selftests: bpf: add selftest for __sk_buff context in BPF_PROG_TEST_RUN

Simple test that sets cb to {1,2,3,4,5} and priority to 6, runs bpf
program that fails if cb is not what we expect and increments cb[i] and
priority. When the test finishes, we check that cb is now {2,3,4,5,6}
and priority is 7.

We also test the sanity checks:
* ctx_in is provided, but ctx_size_in is zero (same for
  ctx_out/ctx_size_out)
* unexpected non-zero fields in __sk_buff return EINVAL
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/prog_tests/skb_ctx.c

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+
+void test_skb_ctx(void)
+{
+	struct __sk_buff skb = {
+		.cb[0] = 1,
+		.cb[1] = 2,
+		.cb[2] = 3,
+		.cb[3] = 4,
+		.cb[4] = 5,
+		.priority = 6,
+	};
+	struct bpf_prog_test_run_attr tattr = {
+		.data_in = &pkt_v4,
+		.data_size_in = sizeof(pkt_v4),
+		.ctx_in = &skb,
+		.ctx_size_in = sizeof(skb),
+		.ctx_out = &skb,
+		.ctx_size_out = sizeof(skb),
+	};
+	struct bpf_object *obj;
+	int err;
+	int i;
+
+	err = bpf_prog_load("./test_skb_ctx.o", BPF_PROG_TYPE_SCHED_CLS, &obj,
+			    &tattr.prog_fd);
+	if (CHECK_ATTR(err, "load", "err %d errno %d\n", err, errno))
+		return;
+
+	/* ctx_in != NULL, ctx_size_in == 0 */
+
+	tattr.ctx_size_in = 0;
+	err = bpf_prog_test_run_xattr(&tattr);
+	CHECK_ATTR(err == 0, "ctx_size_in", "err %d errno %d\n", err, errno);
+	tattr.ctx_size_in = sizeof(skb);
+
+	/* ctx_out != NULL, ctx_size_out == 0 */
+
+	tattr.ctx_size_out = 0;
+	err = bpf_prog_test_run_xattr(&tattr);
+	CHECK_ATTR(err == 0, "ctx_size_out", "err %d errno %d\n", err, errno);
+	tattr.ctx_size_out = sizeof(skb);
+
+	/* non-zero [len, tc_index] fields should be rejected*/
+
+	skb.len = 1;
+	err = bpf_prog_test_run_xattr(&tattr);
+	CHECK_ATTR(err == 0, "len", "err %d errno %d\n", err, errno);
+	skb.len = 0;
+
+	skb.tc_index = 1;
+	err = bpf_prog_test_run_xattr(&tattr);
+	CHECK_ATTR(err == 0, "tc_index", "err %d errno %d\n", err, errno);
+	skb.tc_index = 0;
+
+	/* non-zero [hash, sk] fields should be rejected */
+
+	skb.hash = 1;
+	err = bpf_prog_test_run_xattr(&tattr);
+	CHECK_ATTR(err == 0, "hash", "err %d errno %d\n", err, errno);
+	skb.hash = 0;
+
+	skb.sk = (struct bpf_sock *)1;
+	err = bpf_prog_test_run_xattr(&tattr);
+	CHECK_ATTR(err == 0, "sk", "err %d errno %d\n", err, errno);
+	skb.sk = 0;
+
+	err = bpf_prog_test_run_xattr(&tattr);
+	CHECK_ATTR(err != 0 || tattr.retval,
+		   "run",
+		   "err %d errno %d retval %d\n",
+		   err, errno, tattr.retval);
+
+	CHECK_ATTR(tattr.ctx_size_out != sizeof(skb),
+		   "ctx_size_out",
+		   "incorrect output size, want %lu have %u\n",
+		   sizeof(skb), tattr.ctx_size_out);
+
+	for (i = 0; i < 5; i++)
+		CHECK_ATTR(skb.cb[i] != i + 2,
+			   "ctx_out_cb",
+			   "skb->cb[i] == %d, expected %d\n",
+			   skb.cb[i], i + 2);
+	CHECK_ATTR(skb.priority != 7,
+		   "ctx_out_priority",
+		   "skb->priority == %d, expected %d\n",
+		   skb.priority, 7);
+}

tools/testing/selftests/bpf/progs/test_skb_ctx.c

+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include "bpf_helpers.h"
+
+int _version SEC("version") = 1;
+char _license[] SEC("license") = "GPL";
+
+SEC("skb_ctx")
+int process(struct __sk_buff *skb)
+{
+	#pragma clang loop unroll(full)
+	for (int i = 0; i < 5; i++) {
+		if (skb->cb[i] != i + 1)
+			return 1;
+		skb->cb[i]++;
+	}
+	skb->priority++;
+
+	return 0;
+}