smb: client: introduce reparse mount option

Allow the user to create special files and symlinks by choosing
between WSL and NFS reparse points via 'reparse={nfs,wsl}' mount
options.  If unset or 'reparse=default', the client will default to
creating them via NFS reparse points.

Creating WSL reparse points isn't supported yet, so simply return
error when attempting to mount with 'reparse=wsl' for now.
Signed-off-by: Paulo Alcantara <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/client/cifsglob.h

@@ -153,6 +153,12 @@ enum securityEnum {
 	Kerberos,		/* Kerberos via SPNEGO */
 };
 
+enum cifs_reparse_type {
+	CIFS_REPARSE_TYPE_NFS,
+	CIFS_REPARSE_TYPE_WSL,
+	CIFS_REPARSE_TYPE_DEFAULT = CIFS_REPARSE_TYPE_NFS,
+};
+
 struct session_key {
 	unsigned int len;
 	char *response;

fs/smb/client/connect.c

@@ -2803,6 +2803,8 @@ compare_mount_options(struct super_block *sb, struct cifs_mnt_data *mnt_data)
 		return 0;
 	if (old->ctx->closetimeo != new->ctx->closetimeo)
 		return 0;
+	if (old->ctx->reparse_type != new->ctx->reparse_type)
+		return 0;
 
 	return 1;
 }

fs/smb/client/fs_context.c

@@ -174,6 +174,7 @@ const struct fs_parameter_spec smb3_fs_parameters[] = {
 	fsparam_string("vers", Opt_vers),
 	fsparam_string("sec", Opt_sec),
 	fsparam_string("cache", Opt_cache),
+	fsparam_string("reparse", Opt_reparse),
 
 	/* Arguments that should be ignored */
 	fsparam_flag("guest", Opt_ignore),
@@ -296,6 +297,35 @@ cifs_parse_cache_flavor(struct fs_context *fc, char *value, struct smb3_fs_conte
 	return 0;
 }
 
+static const match_table_t reparse_flavor_tokens = {
+	{ Opt_reparse_default,	"default" },
+	{ Opt_reparse_nfs,	"nfs" },
+	{ Opt_reparse_wsl,	"wsl" },
+	{ Opt_reparse_err,	NULL },
+};
+
+static int parse_reparse_flavor(struct fs_context *fc, char *value,
+				struct smb3_fs_context *ctx)
+{
+	substring_t args[MAX_OPT_ARGS];
+
+	switch (match_token(value, reparse_flavor_tokens, args)) {
+	case Opt_reparse_default:
+		ctx->reparse_type = CIFS_REPARSE_TYPE_DEFAULT;
+		break;
+	case Opt_reparse_nfs:
+		ctx->reparse_type = CIFS_REPARSE_TYPE_NFS;
+		break;
+	case Opt_reparse_wsl:
+		cifs_errorf(fc, "unsupported reparse= option: %s\n", value);
+		return 1;
+	default:
+		cifs_errorf(fc, "bad reparse= option: %s\n", value);
+		return 1;
+	}
+	return 0;
+}
+
 #define DUP_CTX_STR(field)						\
 do {									\
 	if (ctx->field) {						\
@@ -1566,6 +1596,10 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
 	case Opt_rdma:
 		ctx->rdma = true;
 		break;
+	case Opt_reparse:
+		if (parse_reparse_flavor(fc, param->string, ctx))
+			goto cifs_parse_mount_err;
+		break;
 	}
 	/* case Opt_ignore: - is ignored as expected ... */
 
@@ -1652,6 +1686,7 @@ int smb3_init_fs_context(struct fs_context *fc)
 	ctx->backupgid_specified = false; /* no backup intent for a group */
 
 	ctx->retrans = 1;
+	ctx->reparse_type = CIFS_REPARSE_TYPE_DEFAULT;
 
 /*
  *	short int override_uid = -1;

fs/smb/client/fs_context.h

@@ -41,6 +41,13 @@ enum {
 	Opt_cache_err
 };
 
+enum cifs_reparse_parm {
+	Opt_reparse_default,
+	Opt_reparse_nfs,
+	Opt_reparse_wsl,
+	Opt_reparse_err
+};
+
 enum cifs_sec_param {
 	Opt_sec_krb5,
 	Opt_sec_krb5i,
@@ -148,6 +155,7 @@ enum cifs_param {
 	Opt_vers,
 	Opt_sec,
 	Opt_cache,
+	Opt_reparse,
 
 	/* Mount options to be ignored */
 	Opt_ignore,
@@ -271,6 +279,7 @@ struct smb3_fs_context {
 	char *leaf_fullpath;
 	struct cifs_ses *dfs_root_ses;
 	bool dfs_automount:1; /* set for dfs automount only */
+	enum cifs_reparse_type reparse_type;
 };
 
 extern const struct fs_parameter_spec smb3_fs_parameters[];