Re: [PATCH bpf-next v2 3/3] selftests/bpf: Add test for xdp_md context in BPF_PROG_TEST_RUN

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Zvi Effron <zeffron@riotgames.com>
Cc: bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Cody Haas <chaas@riotgames.com>,
	Lisa Watanabe <lwatanabe@riotgames.com>
Subject: Re: [PATCH bpf-next v2 3/3] selftests/bpf: Add test for xdp_md context in BPF_PROG_TEST_RUN
Date: Thu, 27 May 2021 20:21:51 -0700	[thread overview]
Message-ID: <CAEf4BzZ-DdZk2rnzc40aoepJn=zS1o=-mdW15uz5Cm4Vi6rH5Q@mail.gmail.com> (raw)
In-Reply-To: <CAC1LvL1JM4Wu43XU4aVHHheChtSQQmKRx7RaVBG5Kmkt-edbBA@mail.gmail.com>

On Thu, May 27, 2021 at 7:11 PM Zvi Effron <zeffron@riotgames.com> wrote:
>
> On Thu, May 27, 2021 at 6:28 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Thu, May 27, 2021 at 1:14 PM Zvi Effron <zeffron@riotgames.com> wrote:
> > >
> > > +
> > > +       /* Data past the end of the kernel's struct xdp_md must be 0 */
> > > +       bad_ctx[sizeof(bad_ctx) - 1] = 1;
> > > +       tattr.ctx_in = bad_ctx;
> > > +       tattr.ctx_size_in = sizeof(bad_ctx);
> > > +       err = bpf_prog_test_run_opts(prog_fd, &tattr);
> > > +       ASSERT_ERR(err, "bpf_prog_test_run(test2)");
> > > +       ASSERT_EQ(errno, 22, "test2-errno");
> >
> > by the time you are checking errno it might get overwritten. If you
> > want to check errno, you have to remember it right after the function
> > returns
>
> Is it sufficient to simply make the errno ASSERT the first thing after
> the function returns? Or would we still need to preserve it into a
> local variable?

Yes, doing ASSERT_EQ(errno, 22, "test2-errno") right after
bpf_prog_test_run_opts() will work reliably.