Function kern_accessat

freebsd/kern/vfs_syscalls.c:2071–2116

}

int
kern_accessat(struct thread *td, int fd, const char *path,
    enum uio_seg pathseg, int flag, int amode)
{
	struct ucred *cred, *usecred;
	struct vnode *vp;
	struct nameidata nd;
	int error;

	if ((flag & ~(AT_EACCESS | AT_RESOLVE_BENEATH)) != 0)
		return (EINVAL);
	if (amode != F_OK && (amode & ~(R_OK | W_OK | X_OK)) != 0)
		return (EINVAL);

	/*
	 * Create and modify a temporary credential instead of one that
	 * is potentially shared (if we need one).
	 */
	cred = td->td_ucred;
	if ((flag & AT_EACCESS) == 0 &&
	    ((cred->cr_uid != cred->cr_ruid ||
	    cred->cr_rgid != cred->cr_groups[0]))) {
		usecred = crdup(cred);
		usecred->cr_uid = cred->cr_ruid;
		usecred->cr_groups[0] = cred->cr_rgid;
		td->td_ucred = usecred;
	} else
		usecred = cred;
	AUDIT_ARG_VALUE(amode);
	NDINIT_ATRIGHTS(&nd, LOOKUP, FOLLOW | LOCKSHARED | LOCKLEAF |
	    AUDITVNODE1 | at2cnpflags(flag, AT_RESOLVE_BENEATH),
	    pathseg, path, fd, &cap_fstat_rights, td);
	if ((error = namei(&nd)) != 0)
		goto out;
	vp = nd.ni_vp;

	error = vn_access(vp, amode, usecred, td);
	NDFREE_NOTHING(&nd);
	vput(vp);
out:
	if (usecred != cred) {
		td->td_ucred = cred;
		crfree(usecred);
	}
	return (error);
}

/*
 * Check access permissions using "effective" credentials.