Currently, it's not allowed to attach an fentry/fexit prog to another fentry/fexit. 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. But currently it seems impossible to load and attach tracing programs in a way that will form such a cycle. Replace "no same type" requirement with verification that no more than one level of attachment nesting is allowed. In this way only one fentry/fexit program could be attached to another fentry/fexit to cover profiling use case, and still no cycle could be formed. The series contains a test for recursive attachment, as well as a fix + test for 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 selftests/bpf: Test re-attachment fix for bpf_tracing_prog_attach Jiri Olsa (1): bpf: Fix re-attachment branch in bpf_tracing_prog_attach include/linux/bpf.h | 1 + kernel/bpf/syscall.c | 19 ++- kernel/bpf/verifier.c | 39 +++--- .../bpf/prog_tests/recursive_attach.c | 113 ++++++++++++++++++ .../selftests/bpf/progs/fentry_recursive.c | 16 +++ .../bpf/progs/fentry_recursive_target.c | 27 +++++ 6 files changed, 200 insertions(+), 15 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
