linux-kernel - [PATCH 3/7] cifs: Fix creating native symlinks pointing to current or parent directory

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Message-Id: <20240929185053.10554-4-pali@kernel.org>
Date: Sun, 29 Sep 2024 20:50:49 +0200
From: Pali Rohár <pali@...nel.org>
To: Steve French <sfrench@...ba.org>,
	Paulo Alcantara <pc@...guebit.com>,
	Ronnie Sahlberg <ronniesahlberg@...il.com>
Cc: linux-cifs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/7] cifs: Fix creating native symlinks pointing to current or parent directory

Calling 'ln -s . symlink' or 'ln -s .. symlink' creates symlink pointing to
some object name which ends with U+F029 unicode codepoint. This is because
trailing dot in the object name is replaced by non-ASCII unicode codepoint.

So Linux SMB client currently is not able to create native symlink pointing
to current or parent directory on Windows SMB server which can be read by
either on local Windows server or by any other SMB client which does not
implement compatible-reverse character replacement.

Fix this problem in cifsConvertToUTF16() function which is doing that
character replacement. Function comment already says that it does not need
to handle special cases '.' and '..', but after introduction of native
symlinks in reparse point form, this handling is needed.

Signed-off-by: Pali Rohár <pali@...nel.org>
---
 fs/smb/client/cifs_unicode.c | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/fs/smb/client/cifs_unicode.c b/fs/smb/client/cifs_unicode.c
index 79d99a913944..4cc6e0896fad 100644
--- a/fs/smb/client/cifs_unicode.c
+++ b/fs/smb/client/cifs_unicode.c
@@ -484,10 +484,21 @@ cifsConvertToUTF16(__le16 *target, const char *source, int srclen,
 			/**
 			 * Remap spaces and periods found at the end of every
 			 * component of the path. The special cases of '.' and
-			 * '..' do not need to be dealt with explicitly because
-			 * they are addressed in namei.c:link_path_walk().
+			 * '..' are need to be handled because of symlinks.
+			 * They are treated as non-end-of-string to avoid
+			 * remapping and breaking symlinks pointing to . or ..
 			 **/
-			if ((i == srclen - 1) || (source[i+1] == '\\'))
+			if ((i == 0 || source[i-1] == '\\') &&
+			    source[i] == '.' &&
+			    (i == srclen-1 || source[i+1] == '\\'))
+				end_of_string = false; /* "." case */
+			else if (i >= 1 &&
+				 (i == 1 || source[i-2] == '\\') &&
+				 source[i-1] == '.' &&
+				 source[i] == '.' &&
+				 (i == srclen-1 || source[i+1] == '\\'))
+				end_of_string = false; /* ".." case */
+			else if ((i == srclen - 1) || (source[i+1] == '\\'))
 				end_of_string = true;
 			else
 				end_of_string = false;
-- 
2.20.1