a66f..f231e2a273a1 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -5574,8 +5574,8 @@ 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)
@@ -5724,6 +5724,9 @@ struct btf *btf_parse_vmlinux(void)
 	struct btf *btf = NULL;
 	int err;
 
+	if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF))
+		return ERR_PTR(-ENOENT);
+
 	env = kzalloc(sizeof(*env), GFP_KERNEL | __GFP_NOWARN);
 	if (!env)
 		return ERR_PTR(-ENOMEM);
diff --git a/kernel/bpf/sysfs_btf.c b/kernel/bpf/sysfs_btf.c
index ef6911aee3bb..fedb54c94cdb 100644
--- a/kernel/bpf/sysfs_btf.c
+++ b/kernel/bpf/sysfs_btf.c
@@ -9,8 +9,8 @@
 #include <linux/sysfs.h>
 
 /* See scripts/link-vmlinux.sh, gen_btf() func for details */
-extern char __weak __start_BTF[];
-extern char __weak __stop_BTF[];
+extern char __start_BTF[];
+extern char __stop_BTF[];
 
 static ssize_t
 btf_vmlinux_read(struct file *file, struct kobject *kobj,
@@ -32,7 +32,7 @@ static int __init btf_vmlinux_init(void)
 {
 	bin_attr_btf_vmlinux.size = __stop_BTF - __start_BTF;
 
-	if (!__start_BTF || bin_attr_btf_vmlinux.size == 0)
+	if (bin_attr_btf_vmlinux.size == 0)
 		return 0;
 
 	btf_kobj = kobject_create_and_add("btf", kernel_kobj);
-- 
2.43.5