btrfs: store a fscrypt extent context per normal file extent

In order to encrypt data, each file extent must have its own persistent
fscrypt_extent_context, which is then provided to fscrypt upon request.
This is only needed for encrypted extents and is of variable size on
disk, so file extents must additionally keep track of their actual
length.

This puts the long-preserved 1-byte encryption field to work. Right now we
don't anticipate very many encryption policies, so 2 bits should be
ample; similarly right now we can't imagine a extent context larger than
fscrypt's current inode contexts, which are 40 bytes, so 6 bits is ample
to store the extent context's size; and therefore we can pack these
together into the one-byte encryption field without touching other space
reserved for future use.

Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>

fs/btrfs/accessors.h

@@ -963,6 +963,16 @@ BTRFS_SETGET_STACK_FUNCS(stack_file_extent_disk_num_bytes,
 			 struct btrfs_file_extent_item, disk_num_bytes, 64);
 BTRFS_SETGET_STACK_FUNCS(stack_file_extent_compression,
 			 struct btrfs_file_extent_item, compression, 8);
+BTRFS_SETGET_STACK_FUNCS(stack_file_extent_encryption,
+			 struct btrfs_file_extent_item, encryption, 8);
+static inline u8 btrfs_stack_file_extent_encryption_ctxsize(
+	struct btrfs_file_extent_item *e)
+{
+	u8 ctxsize;
+
+	btrfs_unpack_encryption(e->encryption, NULL, &ctxsize);
+	return ctxsize;
+}
 
 static inline unsigned long
 btrfs_file_extent_inline_start(const struct btrfs_file_extent_item *e)
@@ -995,6 +1005,25 @@ BTRFS_SETGET_FUNCS(file_extent_encryption, struct btrfs_file_extent_item,
 BTRFS_SETGET_FUNCS(file_extent_other_encoding, struct btrfs_file_extent_item,
 		   other_encoding, 16);
 
+static inline u8
+btrfs_file_extent_encryption_ctxsize(const struct extent_buffer *eb,
+				     struct btrfs_file_extent_item *e)
+{
+	u8 ctxsize;
+
+	btrfs_unpack_encryption(btrfs_file_extent_encryption(eb, e),
+				NULL, &ctxsize);
+	return ctxsize;
+}
+
+static inline u8
+btrfs_file_extent_ctxsize_from_item(const struct extent_buffer *leaf,
+				    const struct btrfs_path *path)
+{
+	return (btrfs_item_size(leaf, path->slots[0]) -
+		sizeof(struct btrfs_file_extent_item));
+}
+
 /*
  * this returns the number of bytes used by the item on disk, minus the
  * size of any extent headers.  If a file is compressed on disk, this is

fs/btrfs/ctree.h

@@ -37,6 +37,7 @@
 #include "block-rsv.h"
 #include "locking.h"
 #include "misc.h"
+#include "fscrypt.h"
 
 struct btrfs_trans_handle;
 struct btrfs_transaction;
@@ -1088,6 +1089,8 @@ struct btrfs_replace_extent_info {
 	u64 file_offset;
 	/* Pointer to a file extent item of type regular or prealloc. */
 	char *extent_buf;
+	/* The length of @extent_buf */
+	u32 extent_buf_size;
 	/*
 	 * Set to true when attempting to replace a file range with a new extent
 	 * described by this structure, set to false when attempting to clone an

fs/btrfs/extent_map.c

@@ -216,6 +216,13 @@ static int mergable_maps(struct extent_map *prev, struct extent_map *next)
 	if (!list_empty(&prev->list) || !list_empty(&next->list))
 		return 0;
 
+	/*
+	 * Don't merge adjacent maps with different fscrypt_contexts.
+	 */
+	if (!memcmp(&prev->fscrypt_context, &next->fscrypt_context,
+		    sizeof(next->fscrypt_context)))
+		return 0;
+
 	ASSERT(next->block_start != EXTENT_MAP_DELALLOC &&
 	       prev->block_start != EXTENT_MAP_DELALLOC);
 

fs/btrfs/extent_map.h

@@ -5,6 +5,7 @@
 
 #include <linux/rbtree.h>
 #include <linux/refcount.h>
+#include "fscrypt.h"
 
 #define EXTENT_MAP_LAST_BYTE ((u64)-4)
 #define EXTENT_MAP_HOLE ((u64)-3)
@@ -27,6 +28,8 @@ enum {
 	EXTENT_FLAG_FS_MAPPING,
 	/* This em is merged from two or more physically adjacent ems */
 	EXTENT_FLAG_MERGED,
+	/* This em has a fscrypt extent context */
+	EXTENT_FLAG_ENCRYPTED,
 };
 
 struct extent_map {
@@ -50,6 +53,7 @@ struct extent_map {
 	 */
 	u64 generation;
 	unsigned long flags;
+	struct btrfs_fscrypt_extent_context fscrypt_context;
 	/* Used for chunk mappings, flag EXTENT_FLAG_FS_MAPPING must be set */
 	struct map_lookup *map_lookup;
 	refcount_t refs;

