Bump efivarfs_get_variable throughput to 50 variables/second from 33⅓

Read the whole variable at once, then copy out the attributes
(for a description of efivarfs rate limiting, see rhboot#258)

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>

Author: nabijaczleweli

src/efivarfs.c

@@ -241,7 +241,6 @@ efivarfs_get_variable(efi_guid_t guid, const char *name, uint8_t **data,
 	__typeof__(errno) errno_value;
 	int ret = -1;
 	size_t size = 0;
-	uint32_t ret_attributes = 0;
 	uint8_t *ret_data;
 	int fd = -1;
 	char *path = NULL;
@@ -259,21 +258,23 @@ efivarfs_get_variable(efi_guid_t guid, const char *name, uint8_t **data,
 		goto err;
 	}
 
-	rc = read(fd, &ret_attributes, sizeof (ret_attributes));
+	rc = read_file(fd, &ret_data, &size);
 	if (rc < 0) {
-		efi_error("read failed");
+		efi_error("read_file failed");
 		goto err;
 	}
+	--size; // read_file pads out 1 extra byte to NUL
 
-	rc = read_file(fd, &ret_data, &size);
-	if (rc < 0) {
-		efi_error("read_file failed");
+	if (size < sizeof (*attributes)) {
+		efi_error("no attributes");
 		goto err;
 	}
 
-	*attributes = ret_attributes;
+	memcpy(attributes, ret_data, sizeof (*attributes));
+	memmove(ret_data, ret_data + sizeof (*attributes), size - sizeof (*attributes));
+
 	*data = ret_data;
-	*data_size = size - 1; // read_file pads out 1 extra byte to NUL it */
+	*data_size = size - sizeof (*attributes);
 
 	ret = 0;
 err: