Re: [PATCH v2 3/3] btf: Avoid weak external references

[Jiri Olsa]

On Tue, Apr 09, 2024 at 05:01:36PM +0200, Ard Biesheuvel wrote:
> 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 pass, and superseded by the actual definitions in the subsequent ones.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>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  include/asm-generic/vmlinux.lds.h | 9 +++++++++
>  kernel/bpf/btf.c                  | 4 ++--
>  kernel/bpf/sysfs_btf.c            | 6 +++---
>  3 files changed, 14 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

hi,
I'm getting following compilation fail when CONFIG_DEBUG_INFO_BTF is disabled

	[jolsa@krava linux-qemu]$ make 
	  CALL    scripts/checksyscalls.sh
	  DESCEND objtool
	  INSTALL libsubcmd_headers
	  UPD     include/generated/utsversion.h
	  CC      init/version-timestamp.o
	  LD      .tmp_vmlinux.kallsyms1
	ld: kernel/bpf/btf.o: in function `btf_parse_vmlinux':
	/home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5988: undefined reference to `__start_BTF'
	ld: /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5989: undefined reference to `__stop_BTF'
	ld: /home/jolsa/kernel/linux-qemu/kernel/bpf/btf.c:5989: undefined reference to `__start_BTF'
	make[2]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1
	make[1]: *** [/home/jolsa/kernel/linux-qemu/Makefile:1160: vmlinux] Error 2
	make: *** [Makefile:240: __sub-make] Error 2

maybe the assumption was that kernel/bpf/btf.o is compiled only
for CONFIG_DEBUG_INFO_BTF, but it's actually:

  obj-$(CONFIG_BPF_SYSCALL) += btf.o memalloc.o

I guess we just need !CONFIG_DEBUG_INFO_BTF version of btf_parse_vmlinux
function

jirka

> +
>  /*
>   * Read only Data
>   */
> 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)
> 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);
> -- 
> 2.44.0.478.gd926399ef9-goog
> 
>