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On Tue, Jun 28, 2005 at 10:54:34AM -0700, Jason Thorpe wrote:
> On Jun 28, 2005, at 6:28 AM, Reinoud Zandijk wrote:
> It should be pretty straight-forward to encapsulate this type of sub- 
> file handling into the file system itself... OTOH, it might map  
> pretty well to what Gordon wants with openat() and friends.

On the ''2004 NAS Industry Conference'' paper of Gordon Waithofer 
(http://www.nasconf.com/pres04/waidhofer.pdf) he proposes to do Linux's 
getxattr/setxattr as ioctl-style attributes and use Solaris form of 
subfiles though openat(,X_ATTR) or attropen(3C) renamed to opensf() (for 
open-sub-file) to remove confusion with attributes. We then don't need the 
`..namedfork' hack anymore if we define a `list sub files' primitive using 
readdir as shown below:


For a summary of Solaris's `attribute' interface :
   "http://docs.sun.com/app/docs/doc/817-0699/6mgfuai80?a=view"

According to Solaris opening a sub-file goes with :
  attrfd = attropen("test", dp->d_name, O_RDONLY);
  --or--
  attrfd = openat(attrdirfd, dp->d_name, O_RDONLY /* ,mode ?*/);


BUT will this work with subfiles associated with directories? shouldn't the 
X_ATTR flags be set in above example of openat?


Listing the associated sub-files goes like :
  attrdirfd = attropen("test", ".", O_RDONLY);
  dirp = fdopendir(attrdirfd);
  while (dp = readdir(dirp)) {
  ...

note that the name attr is confusing again for it is about subfiles and not
about attributes !

where "The attropen() function is a convenience function and is equivalent 
to the following sequence of operations:

  fd = open (path, O_RDONLY);
  attrfd = openat(fd, attrpath, oflag|O_XATTR, mode);
  close(fd);

The set of existing attributes can be browsed by calling attropen() 
with "." as the attribute name. The list of attributes is obtained by 
calling getdents(2) (or fdopendir(3C) followed by readdir(3C), see below) 
on the returned file descriptor."




So to properly support extended attributes IMHO :

1) use getxattr/setxattr with named arguments and defined structures. All 
   conversion to/from on-disc structures are done in the fs. We could also 
   use this to get/set ACL API ``for free'' like linux does.
2) if noone does it, start a named attribute definition reference document 
   and hopefully try to get it synced with Linux and the other bsd's.
3) update our pax/tar to preserve ext. attributes



To properly use subfiles (streamdirs) IMHO :

1) rather use a dedicated opensf()/openat(,X_ATTR) ... than name mangling.
2) extend libc with fdopendir() implementation
3) we could alias opensf() to openat(,X_ATTR) for portability or vice 
   versa
4) once the files are opened, all goes trough the normal FS.
5) update our pax/tar to preserve subfiles.



Thoughts?
Reinoud


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