Add handlers which read fs caps from the lower or upper filesystem and
write/remove fs caps to the upper filesystem, performing copy-up as
necessary.

While it doesn't make sense to use fscaps on directories, nothing in the
kernel actually prevents setting or getting fscaps xattrs for directory
inodes. If we omit fscaps handlers in ovl_dir_inode_operations then the
generic handlers will be used. These handlers will use the xattr inode
operations, bypassing any idmapping on lower mounts, so fscaps handlers
are also installed for ovl_dir_inode_operations.

Signed-off-by: Seth Forshee (DigitalOcean) <sforshee@kernel.org>
---
 fs/overlayfs/dir.c       |  3 ++
 fs/overlayfs/inode.c     | 84 ++++++++++++++++++++++++++++++++++++++++++++++++
 fs/overlayfs/overlayfs.h |  6 ++++
 3 files changed, 93 insertions(+)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index aab3f5d93556..d9ab3c9ce10a 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c

@@ -1303,6 +1303,9 @@ const struct inode_operations ovl_dir_inode_operations = {
        .get_inode_acl  = ovl_get_inode_acl,
        .get_acl        = ovl_get_acl,
        .set_acl        = ovl_set_acl,
+       .get_fscaps     = ovl_get_fscaps,
+       .set_fscaps     = ovl_set_fscaps,
+       .remove_fscaps  = ovl_remove_fscaps,
        .update_time    = ovl_update_time,
        .fileattr_get   = ovl_fileattr_get,
        .fileattr_set   = ovl_fileattr_set,

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index c63b31a460be..82fc6e479d45 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c

@@ -568,6 +568,87 @@ int ovl_set_acl(struct mnt_idmap *idmap, struct dentry *dentry,
 }
 #endif

+int ovl_get_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
+                  struct vfs_caps *caps)
+{
+       int err;
+       const struct cred *old_cred;
+       struct path realpath;
+
+       ovl_path_real(dentry, &realpath);
+       old_cred = ovl_override_creds(dentry->d_sb);
+       err = vfs_get_fscaps(mnt_idmap(realpath.mnt), realpath.dentry, caps);
+       revert_creds(old_cred);
+       return err;
+}
+
+int ovl_set_fscaps(struct mnt_idmap *idmap, struct dentry *dentry,
+                  const struct vfs_caps *caps, int flags)
+{
+       int err;
+       struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
+       struct dentry *upperdentry = ovl_dentry_upper(dentry);
+       struct dentry *realdentry = upperdentry ?: ovl_dentry_lower(dentry);
+       const struct cred *old_cred;
+
+       err = ovl_want_write(dentry);
+       if (err)
+               goto out;
+
+       if (!upperdentry) {
+               err = ovl_copy_up(dentry);
+               if (err)
+                       goto out_drop_write;
+
+               realdentry = ovl_dentry_upper(dentry);
+       }
+
+       old_cred = ovl_override_creds(dentry->d_sb);
+       err = vfs_set_fscaps(ovl_upper_mnt_idmap(ofs), realdentry, caps, flags);
+       revert_creds(old_cred);
+
+       /* copy c/mtime */
+       ovl_copyattr(d_inode(dentry));
+
+out_drop_write:
+       ovl_drop_write(dentry);
+out:
+       return err;
+}
+
+int ovl_remove_fscaps(struct mnt_idmap *idmap, struct dentry *dentry)
+{
+       int err;
+       struct ovl_fs *ofs = OVL_FS(dentry->d_sb);
+       struct dentry *upperdentry = ovl_dentry_upper(dentry);
+       struct dentry *realdentry = upperdentry ?: ovl_dentry_lower(dentry);
+       const struct cred *old_cred;
+
+       err = ovl_want_write(dentry);
+       if (err)
+               goto out;
+
+       if (!upperdentry) {
+               err = ovl_copy_up(dentry);
+               if (err)
+                       goto out_drop_write;
+
+               realdentry = ovl_dentry_upper(dentry);
+       }
+