Linux Kernel Architecture

Chapter 11: Extended Attributes and Access Control Lists

Array of

pointers

xattr_handler

s_xattr

struct super_block

Figure 11-1: Data structures used for the

generic xattr implementation.

SystemCalls

Recall that there are three system calls for each extended attribute operation (get, set, and list), which

differ in how the destination is specified. To avoid code duplication, the system calls are structured into

two parts:

1. Find the instance ofdentryassociated with the target object.


2. Delegate further processing to a function common to all three calls.

Looking up thedentryinstance is performed byuser_path_walk,byuser_path_walk_link,orbyread-

ing thedentrypointer contained in thefileinstance, depending on which system call was used. After

this, a common basis for all three system call variants has been established.

In the case ofsetxattr, the common function used for further processing issetxattr; the associated

code flow diagram is shown in Figure 11-2.

Copy name and value from userspace

setxattr

vfs_setxattr

Figure 11-2: Code flow diagram for

setxattr.

First, the routine copies both the name and the attribute value from userspace to kernel space. Since the

value of the extended attribute can have arbitrary contents, the size is not predetermined. The system call

has an explicit size parameter to indicate how many bytes are supposed to be read in. To avoid abuse of

kernel memory, it is ensured that the size of name and value does not exceed the limits imposed by the

following quantities:

limits.h

#define XATTR_NAME_MAX 255 /* # chars in an extended attribute name */

#define XATTR_SIZE_MAX 65536 /* size of an extended attribute value (64k) */

After this preparation step, further processing is delegated tovfs_setxattr. The associated code flow

diagram is shown in Figure 11-3.