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From:   Florent Revest <revest@chromium.org>
Date:   Thu, 15 Apr 2021 11:38:04 +0200
Message-ID: <CABRcYm+SFfqzf8Qkt+f3YvXH4pRuA1ckU+YajF3+=B+jRtxs8Q@mail.gmail.com>
Subject: Re: [PATCH bpf-next v3 6/6] selftests/bpf: Add a series of tests for bpf_snprintf
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
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 Thu, Apr 15, 2021 at 12:16 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> 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.

Thanks :) v4 has negative fmt tests