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>

1 files changed, 21 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/test_skb_ctx.c b/tools/testing/selftests/bpf/progs/test_skb_ctx.c
new file mode 100644
index 000000000000..7a80960d7df1
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_skb_ctx.c
@@ -0,0 +1,21 @@
+// 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;
+}