VACCESS(9) BSD Kernel Developer's Manual VACCESS(9)NAMEvaccess — generate an access control decision using vnode parameters
SYNOPSIS
#include <sys/param.h>
#include <sys/vnode.h>
int
vaccess(enum vtype type, mode_t file_mode, uid_t file_uid,
gid_t file_gid, accmode_t accmode, struct ucred *cred,
int *privused);
DESCRIPTION
This call implements the logic for the UNIX discretionary file security
model common to many file systems in FreeBSD. It accepts the vnodes type
type, permissions via file_mode, owning UID file_uid, owning GID
file_gid, desired access mode accmode, requesting credential cred, and an
optional call-by-reference int pointer returning whether or not privilege
was required for successful evaluation of the call; the privused pointer
may be set to NULL by the caller in order not to be informed of privilege
information, or it may point to an integer that will be set to 1 if priv‐
ilege is used, and 0 otherwise.
This call is intended to support implementations of VOP_ACCESS(9), which
will use their own access methods to retrieve the vnode properties, and
then invoke vaccess() in order to perform the actual check. Implementa‐
tions of VOP_ACCESS(9) may choose to implement additional security mecha‐
nisms whose results will be composed with the return value.
The algorithm used by vaccess() selects a component of the file permis‐
sion bits based on comparing the passed credential, file owner, and file
group. If the credential's effective UID matches the file owner, then
the owner component of the permission bits is selected. If the UID does
not match, then the credential's effective GID, followed by additional
groups, are compared with the file group—if there is a match, then the
group component of the permission bits is selected. If neither the cre‐
dential UID or GIDs match the passed file owner and group, then the other
component of the permission bits is selected.
Once appropriate protections are selected for the current credential, the
requested access mode, in combination with the vnode type, will be com‐
pared with the discretionary rights available for the credential. If the
rights granted by discretionary protections are insufficient, then super-
user privilege, if available for the credential, will also be considered.
RETURN VALUESvaccess() will return 0 on success, or a non-zero error value on failure.
ERRORS
[EACCES] Permission denied. An attempt was made to access a
file in a way forbidden by its file access permis‐
sions.
[EPERM] Operation not permitted. An attempt was made to per‐
form an operation limited to processes with appropri‐
ate privileges or to the owner of a file or other
resource.
SEE ALSOvaccess_acl_nfs4(9), vaccess_acl_posix1e(9), vnode(9), VOP_ACCESS(9)AUTHORS
This manual page and the current implementation of vaccess() were written
by Robert Watson.
BSD September 18, 2009 BSD
