From 7bda7c75748f36b0a50f93e46144d5a4de4974ad Mon Sep 17 00:00:00 2001
From: Amin Hassani <ahassani@google.com>
Date: Thu, 15 Dec 2016 10:43:15 -0800
Subject: [PATCH] mksquashfs 4K aligns the files inside the squashfs image

Files inside a squashfs image are not necessarily 4k (4096)
aligned. This patch starts each file in a 4k aligned address and pads
zero to the end of the file until it reaches the next 4k aligned
address. This will not change the size of the compressed
blocks (especially the last one) and hence it will not change how the
files are being loaded in kernel or unsquashfs. However on average this
increases the size of the squashfs image which can be calculated by the
following formula:

increased_size = (number_of_unfragmented_files_in_image + number of fragments) * 2048

The 4k alignment can be enabled by flag '-4k-align'
---
diff -u a/squashfs-tools/mksquashfs.c b/squashfs-tools/mksquashfs.c
--- a/squashfs-tools/mksquashfs.c	2019-07-06 15:50:22.214873176 +0000
+++ b/squashfs-tools/mksquashfs.c	2019-07-06 15:51:22.244802582 +0000
@@ -100,7 +100,9 @@
 int use_regex = FALSE;
 int nopad = FALSE;
 int exit_on_error = FALSE;
+int do_4k_align = FALSE;
 static off_t squashfs_start_offset = 0;
+#define ALIGN_UP(bytes, size) (bytes = (bytes + size - 1) & ~(size - 1))
 
 long long global_uid = -1, global_gid = -1;
 
@@ -1495,6 +1497,9 @@
 	 * queue at this time.
 	 */
 	while(!queue_empty(locked_fragment)) {
+		// 4k align the start of remaining queued fragments.
+		if(do_4k_align)
+			ALIGN_UP(bytes, 4096);
 		write_buffer = queue_get(locked_fragment);
 		frg = write_buffer->block;	
 		size = SQUASHFS_COMPRESSED_SIZE_BLOCK(fragment_table[frg].size);
@@ -2414,6 +2419,9 @@
 	compressed_size = SQUASHFS_COMPRESSED_SIZE_BLOCK(c_byte);
 	write_buffer->size = compressed_size;
 	if(fragments_locked == FALSE) {
+		// 4k align the start of each fragment.
+		if(do_4k_align)
+			ALIGN_UP(bytes, 4096);
 		fragment_table[file_buffer->block].size = c_byte;
 		fragment_table[file_buffer->block].start_block = bytes;
 		write_buffer->block = bytes;
@@ -2728,6 +2736,10 @@
 	long long sparse = 0;
 	struct file_buffer *fragment_buffer = NULL;
 
+	// 4k align the start of each file.
+	if(do_4k_align)
+		ALIGN_UP(bytes, 4096);
+
 	if(pre_duplicate(read_size))
 		return write_file_blocks_dup(inode, dir_ent, read_buffer, dup);
 
@@ -4808,6 +4820,7 @@
 		"compressed", no_fragments ? "no" : noF ? "uncompressed" :
 		"compressed", no_xattrs ? "no" : noX ? "uncompressed" :
 		"compressed", noI || noId ? "uncompressed" : "compressed");
+	printf("\t4k %saligned\n", do_4k_align ? "" : "un");
 	printf("\tduplicates are %sremoved\n", duplicate_checking ? "" :
 		"not ");
 	printf("Filesystem size %.2f Kbytes (%.2f Mbytes)\n", bytes / 1024.0,
@@ -5570,6 +5583,8 @@
 			root_name = argv[i];
 		} else if(strcmp(argv[i], "-version") == 0) {
 			VERSION();
+		} else if(strcmp(argv[i], "-4k-align") == 0) {
+			do_4k_align = TRUE;
 		} else {
 			ERROR("%s: invalid option\n\n", argv[0]);
 printOptions:
@@ -5613,6 +5628,7 @@
 			ERROR("\t\t\tdirectory containing that directory, "
 				"rather than the\n");
 			ERROR("\t\t\tcontents of the directory\n");
+			ERROR("-4k-align\t\tenables 4k alignment of all files\n");
 			ERROR("\nFilesystem filter options:\n");
 			ERROR("-p <pseudo-definition>\tAdd pseudo file "
 				"definition\n");