Compatibility with overlayfs

[Gao Xiang]

On Sat, Nov 30, 2019 at 09:29:04AM +0800, Gao Xiang via Linux-erofs wrote:
> Hi David,
> 
> On Fri, Nov 29, 2019 at 03:22:15PM -0500, David Michael wrote:
> > Hi,
> > 
> > I tried to test EROFS on Linux 5.4 as the root file system and mounted
> > a writable overlay (with upper layer on tmpfs) over /etc, but I get
> > ENODATA errors when attempting to modify files.  For example, adding a
> > user results in "Failed to take /etc/passwd lock: No data available".
> > Files can be modified after unlinking and restoring them so they're
> > created on the upper layer.  This is not necessary with other
> > read-only file systems (at least squashfs or ext4 with the read-only
> > feature).  I tried while forcing compact and extended inodes.
> > 
> > Is EROFS intended to be usable as a lower layer with overlayfs?
> 
> Yes, and overlayfs were used on our smartphones for development use
> only as well. I think it's weird, I will try it on the latest kernel
> now, and see if I can reproduce this issue soon...
> 
> Thanks for your report!
> 
> Thanks,
> Gao Xiang

I have reproduced this issue -- That is due to erofs will return an
unexpected -ENODATA when calling listxattr without xattr and cause
copy_up fail:

int ovl_copy_xattr(struct dentry *old, struct dentry *new)
{
	ssize_t list_size, size, value_size = 0;
	char *buf, *name, *value = NULL;
	int uninitialized_var(error);
	size_t slen;

	if (!(old->d_inode->i_opflags & IOP_XATTR) ||
	    !(new->d_inode->i_opflags & IOP_XATTR))
		return 0;

	list_size = vfs_listxattr(old, NULL, 0);  <- no xattr
	if (list_size <= 0) {
		if (list_size == -EOPNOTSUPP)
			return 0;
		return list_size;    <- will return -ENODATA
	}

since our products using xattr enabled EROFS with overlayfs,
so we didn't observe this issue before. So could you try
the following patch (If it can resolve the issue, I will
send it for 5.5-rc2 and backport to all stable version)?
Look forward to your feekback.

diff --git a/fs/erofs/xattr.c b/fs/erofs/xattr.c
index a13a78725c57..dd328c87dda7 100644
--- a/fs/erofs/xattr.c
+++ b/fs/erofs/xattr.c
@@ -649,8 +649,11 @@ ssize_t erofs_listxattr(struct dentry *dentry,
 	struct listxattr_iter it;
 
 	ret = init_inode_xattrs(d_inode(dentry));
-	if (ret)
+	if (ret) {
+		if (ret == -ENODATA)
+			return 0;
 		return ret;
+	}
 
 	it.dentry = dentry;
 	it.buffer = buffer;