Hello, On Wed, Aug 06, 2025 at 01:03:01AM +0100, Sam James wrote: > When exploring building bpf_skel with GCC's BPF support, there was a > buid failure because of bpf_core_field_exists vs the mem_hops bitfield: > ``` > In file included from util/bpf_skel/sample_filter.bpf.c:6: > util/bpf_skel/sample_filter.bpf.c: In function 'perf_get_sample': > tools/perf/libbpf/include/bpf/bpf_core_read.h:169:42: error: cannot take address of bit-field 'mem_hops' > 169 | #define ___bpf_field_ref1(field) (&(field)) > | ^ > tools/perf/libbpf/include/bpf/bpf_helpers.h:222:29: note: in expansion of macro '___bpf_field_ref1' > 222 | #define ___bpf_concat(a, b) a ## b > | ^ > tools/perf/libbpf/include/bpf/bpf_helpers.h:225:29: note: in expansion of macro '___bpf_concat' > 225 | #define ___bpf_apply(fn, n) ___bpf_concat(fn, n) > | ^~~~~~~~~~~~~ > tools/perf/libbpf/include/bpf/bpf_core_read.h:173:9: note: in expansion of macro '___bpf_apply' > 173 | ___bpf_apply(___bpf_field_ref, ___bpf_narg(args))(args) > | ^~~~~~~~~~~~ > tools/perf/libbpf/include/bpf/bpf_core_read.h:188:39: note: in expansion of macro '___bpf_field_ref' > 188 | __builtin_preserve_field_info(___bpf_field_ref(field), BPF_FIELD_EXISTS) > | ^~~~~~~~~~~~~~~~ > util/bpf_skel/sample_filter.bpf.c:167:29: note: in expansion of macro 'bpf_core_field_exists' > 167 | if (bpf_core_field_exists(data->mem_hops)) > | ^~~~~~~~~~~~~~~~~~~~~ > cc1: error: argument is not a field access > ``` > > ___bpf_field_ref1 was adapted for GCC in 12bbcf8e840f40b82b02981e96e0a5fbb0703ea9 > but the trick added for compatibility in 3a8b8fc3174891c4c12f5766d82184a82d4b2e3e > isn't compatible with that as an address is used as an argument. > > Workaround this by calling __builtin_preserve_field_info directly as the > bpf_core_field_exists macro does, but without the ___bpf_field_ref use. IIUC GCC doesn't support bpf_core_fields_exists() for bitfield members, right? Is it gonna change in the future? > > Link: https://gcc.gnu.org/PR121420 > Co-authored-by: Andrew Pinski <quic_apinski@xxxxxxxxxxx> > Signed-off-by: Sam James <sam@xxxxxxxxxx> > --- > tools/perf/util/bpf_skel/sample_filter.bpf.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/tools/perf/util/bpf_skel/sample_filter.bpf.c b/tools/perf/util/bpf_skel/sample_filter.bpf.c > index b195e6efeb8be..e5666d4c17228 100644 > --- a/tools/perf/util/bpf_skel/sample_filter.bpf.c > +++ b/tools/perf/util/bpf_skel/sample_filter.bpf.c > @@ -164,7 +164,7 @@ static inline __u64 perf_get_sample(struct bpf_perf_event_data_kern *kctx, > if (entry->part == 8) { > union perf_mem_data_src___new *data = (void *)&kctx->data->data_src; > > - if (bpf_core_field_exists(data->mem_hops)) > + if (__builtin_preserve_field_info(data->mem_hops, BPF_FIELD_EXISTS)) I believe those two are equivalent (maybe worth a comment?). But it'd be great if BPF/clang folks can review if it's ok. Anyway, I can build it with clang. Tested-by: Namhyung Kim <namhyung@xxxxxxxxxx> Thanks, Namhyung > return data->mem_hops; > > return 0; > -- > 2.50.1 >
