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[[!meta title="Implement xattr Support"]]

Extended attributes (xattr) are a standardized, generic method for storing
additional metadata along with a file (inode). Most modern UNIX filesystems
support xattrs.

In general, xattrs should be used sparingly, as they are less transparent than
data stored as explicit file contents; however, there are some cases where they
really make sense. The Hurd's variant of ext2 presently uses some additional
fields in the inode to store Hurd-specific metadata: most notable passive
translator settings. As these fields are Hurd-specific, they can't be accessed
by the standard methods from Linux for example, so it's not possible to fully
work with a Hurd filesystem on GNU/Linux (copy, backup etc.); and also, even
when on Hurd, only tools that explicitely support the Hurd-specific information
can handle them.

Using extended attributes instead of custom fields for the Hurd-specific
information would be very helpful.

The most important goal of this project thus is to make the Hurd ext2fs server
able to store and read the Hurd-specific information with extended attributes
instead of the custom fields, so it become accessible from other systems. Being
able to access the information through the standard xattr API instead of
Hurd-specific calls is also desirable. (And in turn requires implementing the
generic xattr API first, which can be useful for other purposes as well.)

Completing this project will require digging into some parts of the Hurd, but
it should be quite doable without previous Hurd experience. Some experience
with xattrs might help a bit, but shouldn't be really necessary either.

Some previous work on xattr support is available in [[!GNU_Savannah_patch
5126]], and might serve as a starting point.

Possible mentors: Samuel Thibault (youpi)

Exercise: Implement support for different inode sizes (other than 128 bytes) in
Hurd's ext2fs.