Linux Kernel Architecture

Chapter 11: Extended Attributes and Access Control Lists

the field is not relevant right now, it will become important should an incompatible future version be

developed.

The in-memory representation of every Ext3 inode is augmented with two fields that are relevant for the

ACL implementation:

<ext3_fs_i.h>

struct ext3_inode_info {

...

#ifdef CONFIG_EXT3_FS_POSIX_ACL

struct posix_acl *i_acl;

struct posix_acl *i_default_acl;

#endif

...

}

Whilei_aclpoints to theposix_aclinstance for a regular ACL list associated with an inode,

i_default_aclpoints to the default ACL that may be associated with a directory and is inherited

by subdirectories. Since all information is storedin extended attributes on disk, no extension of the

disk-basedstruct ext3_inodeis necessary.

Note that the kernel does not automatically construct the ACL information for every inode; if the infor-

mation is not present in memory, the fields are set toEXT3_ACL_NOT_CACHED[defined as(void*)-1].

ConversionbetweenOn-Diskand In-MemoryRepresentation

Two conversion functions are available to switch between the on-disk and the in-memory representation:

ext3_acl_to_diskandext3_acl_from_disk. Both are implemented infs/ext3/acl.c.

The latter one takes the raw data as read from the information contained in the extended inode, strips off

the header, and converts the data from little endian format into a format suitable for the system’s CPU

for every entry in the list of ACLs.

The counterpartext3_acl_to_diskworks similarly: It iterates over all entries of a given instance of

posix_acland converts the contained data from the CPU-specific format to little endian numbers with

appropriate lengths.

InodeInitialization

When a new inode is created withext3_new_inode, the initialization of the ACLs is delegated to

ext3_init_acl. In addition to the transaction handle and the instance ofstruct inodefor the new

inode, the function also expects a pointer to the inode of the directory in which the new entry is created:

fs/ext3/acl.c

int

ext3_init_acl(handle_t *handle, struct inode *inode, struct inode *dir)

{

struct posix_acl *acl = NULL;

int error = 0;

if (!S_ISLNK(inode->i_mode)) {