fscrypt: clean up and improve dentry revalidation

Make various improvements to fscrypt dentry revalidation:

- Don't try to handle the case where the per-directory key is removed,
  as this can't happen without the inode (and dentries) being evicted.

- Flag ciphertext dentries rather than plaintext dentries, since it's
  ciphertext dentries that need the special handling.

- Avoid doing unnecessary work for non-ciphertext dentries.

- When revalidating ciphertext dentries, try to set up the directory's
  i_crypt_info to make sure the key is really still absent, rather than
  invalidating all negative dentries as the previous code did.  An old
  comment suggested we can't do this for locking reasons, but AFAICT
  this comment was outdated and it actually works fine.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

2 files changed, 3 insertions, 5 deletions

diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index 60996e64c579..9b3b75d3bd21 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -212,7 +212,7 @@ struct dentry_operations {
 
 #define DCACHE_MAY_FREE			0x00800000
 #define DCACHE_FALLTHRU			0x01000000 /* Fall through to lower layer */
-#define DCACHE_ENCRYPTED_WITH_KEY	0x02000000 /* dir is encrypted with a valid key */
+#define DCACHE_ENCRYPTED_NAME		0x02000000 /* Encrypted name (dir key was unavailable) */
 #define DCACHE_OP_REAL			0x04000000
 
 #define DCACHE_PAR_LOOKUP		0x10000000 /* being looked up (with parent locked shared) */
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index ec8ab7108599..09e368a515d1 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -545,10 +545,8 @@ static inline int fscrypt_prepare_rename(struct inode *old_dir,
  * filenames are presented in encrypted form.  Therefore, we'll try to set up
  * the directory's encryption key, but even without it the lookup can continue.
  *
- * To allow invalidating stale dentries if the directory's encryption key is
- * added later, we also install a custom ->d_revalidate() method and use the
- * DCACHE_ENCRYPTED_WITH_KEY flag to indicate whether a given dentry is a
- * plaintext name (flag set) or a ciphertext name (flag cleared).
+ * This also installs a custom ->d_revalidate() method which will invalidate the
+ * dentry if it was created without the key and the key is later added.
  *
  * Return: 0 on success, -errno if a problem occurred while setting up the
  * encryption key