kernfs: add exportfs operations

Now we have the facilities to implement exportfs operations. The idea is
cgroup can export the fhandle info to userspace, then userspace uses
fhandle to find the cgroup name. Another example is userspace can get
fhandle for a cgroup and BPF uses the fhandle to filter info for the
cgroup.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 56 insertions, 0 deletions

diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
index acd542625fd8..fa323589704f 100644
--- a/fs/kernfs/mount.c
+++ b/fs/kernfs/mount.c
@@ -16,6 +16,7 @@
 #include <linux/pagemap.h>
 #include <linux/namei.h>
 #include <linux/seq_file.h>
+#include <linux/exportfs.h>
 
 #include "kernfs-internal.h"
 
@@ -64,6 +65,59 @@ const struct super_operations kernfs_sops = {
         .show_path      = kernfs_sop_show_path,
 };
 
+static struct inode *kernfs_fh_get_inode(struct super_block *sb,
+                u64 ino, u32 generation)
+{
+        struct kernfs_super_info *info = kernfs_info(sb);
+        struct inode *inode;
+        struct kernfs_node *kn;
+
+        if (ino == 0)
+                return ERR_PTR(-ESTALE);
+
+        kn = kernfs_find_and_get_node_by_ino(info->root, ino);
+        if (!kn)
+                return ERR_PTR(-ESTALE);
+        inode = kernfs_get_inode(sb, kn);
+        kernfs_put(kn);
+        if (IS_ERR(inode))
+                return ERR_CAST(inode);
+
+        if (generation && inode->i_generation != generation) {
+                /* we didn't find the right inode.. */
+                iput(inode);
+                return ERR_PTR(-ESTALE);
+        }
+        return inode;
+}
+
+static struct dentry *kernfs_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,
+                                    kernfs_fh_get_inode);
+}
+
+static struct dentry *kernfs_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,
+                                    kernfs_fh_get_inode);
+}
+
+static struct dentry *kernfs_get_parent_dentry(struct dentry *child)
+{
+        struct kernfs_node *kn = kernfs_dentry_node(child);
+
+        return d_obtain_alias(kernfs_get_inode(child->d_sb, kn->parent));
+}
+
+static const struct export_operations kernfs_export_ops = {
+        .fh_to_dentry   = kernfs_fh_to_dentry,
+        .fh_to_parent   = kernfs_fh_to_parent,
+        .get_parent     = kernfs_get_parent_dentry,
+};
+
 /**
  * kernfs_root_from_sb - determine kernfs_root associated with a super_block
  * @sb: the super_block in question
@@ -159,6 +213,8 @@ static int kernfs_fill_super(struct super_block *sb, unsigned long magic)
         sb->s_magic = magic;
         sb->s_op = &kernfs_sops;
         sb->s_xattr = kernfs_xattr_handlers;
+        if (info->root->flags & KERNFS_ROOT_SUPPORT_EXPORTOP)
+                sb->s_export_op = &kernfs_export_ops;
         sb->s_time_gran = 1;
 
         /* get root inode, initialize and unlock it */