selftests/bpf: Fix sockmap_multi_channels reliability

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>
Link: https://lore.kernel.org/r/20260312072549.6766-1-jiayuan.chen@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c b/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
index dd3c757859f6b288cb120229b246c023d33ec0e4..d2846579285f2e7707c864e3fe600414e203425d 100644 (file)
--- 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);