fscrypt: cache decrypted symlink target in ->i_link

Path lookups that traverse encrypted symlink(s) are very slow because
each encrypted symlink needs to be decrypted each time it's followed.
This also involves dropping out of rcu-walk mode.

Make encrypted symlinks faster by caching the decrypted symlink target
in ->i_link.  The first call to fscrypt_get_symlink() sets it.  Then,
the existing VFS path lookup code uses the non-NULL ->i_link to take the
fast path where ->get_link() isn't called, and lookups in rcu-walk mode
remain in rcu-walk mode.

Also set ->i_link immediately when a new encrypted symlink is created.

To safely free the symlink target after an RCU grace period has elapsed,
introduce a new function fscrypt_free_inode(), and make the relevant
filesystems call it just before actually freeing the inode.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

1 files changed, 33 insertions, 7 deletions

diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index 042d5b44f4ed..2dc22549d724 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -189,11 +189,9 @@ int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
         sd->len = cpu_to_le16(ciphertext_len);
 
         err = fname_encrypt(inode, &iname, sd->encrypted_path, ciphertext_len);
-        if (err) {
-                if (!disk_link->name)
-                        kfree(sd);
-                return err;
-        }
+        if (err)
+                goto err_free_sd;
+
         /*
          * Null-terminating the ciphertext doesn't make sense, but we still
          * count the null terminator in the length, so we might as well
@@ -201,9 +199,20 @@ int __fscrypt_encrypt_symlink(struct inode *inode, const char *target,
          */
         sd->encrypted_path[ciphertext_len] = '\0';
 
+        /* Cache the plaintext symlink target for later use by get_link() */
+        err = -ENOMEM;
+        inode->i_link = kmemdup(target, len + 1, GFP_NOFS);
+        if (!inode->i_link)
+                goto err_free_sd;
+
         if (!disk_link->name)
                 disk_link->name = (unsigned char *)sd;
         return 0;
+
+err_free_sd:
+        if (!disk_link->name)
+                kfree(sd);
+        return err;
 }
 EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
 
@@ -212,7 +221,7 @@ EXPORT_SYMBOL_GPL(__fscrypt_encrypt_symlink);
  * @inode: the symlink inode
  * @caddr: the on-disk contents of the symlink
  * @max_size: size of @caddr buffer
- * @done: if successful, will be set up to free the returned target
+ * @done: if successful, will be set up to free the returned target if needed
  *
  * If the symlink's encryption key is available, we decrypt its target.
  * Otherwise, we encode its target for presentation.
@@ -227,12 +236,18 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
 {
         const struct fscrypt_symlink_data *sd;
         struct fscrypt_str cstr, pstr;
+        bool has_key;
         int err;
 
         /* This is for encrypted symlinks only */
         if (WARN_ON(!IS_ENCRYPTED(inode)))
                 return ERR_PTR(-EINVAL);
 
+        /* If the decrypted target is already cached, just return it. */
+        pstr.name = READ_ONCE(inode->i_link);
+        if (pstr.name)
+                return pstr.name;
+
         /*
          * Try to set up the symlink's encryption key, but we can continue
          * regardless of whether the key is available or not.
@@ -240,6 +255,7 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
         err = fscrypt_get_encryption_info(inode);
         if (err)
                 return ERR_PTR(err);
+        has_key = fscrypt_has_encryption_key(inode);
 
         /*
          * For historical reasons, encrypted symlink targets are prefixed with
@@ -271,7 +287,17 @@ const char *fscrypt_get_symlink(struct inode *inode, const void *caddr,
                 goto err_kfree;
 
         pstr.name[pstr.len] = '\0';
-        set_delayed_call(done, kfree_link, pstr.name);
+
+        /*
+         * Cache decrypted symlink targets in i_link for later use.  Don't cache
+         * symlink targets encoded without the key, since those become outdated
+         * once the key is added.  This pairs with the READ_ONCE() above and in
+         * the VFS path lookup code.
+         */
+        if (!has_key ||
+            cmpxchg_release(&inode->i_link, NULL, pstr.name) != NULL)
+                set_delayed_call(done, kfree_link, pstr.name);
+
         return pstr.name;
 
 err_kfree: