[PATCH v3 3/3] btf: Avoid weak external references

[Ard Biesheuvel <ardb+git@xxxxxxxxxx>]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

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pass, and superseded by the actual definitions in the subsequent ones.

Make the preliminary definitions dependent on CONFIG_DEBUG_INFO_BTF so
that inadvertent references to this section will trigger a link failure
if they occur in code that does not honour CONFIG_DEBUG_INFO_BTF.

Note that Clang will notice that taking the address of__start_BTF cannot
yield NULL any longer, so testing for that condition is no longer
needed.

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Acked-by: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 include/asm-generic/vmlinux.lds.h | 9 +++++++++
 kernel/bpf/btf.c                  | 7 +++++--
 kernel/bpf/sysfs_btf.c            | 6 +++---
 3 files changed, 17 insertions(+), 5 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e8449be62058..4cb3d88449e5 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -456,6 +456,7 @@
  * independent code.
  */
 #define PRELIMINARY_SYMBOL_DEFINITIONS					\
+	PRELIMINARY_BTF_DEFINITIONS					\
 	PROVIDE(kallsyms_addresses = .);				\
 	PROVIDE(kallsyms_offsets = .);					\
 	PROVIDE(kallsyms_names = .);					\
@@ -466,6 +467,14 @@
 	PROVIDE(kallsyms_markers = .);					\
 	PROVIDE(kallsyms_seqs_of_names = .);
 
+#ifdef CONFIG_DEBUG_INFO_BTF
+#define PRELIMINARY_BTF_DEFINITIONS					\
+	PROVIDE(__start_BTF = .);					\
+	PROVIDE(__stop_BTF = .);
+#else
+#define PRELIMINARY_BTF_DEFINITIONS
+#endif
+
 /*
  * Read only Data
  */
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);
-- 
2.44.0.683.g7961c838ac-goog