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[v4,3/3] btf: Avoid weak external references

Message ID 20240415162041.2491523-8-ardb+git@google.com (mailing list archive)
State New
Headers show
Series kbuild: Avoid weak external linkage where possible | expand

Commit Message

Ard Biesheuvel April 15, 2024, 4:20 p.m. UTC 
From: Ard Biesheuvel <ardb@kernel.org>

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.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 kernel/bpf/btf.c       | 7 +++++--
 kernel/bpf/sysfs_btf.c | 6 +++---
 2 files changed, 8 insertions(+), 5 deletions(-)
diff mbox series

Patch

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 90c4a32d89ff..6d46cee47ae3 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)
@@ -5971,6 +5971,9 @@  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);