vsock/test: fix MSG_PEEK handling in recv_buf()

`recv_buf` does not handle the MSG_PEEK flag correctly: it keeps calling
`recv` until all requested bytes are available or an error occurs.

The problem is how it calculates the number of bytes read: MSG_PEEK
doesn't consume any bytes and will re-read the same bytes from the buffer
head, so summing the return value every time is wrong.

Moreover, MSG_PEEK doesn't consume the bytes in the buffer, so if more
bytes are requested than are available, the loop will never terminate,
because `recv` will never return EOF. For this reason, we need to compare
the number of bytes read with the number of bytes expected.

Add a check: if the MSG_PEEK flag is present, update the byte counter and
break out of the loop only after at least the expected number of bytes
have been received; otherwise, retry after a short delay to avoid
consuming too many CPU cycles.

This allows us to simplify the `test_stream_credit_update_test` by
reusing `recv_buf`, like some other tests already do.

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260415-fix_peek-v4-2-8207e872759e@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 16 insertions, 12 deletions

diff --git a/tools/testing/vsock/util.c b/tools/testing/vsock/util.c
index 1fe1338c79cd..fe316b02a590 100644
--- a/tools/testing/vsock/util.c
+++ b/tools/testing/vsock/util.c
@@ -381,8 +381,14 @@ void send_buf(int fd, const void *buf, size_t len, int flags,
 	}
 }
 
+#define RECV_PEEK_RETRY_USEC (10 * 1000)
+
 /* Receive bytes in a buffer and check the return value.
  *
+ * When MSG_PEEK is set, recv() is retried until it returns at least
+ * expected_ret bytes. The function returns on error, EOF, or timeout
+ * as usual.
+ *
  * expected_ret:
  *  <0 Negative errno (for testing errors)
  *   0 End-of-file
@@ -403,6 +409,15 @@ void recv_buf(int fd, void *buf, size_t len, int flags, ssize_t expected_ret)
 		if (ret <= 0)
 			break;
 
+		if (flags & MSG_PEEK) {
+			if (ret >= expected_ret) {
+				nread = ret;
+				break;
+			}
+			timeout_usleep(RECV_PEEK_RETRY_USEC);
+			continue;
+		}
+
 		nread += ret;
 	} while (nread < len);
 	timeout_end();
diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
index 5bd20ccd9335..bdb0754965df 100644
--- a/tools/testing/vsock/vsock_test.c
+++ b/tools/testing/vsock/vsock_test.c
@@ -1500,18 +1500,7 @@ static void test_stream_credit_update_test(const struct test_opts *opts,
 	}
 
 	/* Wait until there will be 128KB of data in rx queue. */
-	while (1) {
-		ssize_t res;
-
-		res = recv(fd, buf, buf_size, MSG_PEEK);
-		if (res == buf_size)
-			break;
-
-		if (res <= 0) {
-			fprintf(stderr, "unexpected 'recv()' return: %zi\n", res);
-			exit(EXIT_FAILURE);
-		}
-	}
+	recv_buf(fd, buf, buf_size, MSG_PEEK, buf_size);
 
 	/* There is 128KB of data in the socket's rx queue, dequeue first
 	 * 64KB, credit update is sent if 'low_rx_bytes_test' == true.