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 privilege 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.
Implementations of
VOP_ACCESS(9)
may choose to implement additional security mechanisms whose results will
be composed with the return value.
The algorithm used by
vaccess()
selects a component of the file permission 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[em]if there is
a match, then the group component of the permission bits is selected.
If neither the credential 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
compared 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.
