Re: [PATCH bpf-next v3 6/6] selftests/bpf: Add a series of tests for bpf_snprintf

[Florent Revest]

On Wed, Apr 14, 2021 at 1:21 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Mon, Apr 12, 2021 at 8:38 AM Florent Revest <revest@xxxxxxxxxxxx> wrote:
> >
> > This exercises most of the format specifiers.
> >
> > Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
> > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> > ---
>
> 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

> 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
> >
>
> [...]