fscrypt: support decrypting multiple filesystem blocks per page

Rename fscrypt_decrypt_page() to fscrypt_decrypt_pagecache_blocks() and
redefine its behavior to decrypt all filesystem blocks in the given
region of the given page, rather than assuming that the region consists
of just one filesystem block.  Also remove the 'inode' and 'lblk_num'
parameters, since they can be retrieved from the page as it's already
assumed to be a pagecache page.

This is in preparation for allowing encryption on ext4 filesystems with
blocksize != PAGE_SIZE.

This is based on work by Chandan Rajendra.
Reviewed-by: Chandan Rajendra <chandan@linux.ibm.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/bio.c

@@ -33,8 +33,7 @@ static void __fscrypt_decrypt_bio(struct bio *bio, bool done)
 
 	bio_for_each_segment_all(bv, bio, iter_all) {
 		struct page *page = bv->bv_page;
-		int ret = fscrypt_decrypt_page(page->mapping->host, page,
-				PAGE_SIZE, 0, page->index);
+		int ret = fscrypt_decrypt_pagecache_blocks(page, PAGE_SIZE, 0);
 
 		if (ret)
 			SetPageError(page);

fs/crypto/crypto.c

@@ -283,29 +283,47 @@ int fscrypt_encrypt_block_inplace(const struct inode *inode, struct page *page,
 EXPORT_SYMBOL(fscrypt_encrypt_block_inplace);
 
 /**
- * fscrypt_decrypt_page() - Decrypts a page in-place
- * @inode:     The corresponding inode for the page to decrypt.
- * @page:      The page to decrypt. Must be locked.
- * @len:       Number of bytes in @page to be decrypted.
- * @offs:      Start of data in @page.
- * @lblk_num:  Logical block number.
+ * fscrypt_decrypt_pagecache_blocks() - Decrypt filesystem blocks in a pagecache page
+ * @page:      The locked pagecache page containing the block(s) to decrypt
+ * @len:       Total size of the block(s) to decrypt.  Must be a nonzero
+ *		multiple of the filesystem's block size.
+ * @offs:      Byte offset within @page of the first block to decrypt.  Must be
+ *		a multiple of the filesystem's block size.
  *
- * Decrypts page in-place using the ctx encryption context.
+ * The specified block(s) are decrypted in-place within the pagecache page,
+ * which must still be locked and not uptodate.  Normally, blocksize ==
+ * PAGE_SIZE and the whole page is decrypted at once.
  *
- * Called from the read completion callback.
+ * This is for use by the filesystem's ->readpages() method.
  *
- * Return: Zero on success, non-zero otherwise.
+ * Return: 0 on success; -errno on failure
  */
-int fscrypt_decrypt_page(const struct inode *inode, struct page *page,
-			unsigned int len, unsigned int offs, u64 lblk_num)
+int fscrypt_decrypt_pagecache_blocks(struct page *page, unsigned int len,
+				     unsigned int offs)
 {
+	const struct inode *inode = page->mapping->host;
+	const unsigned int blockbits = inode->i_blkbits;
+	const unsigned int blocksize = 1 << blockbits;
+	u64 lblk_num = ((u64)page->index << (PAGE_SHIFT - blockbits)) +
+		       (offs >> blockbits);
+	unsigned int i;
+	int err;
+
 	if (WARN_ON_ONCE(!PageLocked(page)))
 		return -EINVAL;
 
-	return fscrypt_crypt_block(inode, FS_DECRYPT, lblk_num, page, page,
-				   len, offs, GFP_NOFS);
+	if (WARN_ON_ONCE(len <= 0 || !IS_ALIGNED(len | offs, blocksize)))
+		return -EINVAL;
+
+	for (i = offs; i < offs + len; i += blocksize, lblk_num++) {
+		err = fscrypt_crypt_block(inode, FS_DECRYPT, lblk_num, page,
+					  page, blocksize, i, GFP_NOFS);
+		if (err)
+			return err;
+	}
+	return 0;
 }
-EXPORT_SYMBOL(fscrypt_decrypt_page);
+EXPORT_SYMBOL(fscrypt_decrypt_pagecache_blocks);
 
 /**
  * fscrypt_decrypt_block_inplace() - Decrypt a filesystem block in-place

fs/ext4/inode.c

@@ -1232,8 +1232,7 @@ static int ext4_block_write_begin(struct page *page, loff_t pos, unsigned len,
 	if (unlikely(err))
 		page_zero_new_buffers(page, from, to);
 	else if (decrypt)
-		err = fscrypt_decrypt_page(page->mapping->host, page,
-				PAGE_SIZE, 0, page->index);
+		err = fscrypt_decrypt_pagecache_blocks(page, PAGE_SIZE, 0);
 	return err;
 }
 #endif
@@ -4066,8 +4065,8 @@ static int __ext4_block_zero_page_range(handle_t *handle,
 			/* We expect the key to be set. */
 			BUG_ON(!fscrypt_has_encryption_key(inode));
 			BUG_ON(blocksize != PAGE_SIZE);
-			WARN_ON_ONCE(fscrypt_decrypt_page(page->mapping->host,
-						page, PAGE_SIZE, 0, page->index));
+			WARN_ON_ONCE(fscrypt_decrypt_pagecache_blocks(
+						page, PAGE_SIZE, 0));
 		}
 	}
 	if (ext4_should_journal_data(inode)) {

include/linux/fscrypt.h

@@ -112,8 +112,9 @@ extern int fscrypt_encrypt_block_inplace(const struct inode *inode,
 					 struct page *page, unsigned int len,
 					 unsigned int offs, u64 lblk_num,
 					 gfp_t gfp_flags);
-extern int fscrypt_decrypt_page(const struct inode *, struct page *, unsigned int,
-				unsigned int, u64);
+
+extern int fscrypt_decrypt_pagecache_blocks(struct page *page, unsigned int len,
+					    unsigned int offs);
 extern int fscrypt_decrypt_block_inplace(const struct inode *inode,
 					 struct page *page, unsigned int len,
 					 unsigned int offs, u64 lblk_num);
@@ -310,10 +311,9 @@ static inline int fscrypt_encrypt_block_inplace(const struct inode *inode,
 	return -EOPNOTSUPP;
 }
 
-static inline int fscrypt_decrypt_page(const struct inode *inode,
-				       struct page *page,
-				       unsigned int len, unsigned int offs,
-				       u64 lblk_num)
+static inline int fscrypt_decrypt_pagecache_blocks(struct page *page,
+						   unsigned int len,
+						   unsigned int offs)
 {
 	return -EOPNOTSUPP;
 }