Subject: Re: [External] [Patch bpf 2/2] selftests/bpf: Add a BPF selftest for bpf_skb_change_tail()

From: John Fastabend <[email-masked]>
Date: Tue, 26 Nov 2024 23:34:25 -0800

Zijian Zhang wrote:
> On 11/6/24 7:41 PM, Cong Wang wrote:
> > From: Cong Wang <[email-masked]>
> > 
> > As requested by Daniel, we need to add a selftest to cover
> > bpf_skb_change_tail() cases in skb_verdict. Here we test trimming,
> > growing and error cases, and validate its expected return values.
> > 
> > Cc: John Fastabend <[email-masked]>
> > Cc: Daniel Borkmann <[email-masked]>
> > Cc: Zijian Zhang <[email-masked]>
> > Signed-off-by: Cong Wang <[email-masked]>
> > ---
> >   .../selftests/bpf/prog_tests/sockmap_basic.c  | 51 +++++++++++++++++++
> >   .../bpf/progs/test_sockmap_change_tail.c      | 40 +++++++++++++++
> >   2 files changed, 91 insertions(+)
> >   create mode 100644 tools/testing/selftests/bpf/progs/test_sockmap_change_tail.c
> > 
> > diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > index 82bfb266741c..fe735fced836 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
> > @@ -12,6 +12,7 @@
> >   #include "test_sockmap_progs_query.skel.h"
> >   #include "test_sockmap_pass_prog.skel.h"
> >   #include "test_sockmap_drop_prog.skel.h"
> > +#include "test_sockmap_change_tail.skel.h"
> >   #include "bpf_iter_sockmap.skel.h"
> >   
> >   #include "sockmap_helpers.h"
> > @@ -562,6 +563,54 @@ static void test_sockmap_skb_verdict_fionread(bool pass_prog)
> >   		test_sockmap_drop_prog__destroy(drop);
> >   }
> >   
> > +static void test_sockmap_skb_verdict_change_tail(void)
> > +{
> > +	struct test_sockmap_change_tail *skel;
> > +	int err, map, verdict;
> > +	int c1, p1, sent, recvd;
> > +	int zero = 0;
> > +	char b[3];
> > +
> > +	skel = test_sockmap_change_tail__open_and_load();
> > +	if (!ASSERT_OK_PTR(skel, "open_and_load"))
> > +		return;
> > +	verdict = bpf_program__fd(skel->progs.prog_skb_verdict);
> > +	map = bpf_map__fd(skel->maps.sock_map_rx);
> > +
> > +	err = bpf_prog_attach(verdict, map, BPF_SK_SKB_STREAM_VERDICT, 0);
> > +	if (!ASSERT_OK(err, "bpf_prog_attach"))
> > +		goto out;
> > +	err = create_pair(AF_INET, SOCK_STREAM, &c1, &p1);
> > +	if (!ASSERT_OK(err, "create_pair()"))
> > +		goto out;
> > +	err = bpf_map_update_elem(map, &zero, &c1, BPF_NOEXIST);
> > +	if (!ASSERT_OK(err, "bpf_map_update_elem(c1)"))
> > +		goto out_close;
> > +	sent = xsend(p1, "Tr", 2, 0);
> > +	ASSERT_EQ(sent, 2, "xsend(p1)");
> > +	recvd = recv(c1, b, 2, 0);
> > +	ASSERT_EQ(recvd, 1, "recv(c1)");
> > +	ASSERT_EQ(skel->data->change_tail_ret, 0, "change_tail_ret");
> > +
> > +	sent = xsend(p1, "G", 1, 0);
> > +	ASSERT_EQ(sent, 1, "xsend(p1)");
> > +	recvd = recv(c1, b, 2, 0);
> > +	ASSERT_EQ(recvd, 2, "recv(c1)");
> > +	ASSERT_EQ(skel->data->change_tail_ret, 0, "change_tail_ret");
> > +
> > +	sent = xsend(p1, "E", 1, 0);
> > +	ASSERT_EQ(sent, 1, "xsend(p1)");
> > +	recvd = recv(c1, b, 1, 0);
> > +	ASSERT_EQ(recvd, 1, "recv(c1)");
> > +	ASSERT_EQ(skel->data->change_tail_ret, -EINVAL, "change_tail_ret");
> > +
> > +out_close:
> > +	close(c1);
> > +	close(p1);
> > +out:
> > +	test_sockmap_change_tail__destroy(skel);
> > +}
> > +
> >   static void test_sockmap_skb_verdict_peek_helper(int map)
> >   {
> >   	int err, c1, p1, zero = 0, sent, recvd, avail;
> > @@ -927,6 +976,8 @@ void test_sockmap_basic(void)
> >   		test_sockmap_skb_verdict_fionread(true);
> >   	if (test__start_subtest("sockmap skb_verdict fionread on drop"))
> >   		test_sockmap_skb_verdict_fionread(false);
> > +	if (test__start_subtest("sockmap skb_verdict change tail"))
> > +		test_sockmap_skb_verdict_change_tail();
> >   	if (test__start_subtest("sockmap skb_verdict msg_f_peek"))
> >   		test_sockmap_skb_verdict_peek();
> >   	if (test__start_subtest("sockmap skb_verdict msg_f_peek with link"))
> > diff --git a/tools/testing/selftests/bpf/progs/test_sockmap_change_tail.c b/tools/testing/selftests/bpf/progs/test_sockmap_change_tail.c
> > new file mode 100644
> > index 000000000000..2796dd8545eb
> > --- /dev/null
> > +++ b/tools/testing/selftests/bpf/progs/test_sockmap_change_tail.c
> > @@ -0,0 +1,40 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/* Copyright (c) 2024 ByteDance */
> > +#include <linux/bpf.h>
> > +#include <bpf/bpf_helpers.h>
> > +
> > +struct {
> > +	__uint(type, BPF_MAP_TYPE_SOCKMAP);
> > +	__uint(max_entries, 1);
> > +	__type(key, int);
> > +	__type(value, int);
> > +} sock_map_rx SEC(".maps");
> > +
> > +long change_tail_ret = 1;
> > +
> > +SEC("sk_skb")
> > +int prog_skb_verdict(struct __sk_buff *skb)
> > +{
> > +	char *data, *data_end;
> > +
> > +	bpf_skb_pull_data(skb, 1);
> > +	data = (char *)(unsigned long)skb->data;
> > +	data_end = (char *)(unsigned long)skb->data_end;
> > +
> > +	if (data + 1 > data_end)
> > +		return SK_PASS;
> > +
> > +	if (data[0] == 'T') { /* Trim the packet */
> > +		change_tail_ret = bpf_skb_change_tail(skb, skb->len - 1, 0);
> > +		return SK_PASS;
> > +	} else if (data[0] == 'G') { /* Grow the packet */
> > +		change_tail_ret = bpf_skb_change_tail(skb, skb->len + 1, 0);
> > +		return SK_PASS;
> > +	} else if (data[0] == 'E') { /* Error */
> > +		change_tail_ret = bpf_skb_change_tail(skb, 65535, 0);
> > +		return SK_PASS;
> > +	}
> > +	return SK_PASS;
> > +}
> > +
> > +char _license[] SEC("license") = "GPL";
> 
> LGTM!
> 
> I think it will be better if the test could also cover the case you
> indicated in the first patch, where skb_transport_offset is a negative
> value.
> 
> Thanks,
> Zijian
> 

Hi Cong,

I agree it would be great to see the skb_transport_offset is
negative pattern. Could we add it?

Thanks,
John