load_ramdisk_fd: improved IO error handling

- while loop terminates on read error **without** resetting
  `seems_ok` to 0, thus read errors won't be noticed
- no check if bytes_read matches the expected image size
- ram_fd is leaked on write error
- short writes are treated as errors (when in fact they are
  perfectly fine)

Therefore:

- Handle *all* read errors, not just the very first one
- If the image size is known in advance verify if we actually
  read the whole image
- close `ram_fd` in error code paths
- Handle short writes properly

Closes: #40803

tools.c

@@ -294,15 +294,63 @@ static void save_stuff_for_rescue(void)
 }
 
 
+/**
+ * @brief copy exactly len bytes from buffer to file descriptor
+ */
+static int write_exactly(int fd, const char *buf, size_t len)
+{
+	ssize_t dl;
+	while (len > 0) {
+		dl = write(fd, buf, len);
+		if (dl < 0) {
+			log_message("%s: write: %s", __func__, strerror(errno));
+			return -1;
+		}
+		buf += (size_t)dl;
+		len -= (size_t)dl;
+	}
+	return 0;
+}
+
+/*
+ * @brief Copy size bytes from src_fd to dst_fd
+ * @param exact if non-zero check if exactly size bytes have been copied
+ * @return 0 on success, -1 on error
+ * @note if size == 0 keep copying until EOF on src_fd
+ */
+static int copy_loop(int dst_fd, int src_fd, unsigned long size)
+{
+	char buf[32768];
+	unsigned long bytes_written = 0;
+	ssize_t dl;
+	for (;;) {
+		dl = read(src_fd, buf, sizeof(buf));
+		if (dl < 0) {
+			log_message("%s: read: %s", __func__, strerror(errno));
+			return -1;
+		} else if (dl == 0) {
+			break;
+		}
+		if (write_exactly(dst_fd, buf, (size_t)dl) < 0) {
+			return -1;
+		}
+		bytes_written += (size_t)dl;
+		update_progression((int)BYTES2MB(bytes_written));
+	}
+	if (size > 0 && bytes_written != size) {
+		log_message("%s: expected %lu bytes, copied %lu instead\n",
+			__func__, size, bytes_written);
+		errno = EIO;
+		return -1;
+	}
+	return 0;
+}
+
 enum return_type load_ramdisk_fd(int source_fd, unsigned long size)
 {
 	char * ramdisk = "/dev/ram3"; /* warning, verify that this file exists in the initrd, and that root=/dev/ram3 is actually passed to the kernel at boot time */
 	int ram_fd;
-	char buffer[32768];
 	char * wait_msg = "Loading program into memory...";
-	unsigned long bytes_read = 0;
-	ssize_t actually;
-	int seems_ok = 0;
 
 	ram_fd = open(ramdisk, O_WRONLY);
 	if (ram_fd == -1) {
@@ -312,22 +360,8 @@ enum return_type load_ramdisk_fd(int source_fd, unsigned long size)
 	}
 
 	init_progression(wait_msg, (int)BYTES2MB(size));
-
-	while ((actually = read(source_fd, buffer, sizeof(buffer))) > 0) {
-		seems_ok = 1;
-		if (write(ram_fd, buffer, actually) != actually) {
-			log_perror("writing ramdisk");
-			remove_wait_message();
-			return RETURN_ERROR;
-		}
-		bytes_read += actually;
-		update_progression((int)BYTES2MB(bytes_read));
-		if (bytes_read == size)
-			break;
-	}
-
-	if (!seems_ok) {
-		log_message("reading compressed ramdisk: %s", strerror(errno));
+	if (copy_loop(ram_fd, source_fd, size) < 0) {
+		log_message("%s: failed to load image to RAM", __func__);
 		close(ram_fd);
 		remove_wait_message();
 		stg1_error_message("Could not load second stage ramdisk. "
@@ -335,7 +369,6 @@ enum return_type load_ramdisk_fd(int source_fd, unsigned long size)
 				   "(this may be caused by a hardware failure or a Linux kernel bug)");
 		return RETURN_ERROR;
 	}
-
 	end_progression();
 
 	close(ram_fd);