cramfs: implement uncompressed and arbitrary data block positioning

Two new capabilities are introduced here:

- The ability to store some blocks uncompressed.

- The ability to locate blocks anywhere.

Those capabilities can be used independently, but the combination
opens the possibility for execute-in-place (XIP) of program text segments
that must remain uncompressed, and in the MMU case, must have a specific
alignment.  It is even possible to still have the writable data segments
from the same file compressed as they have to be copied into RAM anyway.

This is achieved by giving special meanings to some unused block pointer
bits while remaining compatible with legacy cramfs images.

Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Chris Brandt <chris.brandt@renesas.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 71 insertions, 19 deletions

diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
index bcdccb7a820b..19045453a8f3 100644
--- a/fs/cramfs/inode.c
+++ b/fs/cramfs/inode.c
@@ -618,34 +618,86 @@ static int cramfs_readpage(struct file *file, struct page *page)
 
 	if (page->index < maxblock) {
 		struct super_block *sb = inode->i_sb;
-		u32 blkptr_offset = OFFSET(inode) + page->index*4;
-		u32 start_offset, compr_len;
+		u32 blkptr_offset = OFFSET(inode) + page->index * 4;
+		u32 block_ptr, block_start, block_len;
+		bool uncompressed, direct;
 
-		start_offset = OFFSET(inode) + maxblock*4;
 		mutex_lock(&read_mutex);
-		if (page->index)
-			start_offset = *(u32 *) cramfs_read(sb, blkptr_offset-4,
-				4);
-		compr_len = (*(u32 *) cramfs_read(sb, blkptr_offset, 4) -
-			start_offset);
-		mutex_unlock(&read_mutex);
+		block_ptr = *(u32 *) cramfs_read(sb, blkptr_offset, 4);
+		uncompressed = (block_ptr & CRAMFS_BLK_FLAG_UNCOMPRESSED);
+		direct = (block_ptr & CRAMFS_BLK_FLAG_DIRECT_PTR);
+		block_ptr &= ~CRAMFS_BLK_FLAGS;
+
+		if (direct) {
+			/*
+			 * The block pointer is an absolute start pointer,
+			 * shifted by 2 bits. The size is included in the
+			 * first 2 bytes of the data block when compressed,
+			 * or PAGE_SIZE otherwise.
+			 */
+			block_start = block_ptr << CRAMFS_BLK_DIRECT_PTR_SHIFT;
+			if (uncompressed) {
+				block_len = PAGE_SIZE;
+				/* if last block: cap to file length */
+				if (page->index == maxblock - 1)
+					block_len =
+						offset_in_page(inode->i_size);
+			} else {
+				block_len = *(u16 *)
+					cramfs_read(sb, block_start, 2);
+				block_start += 2;
+			}
+		} else {
+			/*
+			 * The block pointer indicates one past the end of
+			 * the current block (start of next block). If this
+			 * is the first block then it starts where the block
+			 * pointer table ends, otherwise its start comes
+			 * from the previous block's pointer.
+			 */
+			block_start = OFFSET(inode) + maxblock * 4;
+			if (page->index)
+				block_start = *(u32 *)
+					cramfs_read(sb, blkptr_offset - 4, 4);
+			/* Beware... previous ptr might be a direct ptr */
+			if (unlikely(block_start & CRAMFS_BLK_FLAG_DIRECT_PTR)) {
+				/* See comments on earlier code. */
+				u32 prev_start = block_start;
+			       block_start = prev_start & ~CRAMFS_BLK_FLAGS;
+			       block_start <<= CRAMFS_BLK_DIRECT_PTR_SHIFT;
+				if (prev_start & CRAMFS_BLK_FLAG_UNCOMPRESSED) {
+					block_start += PAGE_SIZE;
+				} else {
+					block_len = *(u16 *)
+						cramfs_read(sb, block_start, 2);
+					block_start += 2 + block_len;
+				}
+			}
+			block_start &= ~CRAMFS_BLK_FLAGS;
+			block_len = block_ptr - block_start;
+		}
 
-		if (compr_len == 0)
+		if (block_len == 0)
 			; /* hole */
-		else if (unlikely(compr_len > (PAGE_SIZE << 1))) {
-			pr_err("bad compressed blocksize %u\n",
-				compr_len);
+		else if (unlikely(block_len > 2*PAGE_SIZE ||
+				  (uncompressed && block_len > PAGE_SIZE))) {
+			mutex_unlock(&read_mutex);
+			pr_err("bad data blocksize %u\n", block_len);
 			goto err;
+		} else if (uncompressed) {
+			memcpy(pgdata,
+			       cramfs_read(sb, block_start, block_len),
+			       block_len);
+			bytes_filled = block_len;
 		} else {
-			mutex_lock(&read_mutex);
 			bytes_filled = cramfs_uncompress_block(pgdata,
 				 PAGE_SIZE,
-				 cramfs_read(sb, start_offset, compr_len),
-				 compr_len);
-			mutex_unlock(&read_mutex);
-			if (unlikely(bytes_filled < 0))
-				goto err;
+				 cramfs_read(sb, block_start, block_len),
+				 block_len);
 		}
+		mutex_unlock(&read_mutex);
+		if (unlikely(bytes_filled < 0))
+			goto err;
 	}
 
 	memset(pgdata + bytes_filled, 0, PAGE_SIZE - bytes_filled);