It's become a common pattern to have a collection of small BPF programs
in one BPF object file, each representing one test case. On user-space
side of such tests we maintain a table of program names and expected
failure or success, along with optional expected verifier log message.

This works, but each set of tests reimplement this mundane code over and
over again, which is a waste of time for anyone trying to add a new set
of tests. Furthermore, it's quite error prone as it's way too easy to miss
some entries in these manually maintained test tables (as evidences by
dynptr_fail tests, in which ringbuf_release_uninit_dynptr subtest was
accidentally missed; this is fixed in next patch).

So this patch implements generic test_loader, which accepts skeleton
name and handles the rest of details: opens and loads BPF object file,
making sure each program is tested in isolation. Optionally each test
case can specify expected BPF verifier log message. In case of failure,
tester makes sure to report verifier log, but it also reports verifier
log in verbose mode unconditionally.

Now, the interesting deviation from existing custom implementations is
the use of btf_decl_tag attribute to specify expected-to-fail vs
expected-to-succeed markers and, optionally, expected log message
directly next to BPF program source code, eliminating the need to
manually create and update table of tests.

We define few macros wrapping btf_decl_tag with a convention that all
values of btf_decl_tag start with "comment:" prefix, and then utilizing
a very simple "just_some_text_tag" or "some_key_name=<value>" pattern to
define things like expected success/failure, expected verifier message,
extra verifier log level (if necessary). This approach is demonstrated
by next patch in which two existing sets of failure tests are converted.

Tester supports both expected-to-fail and expected-to-succeed programs,
though this patch set didn't convert any existing expected-to-succeed
programs yet, as existing tests couple BPF program loading with their
further execution through attach or test_prog_run. One way to allow
testing scenarios like this would be ability to specify custom callback,
executed for each successfully loaded BPF program. This is left for
follow up patches, after some more analysis of existing test cases.

This test_loader is, hopefully, a start of a test_verifier-like runner,
but integrated into test_progs infrastructure. It will allow much better
"user experience" of defining low-level verification tests that can take
advantage of all the libbpf-provided nicety features on BPF side: global
variables, declarative maps, etc.  All while having a choice of defining
it in C or as BPF assembly (through __attribute__((naked)) functions and
using embedded asm), depending on what makes most sense in each
particular case. This will be explored in follow up patches as well.
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221207201648.2990661-1-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>