[PATCH bpf v1] selftests/bpf: Fix sockmap_multi_channels reliability

[Jiayuan Chen <jiayuan.chen@linux.dev>]

From: Jiayuan Chen <jiayuan.chen@shopee.com>

Previously I added a FIONREAD test for sockmap, but it can occasionally
fail in CI [1].

The test sends 10 bytes in two segments (2 + 8). For UDP, FIONREAD only
reports the length of the first datagram, not the total queued data.
The original code used recv_timeout() expecting all 10 bytes, but under
high system load, the second datagram may not yet be processed by the
protocol stack, so recv would only return the first 2-byte datagram,
causing a size mismatch failure.

Fix this by receiving exactly the expected bytes (matching FIONREAD) in
the first recv. The remaining datagram is then consumed in a second recv
block, which is only reachable for UDP since TCP's expected already
equals sizeof(buf).

Test:
./test_progs -a sockmap_basic
410/1   sockmap_basic/sockmap create_update_free:OK
...
Summary: 1/35 PASSED, 0 SKIPPED, 0 FAILED

[1] https://github.com/kernel-patches/bpf/actions/runs/22919385910/job/66515395423

Cc: Jiayuan Chen <jiayuan.chen@linux.dev>
Fixes: 17e2ce02bf56 ("selftests/bpf: Add tests for FIONREAD and copied_seq")
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
---
 .../selftests/bpf/prog_tests/sockmap_basic.c    | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
index dd3c757859f6b..d2846579285f2 100644
--- a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
+++ b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
@@ -1298,10 +1298,23 @@ static void test_sockmap_multi_channels(int sotype)
 	avail = wait_for_fionread(p1, expected, IO_TIMEOUT_SEC);
 	ASSERT_EQ(avail, expected, "ioctl(FIONREAD) full return");
 
-	recvd = recv_timeout(p1, rcv, sizeof(rcv), MSG_DONTWAIT, 1);
-	if (!ASSERT_EQ(recvd, sizeof(buf), "recv_timeout(p1)") ||
+	recvd = recv_timeout(p1, rcv, expected, MSG_DONTWAIT, 1);
+	if (!ASSERT_EQ(recvd, expected, "recv_timeout(p1)") ||
 	    !ASSERT_OK(memcmp(buf, rcv, recvd), "data mismatch"))
 		goto end;
+
+	/* process remaining data for udp if secondary data is available */
+	expected = sizeof(buf) - expected;
+	if (expected) {
+		avail = wait_for_fionread(p1, expected, IO_TIMEOUT_SEC);
+		ASSERT_EQ(avail, expected, "second ioctl(FIONREAD) full return");
+
+		recvd = recv_timeout(p1, rcv, expected, MSG_DONTWAIT, 1);
+		if (!ASSERT_EQ(recvd, expected, "second recv_timeout(p1)") ||
+		    !ASSERT_OK(memcmp(buf + sizeof(buf) - expected, rcv, recvd),
+			       "second data mismatch"))
+			goto end;
+	}
 end:
 	if (c0 >= 0)
 		close(c0);
-- 
2.43.0