On Monday, January 6th, 2025 at 10:59 AM, Eduard Zingerman <eddyz87@xxxxxxxxx> wrote: > > > On Mon, 2025-01-06 at 18:54 +0000, Ihor Solodrai wrote: > > > Various compilation errors happen when BPF programs in selftests/bpf > > are built with GCC BPF. For more details see the discussion at [1]. > > > > The changes only affect test_progs-bpf_gcc, which is built only if > > BPF_GCC is set: > > * Pass -std=gnu17 to gcc in order to avoid errors on bool types > > declarations in vmlinux.h > > * Pass -fno-strict-aliasing for tests that trigger uninitialized > > variable warning on BPF_RAW_INSNS [2] > > > > [1] https://lore.kernel.org/bpf/EYcXjcKDCJY7Yb0GGtAAb7nLKPEvrgWdvWpuNzXm2qi6rYMZDixKv5KwfVVMBq17V55xyC-A1wIjrqG3aw-Imqudo9q9X7D7nLU2gWgbN0w=@pm.me/ > > [2] https://lore.kernel.org/bpf/87pll3c8bt.fsf@xxxxxxxxxx/ > > > > CC: Jose E. Marchesi jose.marchesi@xxxxxxxxxx > > Signed-off-by: Ihor Solodrai ihor.solodrai@xxxxx > > > > --- > > > > v1: https://lore.kernel.org/bpf/20250104001751.1869849-1-ihor.solodrai@xxxxx/ > > > > tools/testing/selftests/bpf/Makefile | 6 +++++- > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > diff --git a/tools/testing/selftests/bpf/Makefile b/tools/testing/selftests/bpf/Makefile > > index eb4d21651aa7..b043791fe6db 100644 > > --- a/tools/testing/selftests/bpf/Makefile > > +++ b/tools/testing/selftests/bpf/Makefile > > @@ -69,6 +69,10 @@ progs/timer_crash.c-CFLAGS := -fno-strict-aliasing > > progs/test_global_func9.c-CFLAGS := -fno-strict-aliasing > > progs/verifier_nocsr.c-CFLAGS := -fno-strict-aliasing > > > > +# Uninitialized variable warning on BPF_RAW_INSN > > +progs/verifier_bpf_fastcall.c-CFLAGS := -fno-strict-aliasing > > +progs/verifier_search_pruning.c-CFLAGS := -fno-strict-aliasing > > > Specifying -fno-strict-aliasing for a sub-set of tests is not convenient, > as this list would have to be extended each time __imm_insn macro is used. > Either this flag should be used for test_progs compilation as a whole, > or the macro should be updated to use union as it was suggested previously. > Personally, I don't like the aliasing rules and would prefer -fno-strict-aliasing, > but changing macro is a simple and non-intrusive update, so I think it's a better option. __imm_insn is not the only thing breaking the aliasing rules. An example from bind* tests: In file included from progs/bind4_prog.c:15: progs/bind4_prog.c: In function ‘bind_v4_prog’: progs/bind_prog.h:9:36: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing] 9 | (((volatile __u16 *)&(src))[w] << 16 * w) | ^ progs/bind4_prog.c:138:21: note: in expansion of macro ‘load_word’ 138 | user_ip4 |= load_word(ctx->user_ip4, 0, sizeof(user_ip4)); | ^~~~~~~~~ In file included from progs/bind6_prog.c:15: progs/bind6_prog.c: In function ‘bind_v6_prog’: progs/bind_prog.h:9:36: error: dereferencing type-punned pointer will break strict-aliasing rules [-Werror=strict-aliasing] 9 | (((volatile __u16 *)&(src))[w] << 16 * w) | ^ progs/bind6_prog.c:151:29: note: in expansion of macro ‘load_word’ 151 | user_ip6 |= load_word(ctx->user_ip6[i], 0, sizeof(user_ip6)); | ^~~~~~~~~ And also: $ grep -rl --include="[Mm]akefile*" 'fno-strict-aliasing' | sort arch/arm64/kernel/vdso32/Makefile arch/loongarch/vdso/Makefile arch/mips/vdso/Makefile arch/parisc/boot/compressed/Makefile arch/powerpc/boot/Makefile arch/s390/purgatory/Makefile arch/x86/boot/compressed/Makefile arch/x86/Makefile drivers/firmware/efi/libstub/Makefile Makefile selftests/bpf/Makefile selftests/kvm/Makefile selftests/net/tcp_ao/Makefile tools/scripts/Makefile.include tools/testing/selftests/bpf/Makefile tools/testing/selftests/kvm/Makefile tools/testing/selftests/net/tcp_ao/Makefile tools/testing/vsock/Makefile tools/virtio/Makefile So yeah, just setting this flag for all tests makes sense. I was wondering how clang handles this, and it turns out -fno-strict-aliasing is true by default in clang [1]: -fno-strict-aliasing Disable optimizations based on strict aliasing rules (default) [1]: https://clang.llvm.org/docs/UsersManual.html > > > # Some utility functions use LLVM libraries > > jit_disasm_helpers.c-CFLAGS = $(LLVM_CFLAGS) > > > > @@ -507,7 +511,7 @@ endef > > # Build BPF object using GCC > > define GCC_BPF_BUILD_RULE > > $(call msg,GCC-BPF,$4,$2) > > - $(Q)$(BPF_GCC) $3 -DBPF_NO_PRESERVE_ACCESS_INDEX -Wno-attributes -O2 -c $1 -o $2 > > + $(Q)$(BPF_GCC) $3 -DBPF_NO_PRESERVE_ACCESS_INDEX -Wno-attributes -O2 -std=gnu17 -c $1 -o $2 > > endef > > > > SKEL_BLACKLIST := btf__% test_pinning_invalid.c test_sk_assign.c > >
