Hi Jiri, On Wed, Jun 26, 2024 at 11:52 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote: > On Tue, Jun 25, 2024 at 12:46:48PM +0200, Geert Uytterhoeven wrote: > > 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? Indeed, __has_attribute(__retain__) returns true, while the attribute is not supported. My test program: cat > /tmp/a.c <<EOF #if __has_attribute(__retain__) #warning __retain__ OK #else #warning No __retain__ #endif int x __attribute__((__retain__)); EOF $ arm-linux-gnueabihf-gcc-11 -c /tmp/a.c # gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04)) /tmp/a.c:2:2: warning: #warning __retain__ OK [-Wcpp] 2 | #warning __retain__ OK | ^~~~~~~ /tmp/a.c:7:1: warning: ‘retain’ attribute ignored [-Wattributes] 7 | int x __attribute__((__retain__)); | ^~~ Oops $ arm-linux-gnueabihf-gcc-12 -c /tmp/a.c # gcc version 12.3.0 (Ubuntu 12.3.0-1ubuntu1~22.04) /tmp/a.c:2:2: warning: #warning __retain__ OK [-Wcpp] 2 | #warning __retain__ OK | ^~~~~~~ Fixed It works fine with the native gcc-11: $ gcc-11 -c /tmp/a.c # gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04) /tmp/a.c:2:2: warning: #warning __retain__ OK [-Wcpp] 2 | #warning __retain__ OK | ^~~~~~~ I gave it a try on all installed gcc-11 compilers. /usr/bin/aarch64-linux-gnu-gcc-11 /usr/bin/alpha-linux-gnu-gcc-11 /usr/bin/arm-linux-gnueabi-gcc-11 /usr/bin/arm-linux-gnueabihf-gcc-11 /usr/bin/hppa64-linux-gnu-gcc-11 /usr/bin/hppa-linux-gnu-gcc-11 /usr/bin/m68k-linux-gnu-gcc-11 /usr/bin/powerpc64le-linux-gnu-gcc-11 /usr/bin/powerpc64-linux-gnu-gcc-11 /usr/bin/powerpc-linux-gnu-gcc-11 /usr/bin/riscv64-linux-gnu-gcc-11 /usr/bin/s390x-linux-gnu-gcc-11 /usr/bin/sh4-linux-gnu-gcc-11 /usr/bin/sparc64-linux-gnu-gcc-11 /usr/bin/x86_64-linux-gnu-gcc-11 /usr/bin/x86_64-linux-gnux32-gcc-11 All of them failed (incl. x32), except for the native x86_64-linux-gnu-gcc-11. It works fine with all installed gcc-12 compilers (arm-linux-gnueabihf-gcc-12, m68k-linux-gnu-gcc-12, x86_64-linux-gnu-gcc-12). With gcc-9, the absence of __retain__ is detected correctly. 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
