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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Tue, 13 Apr 2021 16:21:42 -0700
Message-ID: <CAEf4BzZ6cLio0ZZEkc5iYp9yWg3Fc1ZORBTr85TdoqF-sRU3DQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v3 6/6] selftests/bpf: Add a series of tests for bpf_snprintf
To:     Florent Revest <revest@chromium.org>
Cc:     bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Yonghong Song <yhs@fb.com>, KP Singh <kpsingh@kernel.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        open list <linux-kernel@vger.kernel.org>
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On Mon, Apr 12, 2021 at 8:38 AM Florent Revest <revest@chromium.org> wrote:
>
> This exercises most of the format specifiers.
>
> Signed-off-by: Florent Revest <revest@chromium.org>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> ---

As I mentioned on another patch, we probably need negative tests even
more than positive ones.

I think an easy and nice way to do this is to have a separate BPF
skeleton where fmt string and arguments are provided through read-only
global variables, so that user-space can re-use the same BPF skeleton
to simulate multiple cases. BPF program itself would just call
bpf_snprintf() and store the returned result.

Whether we need to validate the verifier log is up to debate (though
it's not that hard to do by overriding libbpf_print_fn() callback),
I'd be ok at least knowing that some bad format strings are rejected
and don't crash the kernel.


>  .../selftests/bpf/prog_tests/snprintf.c       | 81 +++++++++++++++++++
>  .../selftests/bpf/progs/test_snprintf.c       | 74 +++++++++++++++++
>  2 files changed, 155 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/snprintf.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_snprintf.c
>

[...]