Linux Kernel Architecture

Chapter 11: Extended Attributes and Access Control Lists

The following calling convention is used:

❑ If the data buffer passed to the function isNULL, then remove an existing extended attribute.

❑ If the data buffer contains a value, replace an existing extended attribute or create a new one. The

flagsXATTR_REPLACEandXATTR_CREATEcan be used to indicate that the attribute must or must

not exist before the call as per the documentation in the man pagesetxattr(2).

ext3_xattr_set_handleimplements these requirements by utilizing the previously introduced frame-

work as follows:

1. Find the location of the inode.


2. Useext3_xattr_ibody_findto find the data of the extended attribute. If this fails, search in
   the external data block withext3_xattr_block_find.


3. If no value is given, delete the attribute withext3_xattr_ibody_setorext3_xattrblock
   setdepending on whether the entry is contained in the inode or in a separate data block.


4. If a value was given, useext3xattr*_setto modify the value or create a new value either
   within the inode or on the external data block depending on where enough space is left.

The functionsext3_xattr_ibody_setandext3_xattr_block_sethandle the low-level work of remov-

ing an entry from the data structure described in Section 11.1.2. If no value is given to update, the

functions respectively create a new entry. This is primarily a matter of data structure manipulation and

will not be discussed in detail here.

Listing Extended Attributes

Although the kernel includes a generic function (generic_listxattr) for listing all extended attributes

associated with a file, it is not among the filesystemfavorites: Only the shared memory implementation

makes use of it. So let’s step back a little farther to discuss the operation for Ext3.

Theinode_operationsinstance for Ext3 listsext3_listxattras the handler function for

listxattr. The method is just a one-line wrapper forext3_xattr_list. This routine calls, in turn,

ext3_xattr_ibody_listandext3_xattr_block_list, depending on where extended attributes are

stored. Both functions compute the location of the extended attributes and read the data, but then

delegate the work toext3_xattr_list_entries, which finally does the real work — after all, someone

has to do it! It uses the previously introduced macros to iterate over all extended attributes defined

for the inode, callshandler->listto retrieve the name of the attribute for each entry, and collects the

results in a buffer:

fs/ext3/xattr.c

static int

ext3_xattr_list_entries(struct inode *inode, struct ext3_xattr_entry *entry,

char *buffer, size_t buffer_size)

{

size_t rest = buffer_size;

for (; !IS_LAST_ENTRY(entry); entry = EXT3_XATTR_NEXT(entry)) {

struct xattr_handler *handler =

ext3_xattr_handler(entry->e_name_index);

if (handler) {