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

[Masahiro Yamada]

On Mon, Apr 15, 2024 at 4:58 PM Ard Biesheuvel <ardb+git@xxxxxxxxxx> 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 pass, and superseded by the actual definitions in the subsequent ones.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>
> 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
> +



Is this necessary?



The following code (BOUNDED_SECTION_BY)
produces __start_BTF and __stop_BTF symbols
under the same condition.



/*
 * .BTF
 */
#ifdef CONFIG_DEBUG_INFO_BTF
#define BTF                                                                \
        .BTF : AT(ADDR(.BTF) - LOAD_OFFSET) {                                \
                BOUNDED_SECTION_BY(.BTF, _BTF)                                \
        }                                                                \
        . = ALIGN(4);                                                        \
        .BTF_ids : AT(ADDR(.BTF_ids) - LOAD_OFFSET) {                        \
                *(.BTF_ids)                                                \
        }
#else
#define BTF
#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
>


-- 
Best Regards
Masahiro Yamada