[PATCH v2] smb: client: preserve leading slash for POSIX absolute symlink targets

[Steve French <smfrench@gmail.com>]

When creating a native SMB symbolic link (CIFS_SYMLINK_TYPE_NATIVE) whose
target is an absolute path on a mount that uses POSIX paths, the leading
path separator was silently dropped from the stored symlink target.

create_native_symlink() converted the target to UTF-16 with
cifs_convert_path_to_utf16().  That helper is intended for share-relative
SMB paths and therefore unconditionally strips a leading path separator.
For an absolute POSIX symlink target the leading '/' is significant, so a
target of "/foo/bar" was stored - and read back - as "foo/bar", even
though the reparse point was still flagged as absolute
(SYMLINK_FLAG_RELATIVE cleared).

On a POSIX paths mount the symlink target is stored verbatim, so convert
it directly with cifs_strndup_to_utf16() instead.  This preserves the
leading separator, avoids the leading-backslash stripping that
cifs_convert_path_to_utf16() also performs (a backslash is a valid POSIX
filename character), and uses NO_MAP_UNI_RSVD to match the readback path
in smb2_parse_native_symlink(), which always converts the target with
cifs_strndup_from_utf16() / NO_MAP_UNI_RSVD.  This mirrors how the NFS and
WSL reparse symlink creators convert their targets.

The NT-style absolute symlink handling, which needs the "\??\" prefix and
drive-letter colon preserved, continues to use cifs_convert_path_to_utf16()
together with the existing masking of those bytes.

Fixes: 12b466eb52d9 ("cifs: Fix creating and resolving absolute NT-style symlinks")
Signed-off-by: Steve French <stfrench@microsoft.com>
---
 fs/smb/client/reparse.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/smb/client/reparse.c b/fs/smb/client/reparse.c
index cd1e1eaee67a..5cc5b0410d48 100644
--- a/fs/smb/client/reparse.c
+++ b/fs/smb/client/reparse.c
@@ -67,6 +67,7 @@ static int create_native_symlink(const unsigned int xid, struct inode *inode,
 	char *sym = NULL;
 	struct kvec iov;
 	bool directory;
+	int path_len;
 	int rc = 0;
 
 	if (strlen(symname) > REPARSE_SYM_PATH_MAX)
@@ -168,7 +169,21 @@ static int create_native_symlink(const unsigned int xid, struct inode *inode,
 	if (!(sbflags & CIFS_MOUNT_POSIX_PATHS) && symname[0] == '/')
 		sym[0] = sym[1] = sym[2] = sym[5] = '_';
 
-	path = cifs_convert_path_to_utf16(sym, cifs_sb);
+	/*
+	 * On a POSIX paths mount the symlink target is stored verbatim, so
+	 * convert it with cifs_strndup_to_utf16().  cifs_convert_path_to_utf16()
+	 * must not be used here: it strips a leading path separator (it is
+	 * meant for share-relative SMB paths), which would corrupt an absolute
+	 * POSIX symlink target such as "/foo/bar".  Using NO_MAP_UNI_RSVD also
+	 * matches the readback path in smb2_parse_native_symlink().
+	 */
+	if (sbflags & CIFS_MOUNT_POSIX_PATHS)
+		path = cifs_strndup_to_utf16(sym, strlen(sym), &path_len,
+					     cifs_sb->local_nls,
+					     NO_MAP_UNI_RSVD);
+	else
+		path = cifs_convert_path_to_utf16(sym, cifs_sb);
+
 	if (!path) {
 		rc = -ENOMEM;
 		goto out;
-- 
2.53.0