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@xxxxxxxxx> > Closes: https://lore.kernel.org/r/202401211357.OCX9yllM-lkp@xxxxxxxxx/ > Fixes: 57e7c169cd6a ("bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs") > Cc: stable@xxxxxxxxxxxxxxx # v6.6+ > Signed-off-by: Tony Ambardar <Tony.Ambardar@xxxxxxxxx> 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@xxxxxxxxxx> Reviewed-by: Jiri Olsa <jolsa@xxxxxxxxxx> 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 >
