Re: [RFC PATCH 15/17] selftests/bpf: Add verifier tests for dequeue prog

On Thu, 14 Jul 2022 at 20:54, Andrii Nakryiko [off-list ref] wrote:

On Wed, Jul 13, 2022 at 11:45 PM Kumar Kartikeya Dwivedi
[off-list ref] wrote:

quoted

On Thu, 14 Jul 2022 at 07:38, Andrii Nakryiko [off-list ref] wrote:

quoted

On Wed, Jul 13, 2022 at 4:15 AM Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

From: Kumar Kartikeya Dwivedi <memxor@gmail.com>

Test various cases of direct packet access (proper range propagation,
comparison of packet pointers pointing into separate xdp_frames, and
correct invalidation on packet drop (so that multiple packet pointers
are usable safely in a dequeue program)).

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Toke Høiland-Jørgensen <redacted>
---

Consider writing these tests as plain C BPF code and put them in
test_progs, is there anything you can't express in C and thus requires
test_verifier?

Not really, but in general I like test_verifier because it stays
immune to compiler shenanigans.

In general I dislike them because they are almost incomprehensible. So
unless there is a very particular sequence of low-level BPF assembly
instructions one needs to test, I'd always opt for test_progs as more
maintainable solution.

Things like making sure that verifier rejects invalid use of
particular objects or helpers doesn't seem to rely much on particular
assembly sequence and can and should be expressed with plain C.

quoted

So going forward should test_verifier tests be avoided, and normal C
tests (using SEC("?...")) be preferred for these cases?

In my opinion, yes, unless absolutely requiring low-level assembly to
express conditions which are otherwise hard to express reliably in C.

Ok, fair point. I will replace these with C tests in the next version.

quoted

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 tools/testing/selftests/bpf/test_verifier.c   |  29 +++-
 .../testing/selftests/bpf/verifier/dequeue.c  | 160 ++++++++++++++++++
 2 files changed, 180 insertions(+), 9 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/verifier/dequeue.c

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