[PATCH bpf-next v5 0/4] Relax tracing prog recursive attach rules

[Dmitrii Dolgov <9erthalion6@gmail.com>]

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@kernel.org/

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 +
 include/uapi/linux/bpf.h                      |   1 +
 kernel/bpf/syscall.c                          |  16 +++
 kernel/bpf/verifier.c                         |  33 ++---
 tools/include/uapi/linux/bpf.h                |   1 +
 .../bpf/prog_tests/recursive_attach.c         | 117 ++++++++++++++++++
 .../selftests/bpf/progs/fentry_recursive.c    |  19 +++
 .../bpf/progs/fentry_recursive_target.c       |  31 +++++
 8 files changed, 205 insertions(+), 14 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