Currently, it's not allowed to attach an fentry/fexit prog to another one of the same type. At the same time it's not uncommon to see a tracing program with lots of logic in use, and the attachment limitation prevents usage of fentry/fexit for performance analysis (e.g. with "bpftool prog profile" command) in this case. An example could be falcosecurity libs project that uses tp_btf tracing programs for offloading certain part of logic into tail-called programs, but the use-case is still generic enough -- a tracing program could be complicated and heavy enough to warrant its profiling, yet frustratingly it's not possible to do so use best tooling for that. Following the corresponding discussion [1], the reason for that is to avoid tracing progs call cycles without introducing more complex solutions. Relax "no same type" requirement to "no progs that are already an attach target themselves" for the tracing type. In this way only a standalone tracing program (without any other progs attached to it) could be attached to another one, and no cycle could be formed. Note that currently, due to various limitations, it's actually not possible to form such an attachment cycle the original implementation was prohibiting. It seems that the idea was to make this part robust even in the view of potential future changes. The series contains a test for recursive attachment, as well as a fix + test for what looks like an issue in re-attachment branch of bpf_tracing_prog_attach. When preparing the test for the main change set, I've stumbled upon the possibility to construct a sequence of events when attach_btf would be NULL while computing a trampoline key. It doesn't look like this issue is triggered by the main change, because the reproduces doesn't actually need to have an fentry attachment chain. [1]: https://lore.kernel.org/bpf/20191108064039.2041889-16-ast@xxxxxxxxxx/ Dmitrii Dolgov (3): bpf: Relax tracing prog recursive attach rules selftests/bpf: Add test for recursive attachment of tracing progs bpf, selftest/bpf: Fix re-attachment branch in bpf_tracing_prog_attach include/linux/bpf.h | 2 + include/uapi/linux/bpf.h | 1 + kernel/bpf/syscall.c | 16 ++- kernel/bpf/verifier.c | 19 ++- tools/bpf/bpftool/prog.c | 3 + tools/include/uapi/linux/bpf.h | 1 + .../bpf/prog_tests/recursive_attach.c | 133 ++++++++++++++++++ .../selftests/bpf/progs/fentry_recursive.c | 19 +++ .../bpf/progs/fentry_recursive_target.c | 31 ++++ 9 files changed, 220 insertions(+), 5 deletions(-) create mode 100644 tools/testing/selftests/bpf/prog_tests/recursive_attach.c create mode 100644 tools/testing/selftests/bpf/progs/fentry_recursive.c create mode 100644 tools/testing/selftests/bpf/progs/fentry_recursive_target.c base-commit: 40d0eb0259ae77ace3e81d7454d1068c38bc95c2 -- 2.41.0
