cifs: Add support for creating NFS-style symlinks

CIFS client is currently able to parse NFS-style symlinks, but is not able
to create them. This functionality is useful when the mounted SMB share is
used also by Windows NFS server (on Windows Server 2012 or new). It allows
interop of symlinks between SMB share mounted by Linux CIFS client and same
export from Windows NFS server mounted by some NFS client.

New symlinks would be created in NFS-style only in case the mount option
-o reparse=nfs is specified, which is not by default. So default CIFS
mounts are not affected by this change.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: pali

fs/smb/client/reparse.c

@@ -409,6 +409,8 @@ static int create_native_socket(const unsigned int xid, struct inode *inode,
 
 static int nfs_set_reparse_buf(struct reparse_nfs_data_buffer *buf,
 			       mode_t mode, dev_t dev,
+			       __le16 *symname_utf16,
+			       int symname_utf16_len,
 			       struct kvec *iov)
 {
 	u64 type;
@@ -419,13 +421,18 @@ static int nfs_set_reparse_buf(struct reparse_nfs_data_buffer *buf,
 	switch ((type = reparse_mode_nfs_type(mode))) {
 	case NFS_SPECFILE_BLK:
 	case NFS_SPECFILE_CHR:
-		dlen = sizeof(__le64);
+		dlen = 2 * sizeof(__le32);
+		((__le32 *)buf->DataBuffer)[0] = cpu_to_le32(MAJOR(dev));
+		((__le32 *)buf->DataBuffer)[1] = cpu_to_le32(MINOR(dev));
+		break;
+	case NFS_SPECFILE_LNK:
+		dlen = symname_utf16_len;
+		memcpy(buf->DataBuffer, symname_utf16, symname_utf16_len);
 		break;
 	case NFS_SPECFILE_FIFO:
 	case NFS_SPECFILE_SOCK:
 		dlen = 0;
 		break;
-	case NFS_SPECFILE_LNK: /* TODO: add support for NFS symlinks */
 	default:
 		return -EOPNOTSUPP;
 	}
@@ -435,8 +442,6 @@ static int nfs_set_reparse_buf(struct reparse_nfs_data_buffer *buf,
 	buf->InodeType = cpu_to_le64(type);
 	buf->ReparseDataLength = cpu_to_le16(len + dlen -
 					     sizeof(struct reparse_data_buffer));
-	*(__le64 *)buf->DataBuffer = cpu_to_le64(((u64)MINOR(dev) << 32) |
-						 MAJOR(dev));
 	iov->iov_base = buf;
 	iov->iov_len = len + dlen;
 	return 0;
@@ -447,21 +452,42 @@ static int mknod_nfs(unsigned int xid, struct inode *inode,
 		     const char *full_path, umode_t mode, dev_t dev,
 		     const char *symname)
 {
+	struct cifs_sb_info *cifs_sb = CIFS_SB(inode->i_sb);
 	struct cifs_open_info_data data;
-	struct reparse_nfs_data_buffer *p;
+	struct reparse_nfs_data_buffer *p = NULL;
+	__le16 *symname_utf16 = NULL;
+	int symname_utf16_len = 0;
 	struct inode *new;
 	struct kvec iov;
 	__u8 buf[sizeof(*p) + sizeof(__le64)];
 	int rc;
 
-	p = (struct reparse_nfs_data_buffer *)buf;
-	rc = nfs_set_reparse_buf(p, mode, dev, &iov);
+	if (S_ISLNK(mode)) {
+		symname_utf16 = cifs_strndup_to_utf16(symname, strlen(symname),
+						      &symname_utf16_len,
+						      cifs_sb->local_nls,
+						      NO_MAP_UNI_RSVD);
+		if (!symname_utf16) {
+			rc = -ENOMEM;
+			goto out;
+		}
+		symname_utf16_len -= 2; /* symlink is without trailing wide-nul */
+		p = kzalloc(sizeof(*p) + symname_utf16_len, GFP_KERNEL);
+		if (!p) {
+			rc = -ENOMEM;
+			goto out;
+		}
+	} else {
+		p = (struct reparse_nfs_data_buffer *)buf;
+	}
+	rc = nfs_set_reparse_buf(p, mode, dev, symname_utf16, symname_utf16_len, &iov);
 	if (rc)
-		return rc;
+		goto out;
 
 	data = (struct cifs_open_info_data) {
 		.reparse_point = true,
 		.reparse = { .tag = IO_REPARSE_TAG_NFS, .buf = (struct reparse_data_buffer *)p, },
+		.symlink_target = kstrdup(symname, GFP_KERNEL),
 	};
 
 	new = smb2_get_reparse_inode(&data, inode->i_sb, xid,
@@ -471,6 +497,11 @@ static int mknod_nfs(unsigned int xid, struct inode *inode,
 	else
 		rc = PTR_ERR(new);
 	cifs_free_open_info(&data);
+out:
+	if (S_ISLNK(mode)) {
+		kfree(symname_utf16);
+		kfree(p);
+	}
 	return rc;
 }