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

[Sun Jian <sun.jian.kdev@gmail.com>]

Add a test case for BPF_PROG_TEST_RUN with a non-linear skb and a short
data_out buffer.

The test verifies that test_run returns -ENOSPC, reports the full packet
length through data_size_out, and copies the packet prefix into data_out.
The test uses a 100-byte data_out buffer with a 64-byte linear head, so the
expected output spans both the skb head and the first fragment.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 .../selftests/bpf/prog_tests/skb_load_bytes.c | 35 +++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/skb_load_bytes.c b/tools/testing/selftests/bpf/prog_tests/skb_load_bytes.c
index d7f83c0a40a5..134be0ea8ed7 100644
--- a/tools/testing/selftests/bpf/prog_tests/skb_load_bytes.c
+++ b/tools/testing/selftests/bpf/prog_tests/skb_load_bytes.c
@@ -3,6 +3,39 @@
 #include <network_helpers.h>
 #include "skb_load_bytes.skel.h"
 
+#define NONLINEAR_PKT_LEN 9000
+#define NONLINEAR_HEAD_LEN 64
+#define SHORT_OUT_LEN 100
+
+static void test_nonlinear_data_out_partial(int prog_fd)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, tattr);
+	__u8 pkt[NONLINEAR_PKT_LEN];
+	__u8 out[SHORT_OUT_LEN];
+	struct __sk_buff skb = {};
+	int err, i;
+
+	for (i = 0; i < sizeof(pkt); i++)
+		pkt[i] = i & 0xff;
+
+	memset(out, 0xa5, sizeof(out));
+
+	skb.data_end = NONLINEAR_HEAD_LEN;
+
+	tattr.data_in = pkt;
+	tattr.data_size_in = sizeof(pkt);
+	tattr.data_out = out;
+	tattr.data_size_out = sizeof(out);
+	tattr.ctx_in = &skb;
+	tattr.ctx_size_in = sizeof(skb);
+
+	err = bpf_prog_test_run_opts(prog_fd, &tattr);
+
+	ASSERT_EQ(err, -ENOSPC, "nonlinear_partial_err");
+	ASSERT_EQ(tattr.data_size_out, sizeof(pkt), "nonlinear_partial_data_size_out");
+	ASSERT_OK(memcmp(out, pkt, sizeof(out)), "nonlinear_partial_data_out");
+}
+
 void test_skb_load_bytes(void)
 {
 	struct skb_load_bytes *skel;
@@ -40,6 +73,8 @@ void test_skb_load_bytes(void)
 	if (!ASSERT_EQ(test_result, 0, "offset 10"))
 		goto out;
 
+	test_nonlinear_data_out_partial(prog_fd);
+
 out:
 	skb_load_bytes__destroy(skel);
 }
-- 
2.43.0