On Thu, May 8, 2025 at 6:22 AM Alan Maguire <alan.maguire@xxxxxxxxxx> wrote: > > When testing v1 of [1] we noticed that functions with 0-sized structs > as parameters were not part of BTF encoding; this was fixed in v2. > However we need to make sure we handle such zero-sized structs > correctly since they confound the calling convention expectations - > no registers are used for the empty struct so this has knock-on effects > for subsequent register-parameter matching. Do you have a list (or at least an example) of the function we are talking about, just curious to see what's that. The question I have is whether it's safe to assume that regardless of architecture we can assume that zero-sized struct has no effect on register allocation (which would seem logical, but is that true for all ABIs). BTW, while looking at patch #2, I noticed that btf_distill_func_proto() disallows functions returning struct-by-value, which seems overly aggressive, at least for structs of up to 8 bytes. So maybe if we can validate that both cases are not introducing any new quirks across all supported architectures, we can solve both limitations? P.S., oh, and s390x selftest (test_struct_args) isn't happy, please check. > > Patch 1 updates BPF_PROG2() to handle the zero-sized struct case. > Patch 2 makes 0-sized structs a special case, allowing them to exist > as parameter representations in BTF without failing verification. > Patch 3 is a selftest that ensures the parameters after the 0-sized > struct are represented correctly. > > [1] https://lore.kernel.org/dwarves/20250502070318.1561924-1-tony.ambardar@xxxxxxxxx/ > > Alan Maguire (3): > libbpf: update BPF_PROG2() to handle empty structs > bpf: allow 0-sized structs as function parameters > selftests/bpf: add 0-length struct testing to tracing_struct tests > > kernel/bpf/btf.c | 2 +- > tools/lib/bpf/bpf_tracing.h | 6 ++++-- > .../selftests/bpf/prog_tests/tracing_struct.c | 2 ++ > tools/testing/selftests/bpf/progs/tracing_struct.c | 11 +++++++++++ > tools/testing/selftests/bpf/test_kmods/bpf_testmod.c | 12 ++++++++++++ > 5 files changed, 30 insertions(+), 3 deletions(-) > > -- > 2.39.3 >
