fscrypt: make it clearer that key_prefix is deprecated

fscrypt_operations::key_prefix should not be set by any filesystems that
aren't setting it already.  This is already documented, but apparently
it's not sufficiently clear, as both ceph and btrfs have tried to set
it.  Rename the field to legacy_key_prefix and improve the documentation
to hopefully make it clearer.

Link: https://lore.kernel.org/r/20230925055451.59499-2-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

5 files changed, 15 insertions, 10 deletions

diff --git a/fs/crypto/keysetup_v1.c b/fs/crypto/keysetup_v1.c
index 75dabd9b27f9..86b48a2b47d1 100644
--- a/fs/crypto/keysetup_v1.c
+++ b/fs/crypto/keysetup_v1.c
@@ -299,6 +299,7 @@ int fscrypt_setup_v1_file_key(struct fscrypt_info *ci, const u8 *raw_master_key)
 
 int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci)
 {
+	const struct super_block *sb = ci->ci_inode->i_sb;
 	struct key *key;
 	const struct fscrypt_key *payload;
 	int err;
@@ -306,8 +307,8 @@ int fscrypt_setup_v1_file_key_via_subscribed_keyrings(struct fscrypt_info *ci)
 	key = find_and_lock_process_key(FSCRYPT_KEY_DESC_PREFIX,
 					ci->ci_policy.v1.master_key_descriptor,
 					ci->ci_mode->keysize, &payload);
-	if (key == ERR_PTR(-ENOKEY) && ci->ci_inode->i_sb->s_cop->key_prefix) {
-		key = find_and_lock_process_key(ci->ci_inode->i_sb->s_cop->key_prefix,
+	if (key == ERR_PTR(-ENOKEY) && sb->s_cop->legacy_key_prefix) {
+		key = find_and_lock_process_key(sb->s_cop->legacy_key_prefix,
 						ci->ci_policy.v1.master_key_descriptor,
 						ci->ci_mode->keysize, &payload);
 	}
diff --git a/fs/ext4/crypto.c b/fs/ext4/crypto.c
index 453d4da5de52..99a4769a53f6 100644
--- a/fs/ext4/crypto.c
+++ b/fs/ext4/crypto.c
@@ -240,7 +240,7 @@ static void ext4_get_ino_and_lblk_bits(struct super_block *sb,
 }
 
 const struct fscrypt_operations ext4_cryptops = {
-	.key_prefix		= "ext4:",
+	.legacy_key_prefix	= "ext4:",
 	.get_context		= ext4_get_context,
 	.set_context		= ext4_set_context,
 	.get_dummy_policy	= ext4_get_dummy_policy,
diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index a8c8232852bb..f60062b558fd 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -3231,7 +3231,7 @@ static struct block_device **f2fs_get_devices(struct super_block *sb,
 }
 
 static const struct fscrypt_operations f2fs_cryptops = {
-	.key_prefix		= "f2fs:",
+	.legacy_key_prefix	= "f2fs:",
 	.get_context		= f2fs_get_context,
 	.set_context		= f2fs_set_context,
 	.get_dummy_policy	= f2fs_get_dummy_policy,
diff --git a/fs/ubifs/crypto.c b/fs/ubifs/crypto.c
index 3125e76376ee..1be3e11da3b3 100644
--- a/fs/ubifs/crypto.c
+++ b/fs/ubifs/crypto.c
@@ -89,7 +89,7 @@ int ubifs_decrypt(const struct inode *inode, struct ubifs_data_node *dn,
 
 const struct fscrypt_operations ubifs_crypt_operations = {
 	.flags			= FS_CFLG_OWN_PAGES,
-	.key_prefix		= "ubifs:",
+	.legacy_key_prefix	= "ubifs:",
 	.get_context		= ubifs_crypt_get_context,
 	.set_context		= ubifs_crypt_set_context,
 	.empty_dir		= ubifs_crypt_empty_dir,
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index c895b12737a1..b0037566ce30 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -73,12 +73,16 @@ struct fscrypt_operations {
 	unsigned int flags;
 
 	/*
-	 * If set, this is a filesystem-specific key description prefix that
-	 * will be accepted for "logon" keys for v1 fscrypt policies, in
-	 * addition to the generic prefix "fscrypt:".  This functionality is
-	 * deprecated, so new filesystems shouldn't set this field.
+	 * This field exists only for backwards compatibility reasons and should
+	 * only be set by the filesystems that are setting it already.  It
+	 * contains the filesystem-specific key description prefix that is
+	 * accepted for "logon" keys for v1 fscrypt policies.  This
+	 * functionality is deprecated in favor of the generic prefix
+	 * "fscrypt:", which itself is deprecated in favor of the filesystem
+	 * keyring ioctls such as FS_IOC_ADD_ENCRYPTION_KEY.  Filesystems that
+	 * are newly adding fscrypt support should not set this field.
 	 */
-	const char *key_prefix;
+	const char *legacy_key_prefix;
 
 	/*
 	 * Get the fscrypt context of the given inode.