From ac507ed9882fd91a94657d68fe9ceac04b957103 Mon Sep 17 00:00:00 2001 From: Tony Ambardar Date: Sat, 1 Jun 2024 00:00:21 -0700 Subject: [PATCH 2/2] bpf: Harden __bpf_kfunc tag against linker kfunc removal 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 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 --- include/linux/btf.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- a/include/linux/btf.h +++ b/include/linux/btf.h @@ -81,7 +81,7 @@ * 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 /* * Return the name of the passed struct, if exists, or halt the build if for