Re: [PATCH v2 bpf-next 6/6] selftests/bpf: add fixed vs rotating verifier log tests

Andrii Nakryiko
2023-03-30T21:07:47Z

On Thu, Mar 30, 2023 at 10:13 AM Lorenz Bauer <lmb@isovalent.com> wrote:
>
> On Wed, Mar 29, 2023 at 12:56 AM Andrii Nakryiko <andrii@kernel.org> wrote:
> >
> > Add selftests validating BPF_LOG_FIXED behavior, which used to be the
> > only behavior, and now default rotating BPF verifier log, which returns
> > just up to last N bytes of full verifier log, instead of returning
> > -ENOSPC.
> >
> > To stress test correctness of in-kernel verifier log logic, we force it
> > to truncate program's verifier log to all lengths from 1 all the way to
> > its full size (about 450 bytes today). This was a useful stress test
> > while developing the feature.
> >
> > For both fixed and rotating log modes we expect -ENOSPC if log contents
> > doesn't fit in user-supplied log buffer.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
>
> Acked-by: Lorenz Bauer <lmb@isovalent.com>
>
> > + /* validate BPF_LOG_FIXED works as verifier log used to work, that is:
> > + * we get -ENOSPC and beginning of the full verifier log. This only
> > + * works for log_level 2 and log_level 1 + failed program. For log
> > + * level 2 we don't reset log at all. For log_level 1 + failed program
> > + * we don't get to verification stats output. With log level 1
> > + * for successful program final result will be just verifier stats.
> > + * But if provided too short log buf, kernel will NULL-out log->ubuf
>
> Out of curiousity: why is ubuf NULLed? Is that something we could change?

This is internal kernel implementation detail. That's how the verifier
signals that there was an error during copy_to_user(). This is
behavior not observable from user space at all. Why would you like to
change that? bpf_attr's log_buf field will never be NULLed out, if
that's what you are worried about.