Linux Kernel Architecture

Chapter 11: Extended Attributes and Access Control Lists

for_each_xattr_handleris a macro that iterates over all entries inhandlersuntil it encounters aNULL

entry. For every array element, the kernel compares the handler prefix with the namespace part of the

attribute name. If there is a match, the appropriate handler has been found.

The generic implementations for the other extendedattribute operations differ only slightly from the

code forgeneric_setxattr:

❑ generic_getxattrcallshandler->getinstead of thehandler->set.

❑ generic_removexattrcallshandler->setbut specifiesNULLfor the value and a size of 0. This

triggers, per convention, removing the attribute.^1

generic_listxattrcan operate in two modes: If aNULLpointer instead of a buffer was passed to the

function to hold the result, the code iterates over allhandlers registered in the superblock and calls the

listmethod for the inode in question; sincelistreturns the number of bytes required to hold the result,

they can be summed up to provide predictions about how much memory is required in total. If a buffer

for the results was specified,generic_listxattragain iterates over all handlers, but this time uses the

buffer to actually store the results.

11.1.2 Implementation in Ext3

Among the citizens in filesystem land, Ext3 is one of the most prominent members because it makes it

understood that support for extended attributes is available and well developed. Examine the following

source code to learn more about the filesystem side of extended attribute implementations. This also

raises a question that has not been touched on: namely, how extended attributes are permanently stored

on disk.

Data Structures

As an exemplary citizen, Ext3 starts with some good advice on coding efficiency and employs the generic

implementation presented above. A number of handler functions are provided, and the following map

makes it possible to access handler functions by their identification number and not by their string iden-

tifier; this simplifies many operations and allows a more efficient use of disk space because rather than

the prefix string, only a simple number needs to be stored:

fs/ext3/xattr.c

static struct xattr_handler *ext3_xattr_handler_map[] = {

[EXT3_XATTR_INDEX_USER] = &ext3_xattr_user_handler,

#ifdef CONFIG_EXT3_FS_POSIX_ACL

[EXT3_XATTR_INDEX_POSIX_ACL_ACCESS] = &ext3_xattr_acl_access_handler,

[EXT3_XATTR_INDEX_POSIX_ACL_DEFAULT] = &ext3_xattr_acl_default_handler,

#endif

[EXT3_XATTR_INDEX_TRUSTED] = &ext3_xattr_trusted_handler,

#ifdef CONFIG_EXT3_FS_SECURITY

[EXT3_XATTR_INDEX_SECURITY] = &ext3_xattr_security_handler,

#endif

};

(^1) Note that both aNULLvalueanda size of 0 must be specified for it is possible to have empty attributes with size 0 and an empty
value string (which differs from a NULLvalue).