generate an access control decision using vnode
, mode_t file_mode
struct ucred *cred
This call implements the logic for the UNIX
discretionary file security model common to many file systems in
. It accepts the vnodes type
, permissions via
, owning UID
, owning GID
, desired access mode
, requesting credential
, and an optional call-by-reference
pointer returning whether or not
privilege was required for successful evaluation of the call; the
pointer may be set to
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
which will use their own access methods to retrieve the vnode properties, and
() in order to perform
the actual check. Implementations of
may choose to implement additional security mechanisms whose results will be
composed with the return value.
The algorithm used by
() 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—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.
() will return 0 on success, or a
non-zero error value on failure.
- Permission denied. An attempt was made to access a file in a way forbidden
by its file access permissions.
- Operation not permitted. An attempt was made to perform an operation
limited to processes with appropriate privileges or to the owner of a file
or other resource.
This manual page and the current implementation of
() were written by