linux-ext4 - [PATCH 24/24] fscrypt: document symlink length restriction

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Date:   Fri, 15 Dec 2017 09:42:25 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     linux-fscrypt@...r.kernel.org
Cc:     "Theodore Y . Ts'o" <tytso@....edu>,
        Jaegeuk Kim <jaegeuk@...nel.org>, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, Eric Biggers <ebiggers@...gle.com>
Subject: [PATCH 24/24] fscrypt: document symlink length restriction

From: Eric Biggers <ebiggers@...gle.com>

Document that encryption reduces the maximum length of a symlink target
slightly.

Signed-off-by: Eric Biggers <ebiggers@...gle.com>
---
 Documentation/filesystems/fscrypt.rst | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
index 776ddc655f79..cfbc18f0d9c9 100644
--- a/Documentation/filesystems/fscrypt.rst
+++ b/Documentation/filesystems/fscrypt.rst
@@ -448,8 +448,14 @@ astute users may notice some differences in behavior:
 
 - The st_size of an encrypted symlink will not necessarily give the
   length of the symlink target as required by POSIX.  It will actually
-  give the length of the ciphertext, which may be slightly longer than
-  the plaintext due to the NUL-padding.
+  give the length of the ciphertext, which will be slightly longer
+  than the plaintext due to NUL-padding and an extra 2-byte overhead.
+
+- The maximum length of an encrypted symlink is 2 bytes shorter than
+  the maximum length of an unencrypted symlink.  For example, on an
+  EXT4 filesystem with a 4K block size, unencrypted symlinks can be up
+  to 4095 bytes long, while encrypted symlinks can only be up to 4093
+  bytes long (both lengths excluding the terminating null).
 
 Note that mmap *is* supported.  This is possible because the pagecache
 for an encrypted file contains the plaintext, not the ciphertext.
-- 
2.15.1