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

[Geert Uytterhoeven <geert@linux-m68k.org>]

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 	Hi Tony,

On Mon, 3 Jun 2024, 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>

Thanks for your patch, which is now commit 7bdcedd5c8fb88e7
("bpf: Harden __bpf_kfunc tag against linker kfunc removal") in
v6.10-rc5.

This is causing build failures on ARM with
CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y:

     net/core/filter.c:11859:1: error: ‘retain’ attribute ignored [-Werror=attributes]
     11859 | {
           | ^
     net/core/filter.c:11872:1: error: ‘retain’ attribute ignored [-Werror=attributes]
     11872 | {
           | ^
     net/core/filter.c:11885:1: error: ‘retain’ attribute ignored [-Werror=attributes]
     11885 | {
           | ^
     net/core/filter.c:11906:1: error: ‘retain’ attribute ignored [-Werror=attributes]
     11906 | {
           | ^
     net/core/filter.c:12092:1: error: ‘retain’ attribute ignored [-Werror=attributes]
     12092 | {
           | ^
     net/core/xdp.c:713:1: error: ‘retain’ attribute ignored [-Werror=attributes]
       713 | {
           | ^
     net/core/xdp.c:736:1: error: ‘retain’ attribute ignored [-Werror=attributes]
       736 | {
           | ^
     net/core/xdp.c:769:1: error: ‘retain’ attribute ignored [-Werror=attributes]
       769 | {
           | ^
     [...]

My compiler is arm-linux-gnueabihf-gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04).

> --- 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();							       \

Gr{oetje,eeting}s,

 						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
 							    -- Linus Torvalds