Merge branch 'relax-tracing-prog-recursive-attach-rules'

Dmitrii Dolgov says:

====================
Relax tracing prog recursive attach rules

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/
====================

Link: https://lore.kernel.org/r/20240103190559.14750-1-9erthalion6@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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