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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Wed, 14 Apr 2021 15:16:00 -0700
Message-ID: <CAEf4Bzb-Xh_JOWsZwC+fNiC20K_9fzrpfiAMHTYM=6k--+SZaw@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 Wed, Apr 14, 2021 at 2:21 AM Florent Revest <revest@chromium.org> wrote:
>
> On Wed, Apr 14, 2021 at 1:21 AM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > 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.
>
> Agreed.
>
> > 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.
>
> Ah, great idea! I was thinking of having one skeleton for each but it
> would be a bit much indeed.
>
> Because the format string needs to be in a read only map though, I
> hope it can be modified from userspace before loading. I'll try it out
> and see :) if it doesn't work I'll just use more skeletons

You need read-only variables (const volatile my_type). Their contents
are statically verified by BPF verifier, yet user-space can pre-setup
it at runtime.

>
> > 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.
>
> Alright :)
>
> >
> > >  .../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
> > >
> >
> > [...]