On Tue, Jun 25, 2024 at 12:46:48PM +0200, Geert Uytterhoeven wrote: > 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@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> > > 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). hum, so it'd mean __has_attribute(__retain__) returns true while gcc still ignores the retain attribute.. like in this bug which seems similar: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99587 but not sure how it got fixed.. any chance you can upgrade gcc and retest? jirka > > > --- 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@xxxxxxxxxxxxxx > > 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
