um: chan_user: retry partial writes

In the next commit, we are going to set the output FD to be blocking.
Once that is done, the write() may be short if an interrupt happens
while it is writing out data. As such, to properly catch an EINTR error,
we need to retry the write.
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20231018123643.1255813-3-benjamin@sipsolutions.netSigned-off-by: Johannes Berg <johannes.berg@intel.com>

arch/um/drivers/chan_user.c

@@ -37,11 +37,23 @@ int generic_read(int fd, __u8 *c_out, void *unused)
 
 int generic_write(int fd, const __u8 *buf, size_t n, void *unused)
 {
+	int written = 0;
 	int err;
 
-	CATCH_EINTR(err = write(fd, buf, n));
-	if (err > 0)
-		return err;
+	/* The FD may be in blocking mode, as such, need to retry short writes,
+	 * they may have been interrupted by a signal.
+	 */
+	do {
+		errno = 0;
+		err = write(fd, buf + written, n - written);
+		if (err > 0) {
+			written += err;
+			continue;
+		}
+	} while (err < 0 && errno == EINTR);
+
+	if (written > 0)
+		return written;
 	else if (errno == EAGAIN)
 		return 0;
 	else if (err == 0)