Re: [PATCH bpf v2 2/2] bpf: Harden __bpf_kfunc tag against linker kfunc removal

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Jun 03, 2024 at 10:23:16PM -0700, Tony Ambardar wrote:
> BPF kfuncs are often not directly referenced and may be inadvertently
> removed by optimization steps during kernel builds, thus the __bpf_kfunc
> tag mitigates against this removal by including the __used macro. However,
> this macro alone does not prevent removal during linking, and may still
> yield build warnings (e.g. on mips64el):
> 
>     LD      vmlinux
>     BTFIDS  vmlinux
>   WARN: resolve_btfids: unresolved symbol bpf_verify_pkcs7_signature
>   WARN: resolve_btfids: unresolved symbol bpf_lookup_user_key
>   WARN: resolve_btfids: unresolved symbol bpf_lookup_system_key
>   WARN: resolve_btfids: unresolved symbol bpf_key_put
>   WARN: resolve_btfids: unresolved symbol bpf_iter_task_next
>   WARN: resolve_btfids: unresolved symbol bpf_iter_css_task_new
>   WARN: resolve_btfids: unresolved symbol bpf_get_file_xattr
>   WARN: resolve_btfids: unresolved symbol bpf_ct_insert_entry
>   WARN: resolve_btfids: unresolved symbol bpf_cgroup_release
>   WARN: resolve_btfids: unresolved symbol bpf_cgroup_from_id
>   WARN: resolve_btfids: unresolved symbol bpf_cgroup_acquire
>   WARN: resolve_btfids: unresolved symbol bpf_arena_free_pages
>     NM      System.map
>     SORTTAB vmlinux
>     OBJCOPY vmlinux.32
> 
> Update the __bpf_kfunc tag to better guard against linker optimization by
> including the new __retain compiler macro, which fixes the warnings above.
> 
> Verify the __retain macro with readelf by checking object flags for 'R':
> 
>   $ readelf -Wa kernel/trace/bpf_trace.o
>   Section Headers:
>     [Nr]  Name              Type     Address  Off  Size ES Flg Lk Inf Al
>   ...
>     [178] .text.bpf_key_put PROGBITS 00000000 6420 0050 00 AXR  0   0  8
>   ...
>   Key to Flags:
>   ...
>     R (retain), D (mbind), p (processor specific)
> 
> Link: https://lore.kernel.org/bpf/ZlmGoT9KiYLZd91S@krava/T/
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/r/202401211357.OCX9yllM-lkp@intel.com/
> Fixes: 57e7c169cd6a ("bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs")
> Cc: stable@vger.kernel.org # v6.6+
> Signed-off-by: Tony Ambardar <Tony.Ambardar@gmail.com>

tested on mips64 cross build and the warnings are gone
and related functions are in the vmlinux

patchset looks good to me

Tested-by: Jiri Olsa <jolsa@kernel.org>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>

thanks,
jirka

> ---
>  include/linux/btf.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index f9e56fd12a9f..7c3e40c3295e 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -82,7 +82,7 @@
>   * as to avoid issues such as the compiler inlining or eliding either a static
>   * kfunc, or a global kfunc in an LTO build.
>   */
> -#define __bpf_kfunc __used noinline
> +#define __bpf_kfunc __used __retain noinline
>  
>  #define __bpf_kfunc_start_defs()					       \
>  	__diag_push();							       \
> -- 
> 2.34.1
>