erofs: make filesystem exportable

Implement export operations in order to make EROFS support accessing
inodes with filehandles so that it can be exported via NFS and used
by overlayfs.

Without this patch, 'exportfs -rv' will report:
exportfs: /root/erofs_mp does not support NFS export

Also tested with unionmount-testsuite and the testcase below passes now:
./run --ov --erofs --verify hard-link

For more details about the testcase, see:
https://github.com/amir73il/unionmount-testsuite/pull/6

Signed-off-by: Hongnan Li <hongnan.li@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220425040712.91685-1-hongnan.li@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/internal.h

@@ -484,7 +484,7 @@ int erofs_getattr(struct user_namespace *mnt_userns, const struct path *path,
 /* namei.c */
 extern const struct inode_operations erofs_dir_iops;
 
-int erofs_namei(struct inode *dir, struct qstr *name,
+int erofs_namei(struct inode *dir, const struct qstr *name,
 		erofs_nid_t *nid, unsigned int *d_type);
 
 /* dir.c */

fs/erofs/namei.c

@@ -165,9 +165,8 @@ out:		/* free if the candidate is valid */
 	return candidate;
 }
 
-int erofs_namei(struct inode *dir,
-		struct qstr *name,
-		erofs_nid_t *nid, unsigned int *d_type)
+int erofs_namei(struct inode *dir, const struct qstr *name, erofs_nid_t *nid,
+		unsigned int *d_type)
 {
 	int ndirents;
 	struct erofs_buf buf = __EROFS_BUF_INITIALIZER;

fs/erofs/super.c

@@ -13,6 +13,7 @@
 #include <linux/fs_context.h>
 #include <linux/fs_parser.h>
 #include <linux/dax.h>
+#include <linux/exportfs.h>
 #include "xattr.h"
 
 #define CREATE_TRACE_POINTS
@@ -577,6 +578,44 @@ static int erofs_init_managed_cache(struct super_block *sb)
 static int erofs_init_managed_cache(struct super_block *sb) { return 0; }
 #endif
 
+static struct inode *erofs_nfs_get_inode(struct super_block *sb,
+					 u64 ino, u32 generation)
+{
+	return erofs_iget(sb, ino, false);
+}
+
+static struct dentry *erofs_fh_to_dentry(struct super_block *sb,
+		struct fid *fid, int fh_len, int fh_type)
+{
+	return generic_fh_to_dentry(sb, fid, fh_len, fh_type,
+				    erofs_nfs_get_inode);
+}
+
+static struct dentry *erofs_fh_to_parent(struct super_block *sb,
+		struct fid *fid, int fh_len, int fh_type)
+{
+	return generic_fh_to_parent(sb, fid, fh_len, fh_type,
+				    erofs_nfs_get_inode);
+}
+
+static struct dentry *erofs_get_parent(struct dentry *child)
+{
+	erofs_nid_t nid;
+	unsigned int d_type;
+	int err;
+
+	err = erofs_namei(d_inode(child), &dotdot_name, &nid, &d_type);
+	if (err)
+		return ERR_PTR(err);
+	return d_obtain_alias(erofs_iget(child->d_sb, nid, d_type == FT_DIR));
+}
+
+static const struct export_operations erofs_export_ops = {
+	.fh_to_dentry = erofs_fh_to_dentry,
+	.fh_to_parent = erofs_fh_to_parent,
+	.get_parent = erofs_get_parent,
+};
+
 static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	struct inode *inode;
@@ -619,6 +658,7 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 
 	sb->s_op = &erofs_sops;
 	sb->s_xattr = erofs_xattr_handlers;
+	sb->s_export_op = &erofs_export_ops;
 
 	if (test_opt(&sbi->opt, POSIX_ACL))
 		sb->s_flags |= SB_POSIXACL;