hostfs: fix the host directory parse when mounting.

hostfs not keep the host directory when mounting. When the host
directory is none (default), fc->source is used as the host root
directory, and this is wrong. Here we use `parse_monolithic` to
handle the old mount path for parsing the root directory. For new
mount path, The `parse_param` is used for the host directory parse.

Reported-and-tested-by: Maciej Żenczykowski <maze@google.com>
Fixes: cd140ce ("hostfs: convert hostfs to use the new mount API")
Link: https://lore.kernel.org/all/CANP3RGceNzwdb7w=vPf5=7BCid5HVQDmz1K5kC9JG42+HVAh_g@mail.gmail.com/
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20240725065130.1821964-1-lihongbo22@huawei.com
[brauner: minor fixes]
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Hongbo Li

fs/hostfs/hostfs_kern.c

@@ -17,6 +17,7 @@
 #include <linux/writeback.h>
 #include <linux/mount.h>
 #include <linux/fs_context.h>
+#include <linux/fs_parser.h>
 #include <linux/namei.h>
 #include "hostfs.h"
 #include <init.h>
@@ -927,7 +928,6 @@ static const struct inode_operations hostfs_link_iops = {
 static int hostfs_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	struct hostfs_fs_info *fsi = sb->s_fs_info;
-	const char *host_root = fc->source;
 	struct inode *root_inode;
 	int err;
 
@@ -941,15 +941,6 @@ static int hostfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	if (err)
 		return err;
 
-	/* NULL is printed as '(null)' by printf(): avoid that. */
-	if (fc->source == NULL)
-		host_root = "";
-
-	fsi->host_root_path =
-		kasprintf(GFP_KERNEL, "%s/%s", root_ino, host_root);
-	if (fsi->host_root_path == NULL)
-		return -ENOMEM;
-
 	root_inode = hostfs_iget(sb, fsi->host_root_path);
 	if (IS_ERR(root_inode))
 		return PTR_ERR(root_inode);
@@ -975,6 +966,58 @@ static int hostfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	return 0;
 }
 
+enum hostfs_parma {
+	Opt_hostfs,
+};
+
+static const struct fs_parameter_spec hostfs_param_specs[] = {
+	fsparam_string_empty("hostfs",		Opt_hostfs),
+	{}
+};
+
+static int hostfs_parse_param(struct fs_context *fc, struct fs_parameter *param)
+{
+	struct hostfs_fs_info *fsi = fc->s_fs_info;
+	struct fs_parse_result result;
+	char *host_root;
+	int opt;
+
+	opt = fs_parse(fc, hostfs_param_specs, param, &result);
+	if (opt < 0)
+		return opt;
+
+	switch (opt) {
+	case Opt_hostfs:
+		host_root = param->string;
+		if (!*host_root)
+			host_root = "";
+		fsi->host_root_path =
+			kasprintf(GFP_KERNEL, "%s/%s", root_ino, host_root);
+		if (fsi->host_root_path == NULL)
+			return -ENOMEM;
+		break;
+	}
+
+	return 0;
+}
+
+static int hostfs_parse_monolithic(struct fs_context *fc, void *data)
+{
+	struct hostfs_fs_info *fsi = fc->s_fs_info;
+	char *host_root = (char *)data;
+
+	/* NULL is printed as '(null)' by printf(): avoid that. */
+	if (host_root == NULL)
+		host_root = "";
+
+	fsi->host_root_path =
+		kasprintf(GFP_KERNEL, "%s/%s", root_ino, host_root);
+	if (fsi->host_root_path == NULL)
+		return -ENOMEM;
+
+	return 0;
+}
+
 static int hostfs_fc_get_tree(struct fs_context *fc)
 {
 	return get_tree_nodev(fc, hostfs_fill_super);
@@ -992,6 +1035,8 @@ static void hostfs_fc_free(struct fs_context *fc)
 }
 
 static const struct fs_context_operations hostfs_context_ops = {
+	.parse_monolithic = hostfs_parse_monolithic,
+	.parse_param	= hostfs_parse_param,
 	.get_tree	= hostfs_fc_get_tree,
 	.free		= hostfs_fc_free,
 };