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.\" Copyright (c) 2001 Robert N. M. Watson
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.Dd August 23, 2022
.Dt VACCESS 9
.Os
.Sh NAME
.Nm vaccess
.Nd generate an access control decision using vnode parameters
.Sh SYNOPSIS
.In sys/param.h
.In sys/vnode.h
.Ft int
.Fo vaccess
.Fa "enum vtype type"
.Fa "mode_t file_mode"
.Fa "uid_t file_uid"
.Fa "gid_t file_gid"
.Fa "accmode_t accmode"
.Fa "struct ucred *cred"
.Fc
.Sh DESCRIPTION
This call implements the logic for the
.Ux
discretionary file security model
common to many file systems in
.Fx .
It accepts the vnodes type
.Fa type ,
permissions via
.Fa file_mode ,
owning UID
.Fa file_uid ,
owning GID
.Fa file_gid ,
desired access mode
.Fa accmode
and requesting credential
.Fa cred .
.Pp
This call is intended to support implementations of
.Xr VOP_ACCESS 9 ,
which will use their own access methods to retrieve the vnode properties,
and then invoke
.Fn vaccess
in order to perform the actual check.
Implementations of
.Xr VOP_ACCESS 9
may choose to implement additional security mechanisms whose results will
be composed with the return value.
.Pp
The algorithm used by
.Fn 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.
.Pp
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.
.Sh RETURN VALUES
.Fn vaccess
will return 0 on success, or a non-zero error value on failure.
.Sh ERRORS
.Bl -tag -width Er
.It Bq Er EACCES
Permission denied.
An attempt was made to access a file in a way forbidden by its file access
permissions.
.It Bq Er EPERM
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.
.El
.Sh SEE ALSO
.Xr vaccess_acl_nfs4 9 ,
.Xr vaccess_acl_posix1e 9 ,
.Xr vnode 9 ,
.Xr VOP_ACCESS 9
.Sh AUTHORS
This manual page and the current implementation of
.Fn vaccess
were written by
.An Robert Watson .