[OpenWrt-Devel] Why default of XATTR in kernel but not userspace?

Daniel Dickinson openwrt at daniel.thecshore.com
Sun Dec 20 03:18:50 EST 2015

Hi all,

I've been working on a patch for making POSIX ACL support and userspace 
support of POSIX ACL and XATTR (extended attributes) a configuration 
option at compile time and noticed something rather wonky:

While for POSIX ACL the default is disabled both at the kernel
level and in userpace (i.e. filesystem modules by default do
not support POSIX ACL and programs don't use libacl).

OTOH for XATTR support the default is for kernel modules and built-ins
to support XATTR, but block mount does not support XATTR so I'm not sure 
any filesystems are actually mounted with xattr support enabled, and, in 
addition, no programs are compiled with xattr support (that is the 
default is not support xattr in userspace by default).

I'm a little puzzled as to why this is and what is acommplished by
having XATTR support by filesystem modules, when it doesn't appear 
anything uses that support (unless overlayfs uses with jffs2 and/or
tmpfs uses it via procd, but the support would fail for anything not 
mounted by procd).

What is exactly is logic/reason for this configuration and what is 
actually going on here?  Is xattr used at all?  If it is, why the mixed 
bag of support vs. lack of support depending on how a filesystem
is mounted?

My plan is to make a config option that, when enabled, makes POSIX ACL 
support in both kernel and userpace the default (with certain exceptions 
like JFFS2 and TMPFS), along with userpace xattr support.
In addition I add to the build options menu (if the above option is 
enabled), the ability to turn on and off POSIX ACL support for each 

(The option if enabled also manages (on by default) NFSv4 (and off by 
default) old NFS ACL support).

In addition I have a patch to fstools that lets block mount mount with 
posix acl and xattr support enabled.

(I make this an ifdef since it's pointless without the appropriate 
kernel support).


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