NAME
fileassoc
—
in-kernel, file system independent,
file meta-data association
SYNOPSIS
#include
<sys/fileassoc.h>
int
fileassoc_register
(const
char *name,
fileassoc_cleanup_cb_t
cleanup_cb, fileassoc_t
*result);
int
fileassoc_deregister
(fileassoc_t
id);
void *
fileassoc_lookup
(struct
vnode *vp, fileassoc_t
id);
int
fileassoc_table_delete
(struct
mount *mp);
int
fileassoc_table_clear
(struct
mount *mp, fileassoc_t
id);
int
fileassoc_table_run
(struct
mount *mp, fileassoc_t
id, fileassoc_cb_t
cb, void
*cookie);
int
fileassoc_file_delete
(struct
vnode *vp);
int
fileassoc_add
(struct
vnode *vp, fileassoc_t
id, void
*data);
int
fileassoc_clear
(struct
vnode *vp, fileassoc_t
id);
DESCRIPTION
The fileassoc
KPI allows association of
meta-data with files independent of file system support for such elaborate
meta-data.
When plugging a new fileassoc to the system, a developer can specify private data to be associated with every file, as well as (potentially different) private data to be associated with every file system mount.
For example, a developer might choose to associate a custom ACL with every file, and a count of total files with ACLs with the mount.
KERNEL PROGRAMMING INTERFACE
Designed with simplicity in mind, the
fileassoc
KPI usually accepts four different types
of parameters to the most commonly used routines:
- struct mount * mp
- Describing a mount on which to take action.
- struct vnode * vp
- Describing a file on which to take action.
- fileassoc_t id
- Describing an id, as returned from a successful call to
fileassoc_register
(). - void * data
- Describing a custom private data block, attached to either a file or a mount.
Before using the fileassoc
KPI it is
important to keep in mind that the interface provides memory management only
for fileassoc
internal memory. Any additional memory
stored in the tables (such as private data structures used by custom
fileassocs) should be allocated and freed by the developer.
fileassoc
provides the ability to specify a “cleanup” routine to
fileassoc_register
()
(see below) to be called whenever an entry for a file or a mount is
deleted.
REGISTRATION AND DEREGISTRATION ROUTINES
These routines allow a developer to allocate a
fileassoc
slot to be used for private data.
fileassoc_register
(name, cleanup_cb, result)- Registers a new fileassoc as name, and returns a
fileassoc_t via result to be
used as identifier in subsequent calls to the
fileassoc
subsystem.fileassoc_register
() returns zero on success. Otherwise, an error number will be returned.If cleanup_cb is not
NULL
, it will be called during delete/clear operations (see routines below) with indication whether the passed data is file- or mount-specific.cleanup_cb should be a function receiving a void * and returning void. See the EXAMPLES section for illustration.
fileassoc_deregister
(id)- Deregisters a
fileassoc
whose id is id.Note that calling
fileassoc_deregister
() only frees the associated slot in thefileassoc
subsystem. It is up to the developer to take care of garbage collection.
LOOKUP ROUTINES
These routines allow lookup of fileassoc
mounts, files, and private data attached to them.
fileassoc_lookup
(vp, id)- Returns the private data for the file/id combination or
NULL
if not found.
MOUNT-WIDE ROUTINES
fileassoc_table_delete
(mp)- Deletes a fileassoc table for mp.
fileassoc_table_clear
(mp, id)- Clear all table entries for fileassoc from
mp.
If specified, the fileassoc's “cleanup routine” will be called with a pointer to the private data structure.
fileassoc_table_run
(mp, id, cb, cookie)- For each entry for id, call cb
with the entry being the first argument, and cookie
being the second argument.
cb is a function returning void and receiving two void * parameters.
FILE-SPECIFIC ROUTINES
fileassoc_file_delete
(vp)- Delete the fileassoc entries for vp.
If specified, the “cleanup routines” of all fileassoc types added will be called with a pointer to the corresponding private data structure and indication of
FILEASSOC_CLEANUP_FILE
.
FILEASSOC-SPECIFIC ROUTINES
fileassoc_add
(vp, id, data)- Add private data in data for
vp, for the fileassoc specified by
id.
If a table for the mount-point vp is on doesn't exist, one will be created automatically.
fileassoc
manages internally the optimal table sizes as tables are modified. fileassoc_clear
(vp, id)- Clear the private data for vp, for the fileassoc
specified by id.
If specified, the fileassoc's “cleanup routine” will be called with a pointer to the private data structure and indication of
FILEASSOC_CLEANUP_FILE
.
EXAMPLES
The following code examples should give you a clue on using
fileassoc
for your purposes.
First, we'll begin with registering a new id. We need to do that to save a slot for private data storage with each mount and/or file:
fileassoc_t myhook_id; int error; error = fileassoc_register("my_hook", myhook_cleanup, &myhook_id); if (error != 0) ...handle error...
In the above example we pass a
myhook_cleanup
() routine. It could look something
like this:
void myhook_cleanup(void *data) { printf("Myhook: Removing entry for file.\n"); ...handle file entry removal... free(data, M_TEMP); }
Another useful thing would be to add our private data to a file. For example, let's assume we keep a custom ACL with each file:
int myhook_acl_add(struct vnode *vp, struct myhook_acl *acl) { int error; error = fileassoc_add(vp, myhook_id, acl); if (error) { printf("Myhook: Could not add ACL.\n"); ...handle error... } printf("Myhook: Added ACL.\n"); return (0); }
Adding an entry will override any entry that previously exists.
Whatever your plug is, eventually you'll want to access the private data you store with each file. To do that you can use the following:
int myhook_acl_access(struct vnode *vp, int access_flags) { struct myhook_acl *acl; acl = fileassoc_lookup(vp, myhook_id); if (acl == NULL) return (0); error = myhook_acl_eval(acl, access_flags); if (error) { printf("Myhook: Denying access based on ACL decision.\n"); return (error); } return (0); }
And, in some cases, it may be desired to remove private data associated with an file:
int error; error = fileassoc_clear(vp, myhook_id); if (error) { printf("Myhook: Error occurred during fileassoc removal.\n"); ...handle error... }
As mentioned previously, the call to
fileassoc_clear
() will result in a call to the
“cleanup routine” specified in the initial call to
fileassoc_register
().
The above should be enough to get you started.
For example usage of fileassoc
, see the
Veriexec code.
CODE REFERENCES
The fileassoc
is implemented within
src/sys/kern/kern_fileassoc.c.
SEE ALSO
HISTORY
The fileassoc
KPI first appeared in
NetBSD 4.0.
AUTHORS
Elad Efrat
<elad@NetBSD.org>
Brett Lymn
<blymn@NetBSD.org>