On Thu, Sep 6, 2012 at 5:56 PM, Andy Whitcroft <apw@xxxxxxxxxxxxx> wrote: > During some testing here we discovered that we could not successfully > use a NFS as the lower layer for overlayfs. There are two separate issues: > > Firstly when using an NFSv4 lower layer we tickle an issue when copying > up the xattrs for the underlying file. NFS uses an xattr system.nfs4_acl > which the upper layer will not store (ext4 for example). This triggers > an EOPNOTSUPP error when trying to copy up the xattrs for the file, > preventing the file being written. I am a little unclear as to whether it > makes sense to generally ignore xattrs we cannot store in the upper layer, > this is based on the assumption the person creating the mount knew what > they were combining. The first patch (for discussion) following this > email avoids this issue by ignoring the xattr if it is not storable. I don't know much about NFSv4 ACL's but I think it may be incompatible with POSIX ACLs in which case copying them up is not possible and ignoring them should be the right thing to do. > > Secondly when using an NFSv3 R/O lower layer the filesystem permissions > check refuses permission to write to the inode which prevents us from > copying it up even though we have a writable upper layer. (With an ext4 > lower layer the inode check will succeed if the inode is writable even > if the filesystem is not.) It is not clear what the right solution is > here. One approach is to check the inode permissions only (avoiding the > filesystem specific permissions op), but it is not clear we can rely on > these for all underlying filesystems. Perhaps this check should only be > used for NFS. Perhaps it needs to be a mount option. The second patch > (for discussion) following this email implements this, using the inode > permissions when the lowerlayer is read-only. This seems to work as > expected in my limited testing. I fear that will create an inconsistency between the read-only and the non-read-only case, even though both should behave the same. I think the cleanest would be to create a mount option to always use generic_permission (on both the lower and the upper fs). That would give us two, slightly different, operating modes but each would be self consistent. Thanks, Miklos -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