fs/btrfs/file-item.c

@@ -1232,6 +1232,7 @@ void btrfs_extent_item_to_extent_map(struct btrfs_inode *inode,
 	em->generation = btrfs_file_extent_generation(leaf, fi);
 	if (type == BTRFS_FILE_EXTENT_REG ||
 	    type == BTRFS_FILE_EXTENT_PREALLOC) {
+		u8 ctxsize;
 		em->start = extent_start;
 		em->len = extent_end - extent_start;
 		em->orig_start = extent_start -
@@ -1247,13 +1248,28 @@ void btrfs_extent_item_to_extent_map(struct btrfs_inode *inode,
 			em->compress_type = compress_type;
 			em->block_start = bytenr;
 			em->block_len = em->orig_block_len;
+		} else if (btrfs_file_extent_encryption(leaf, fi)) {
+			set_bit(EXTENT_FLAG_ENCRYPTED, &em->flags);
+			em->block_start = bytenr;
+			em->block_len = em->orig_block_len;
 		} else {
 			bytenr += btrfs_file_extent_offset(leaf, fi);
 			em->block_start = bytenr;
 			em->block_len = em->len;
 			if (type == BTRFS_FILE_EXTENT_PREALLOC)
 				set_bit(EXTENT_FLAG_PREALLOC, &em->flags);
 		}
+
+		ctxsize = btrfs_file_extent_ctxsize_from_item(leaf, path);
+		ASSERT(ctxsize == btrfs_file_extent_encryption_ctxsize(leaf, fi));
+
+#ifdef CONFIG_FS_ENCRYPTION
+		em->fscrypt_context.len = ctxsize;
+
+		read_extent_buffer(leaf, em->fscrypt_context.buffer,
+				   (unsigned long)fi->fscrypt_context,
+				   ctxsize);
+#endif /* CONFIG_FS_ENCRYPTION */
 	} else if (type == BTRFS_FILE_EXTENT_INLINE) {
 		em->block_start = EXTENT_MAP_INLINE;
 		em->start = extent_start;

fs/btrfs/file.c

@@ -2554,14 +2554,14 @@ static int btrfs_insert_replace_extent(struct btrfs_trans_handle *trans,
 	key.type = BTRFS_EXTENT_DATA_KEY;
 	key.offset = extent_info->file_offset;
 	ret = btrfs_insert_empty_item(trans, root, path, &key,
-				      sizeof(struct btrfs_file_extent_item));
+				      extent_info->extent_buf_size);
 	if (ret)
 		return ret;
 	leaf = path->nodes[0];
 	slot = path->slots[0];
 	write_extent_buffer(leaf, extent_info->extent_buf,
 			    btrfs_item_ptr_offset(leaf, slot),
-			    sizeof(struct btrfs_file_extent_item));
+			    extent_info->extent_buf_size);
 	extent = btrfs_item_ptr(leaf, slot, struct btrfs_file_extent_item);
 	ASSERT(btrfs_file_extent_type(leaf, extent) != BTRFS_FILE_EXTENT_INLINE);
 	btrfs_set_file_extent_offset(leaf, extent, extent_info->data_offset);

fs/btrfs/fscrypt.c

@@ -10,6 +10,7 @@
 #include "messages.h"
 #include "transaction.h"
 #include "xattr.h"
+#include "fscrypt.h"
 
 static int btrfs_fscrypt_get_context(struct inode *inode, void *ctx, size_t len)
 {
@@ -181,9 +182,30 @@ static bool btrfs_fscrypt_empty_dir(struct inode *inode)
 	return inode->i_size == BTRFS_EMPTY_DIR_SIZE;
 }
 
+static int btrfs_fscrypt_get_extent_context(const struct inode *inode,
+					    u64 lblk_num, void *ctx,
+					    size_t len,
+					    size_t *extent_offset,
+					    size_t *extent_length)
+{
+	return len;
+}
+
+static int btrfs_fscrypt_set_extent_context(void *extent, void *ctx,
+					    size_t len)
+{
+	struct btrfs_fscrypt_extent_context *extent_context = extent;
+
+	memcpy(extent_context->buffer, ctx, len);
+	extent_context->len = len;
+	return 0;
+}
+
 const struct fscrypt_operations btrfs_fscrypt_ops = {
 	.key_prefix = "btrfs:",
 	.get_context = btrfs_fscrypt_get_context,
 	.set_context = btrfs_fscrypt_set_context,
 	.empty_dir = btrfs_fscrypt_empty_dir,
+	.get_extent_context = btrfs_fscrypt_get_extent_context,
+	.set_extent_context = btrfs_fscrypt_set_extent_context,
 };

fs/btrfs/fscrypt.h

@@ -5,6 +5,41 @@
 
 #include <linux/fscrypt.h>
 
+#define BTRFS_ENCRYPTION_POLICY_BITS 2
+#define BTRFS_ENCRYPTION_CTXSIZE_BITS 6
+
+#define BTRFS_ENCRYPTION_POLICY_MASK ((1 << BTRFS_ENCRYPTION_POLICY_BITS) - 1)
+#define BTRFS_ENCRYPTION_CTXSIZE_MASK \
+	(((1 << BTRFS_ENCRYPTION_CTXSIZE_BITS) - 1) << \
+		BTRFS_ENCRYPTION_POLICY_BITS)
+
+#ifdef CONFIG_FS_ENCRYPTION
+struct btrfs_fscrypt_extent_context {
+	u8 buffer[FSCRYPT_EXTENT_CONTEXT_MAX_SIZE];
+	u8 len;
+};
+#else
+struct btrfs_fscrypt_extent_context {
+	u8 len;
+};
+#endif
+
+static inline void btrfs_unpack_encryption(u8 encryption,
+					   u8 *policy,
+					   u8 *ctxsize)
+{
+	if (policy)
+		*policy = encryption & BTRFS_ENCRYPTION_POLICY_MASK;
+	if (ctxsize)
+		*ctxsize = ((encryption & BTRFS_ENCRYPTION_CTXSIZE_MASK) >>
+			    BTRFS_ENCRYPTION_POLICY_BITS);
+}
+
+static inline u8 btrfs_pack_encryption(u8 policy, u8 ctxsize)
+{
+	return policy | (ctxsize << BTRFS_ENCRYPTION_POLICY_BITS);
+}
+
 extern const struct fscrypt_operations btrfs_fscrypt_ops;
 
 #endif /* BTRFS_FSCRYPT_H */