Re: [PATCH bpf v3 2/2] selftests/bpf: Cover partial copy of non-linear test_run output

[Paul Chaignon <paul.chaignon@gmail.com>]

On Wed, Jun 17, 2026 at 10:19:52PM +0800, sun jian wrote:
> On Wed, Jun 17, 2026 at 6:31 PM <bot+bpf-ci@kernel.org> wrote:
> >
> > > diff --git a/tools/testing/selftests/bpf/prog_tests/prog_run_opts.c b/tools/testing/selftests/bpf/prog_tests/prog_run_opts.c
> > > index 01f1d1b6715a..9cc898e6a9f7 100644
> > > --- a/tools/testing/selftests/bpf/prog_tests/prog_run_opts.c
> > > +++ b/tools/testing/selftests/bpf/prog_tests/prog_run_opts.c
> > > @@ -4,6 +4,10 @@
> > >
> > >  #include "test_pkt_access.skel.h"
> > >
> > > +#define NONLINEAR_PKT_LEN 9000
> > > +#define NONLINEAR_LINEAR_DATA_LEN 64
> > > +#define SHORT_OUT_LEN 100
> > > +
> >
> > [ ... ]
> >
> > > @@ -20,6 +24,69 @@ static void check_run_cnt(int prog_fd, __u64 run_cnt)
> > >             "incorrect number of repetitions, want %llu have %llu\n", run_cnt, info.run_cnt);
> > >  }
> > >
> > > +static void init_pkt(__u8 *pkt, size_t len)
> > > +{
> > > +     size_t i;
> > > +
> > > +     for (i = 0; i < len; i++)
> > > +             pkt[i] = i & 0xff;
> > > +}
> >
> > A question was raised on v2 about whether pkt_v4 could be reused by
> > reducing the linear area to ETH_HLEN, rather than introducing a custom
> > init_pkt() with a 9000-byte stack packet.
> >
> > Can't we reuse pkt_v4 here by reducing the linear area to ETH_HLEN?
> > The v3 still adds init_pkt() and the NONLINEAR_PKT_LEN packet, so this
> > doesn't seem to have been picked up.
> >
> > > +
> > > +static void test_skb_nonlinear_data_out_partial(struct test_pkt_access *skel)
> > > +{
> > > +     LIBBPF_OPTS(bpf_test_run_opts, topts);
> > > +     __u8 pkt[NONLINEAR_PKT_LEN];
> > > +     __u8 out[SHORT_OUT_LEN];
> > > +     struct __sk_buff skb = {};
> > > +     int prog_fd, err;
> > > +
> > > +     init_pkt(pkt, sizeof(pkt));
> > > +
> > > +     skb.data_end = NONLINEAR_LINEAR_DATA_LEN;
> > > +
> > > +     topts.data_in = pkt;
> > > +     topts.data_size_in = sizeof(pkt);
> > > +     topts.data_out = out;
> > > +     topts.data_size_out = sizeof(out);
> > > +     topts.ctx_in = &skb;
> > > +     topts.ctx_size_in = sizeof(skb);
> > > +
> > > +     prog_fd = bpf_program__fd(skel->progs.tc_pass_prog);
> >
> > [ ... ]
> >
> > > diff --git a/tools/testing/selftests/bpf/progs/test_pkt_access.c b/tools/testing/selftests/bpf/progs/test_pkt_access.c
> > > index bce7173152c6..cd284401eebd 100644
> > > --- a/tools/testing/selftests/bpf/progs/test_pkt_access.c
> > > +++ b/tools/testing/selftests/bpf/progs/test_pkt_access.c
> > > @@ -150,3 +150,15 @@ int test_pkt_access(struct __sk_buff *skb)
> > >
> > >       return TC_ACT_UNSPEC;
> > >  }
> > > +
> > > +SEC("tc")
> > > +int tc_pass_prog(struct __sk_buff *skb)
> > > +{
> > > +     return TC_ACT_OK;
> > > +}
> > > +
> > > +SEC("xdp.frags")
> > > +int xdp_frags_pass_prog(struct xdp_md *ctx)
> > > +{
> > > +     return XDP_PASS;
> > > +}
> >
> > A related suggestion on v2 was that, once pkt_v4 is reused, the existing
> > BPF program could be reused instead of adding new pass-through programs.
> >
> > Could tc_pass_prog and xdp_frags_pass_prog be dropped in favour of the
> > existing program? The v3 still adds both of these, so this point also
> > seems to be open.
> >
> >
> > ---
> > AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> > See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> >
> > CI run summary: https://github.com/kernel-patches/bpf/actions/runs/27680511802
> 
> Hi,
> 
> Thanks for checking this.

Hi Sun Jian,

It would help if you could reply inline instead of at the end of the
messages, especially when there are multiple comments. See [1] for an
explanation of how that works.

1: https://kernelnewbies.org/FirstKernelPatch#Responding_inline

> 
> I tried reusing pkt_v4 and the existing TC program, but they do not fit
> the skb case this test is trying to cover.
> 
> For skb test_run, IPv4/IPv6 inputs with a too-short L3 header in the
> linear area are rejected before bpf_test_finish(). With pkt_v4 and a
> linear area of ETH_HLEN, the test fails with -EINVAL before reaching the
> partial copy-out path. If the linear area is increased enough to pass the
> IPv4 check, pkt_v4 is too small to both trigger the old
> copy_size - frag_size path and verify that the copied prefix spans the
> linear data and the first fragment. pkt_v6 has the same issue: after
> making the IPv6 header linear, only 20 bytes remain in frags.
> 
> The existing test_pkt_access program has its own packet-access coverage
> goals and is not just a pass-through carrier. With such a short linear
> area or small packet fixture, it can fail before the test hits the
> bpf_test_finish()'s partial copy-out path. A pass-through TC program is
> therefore a better fit, because it keeps the test focused on the
> bpf_test_finish() copy-out semantics.

If we're keeping tc_pass_prog() then can't we use pkt_v4 and get rid of
init_pkt?

> 
> For XDP, this object does not have an existing xdp.frags pass-through
> program, so the small XDP frags program is needed to cover the other
> caller of the shared bpf_test_finish() path.
> 
> Thanks,
> Sun Jian