On Wed, Feb 20, 2019 at 7:55 AM Eric Biggers [off-list ref] wrote:

+#define FSCRYPT_FS_KEYRING_DESCRIPTION_SIZE    \
+       (CONST_STRLEN("fscrypt-") + FIELD_SIZEOF(struct super_block, s_id))
+
+#define FSCRYPT_MK_DESCRIPTION_SIZE    (2 * FSCRYPT_KEY_DESCRIPTOR_SIZE + 1)
+
+static void format_fs_keyring_description(
+                       char description[FSCRYPT_FS_KEYRING_DESCRIPTION_SIZE],
+                       const struct super_block *sb)
+{
+       sprintf(description, "fscrypt-%s", sb->s_id);
+}

I fear ->s_id is not the right thing.
For filesystems such as ext4 ->s_id is the name of the backing block device,
so it is per filesysem instance unique.
But this is not guaranteed. For UBIFS ->s_id is just "ubifs", always.
So the names will clash.

-- 
Thanks,
//richard