Function vn_extattr_get

freebsd/kern/vfs_vnops.c:2121–2157

* Simplified in-kernel wrapper calls for extended attribute access. * Both calls pass in a NULL credential, authorizing as "kernel" access. * Set IO_NODELOCKED in ioflg if the vnode is already locked. */

 * Set IO_NODELOCKED in ioflg if the vnode is already locked.
 */
int
vn_extattr_get(struct vnode *vp, int ioflg, int attrnamespace,
    const char *attrname, int *buflen, char *buf, struct thread *td)
{
	struct uio	auio;
	struct iovec	iov;
	int	error;

	iov.iov_len = *buflen;
	iov.iov_base = buf;

	auio.uio_iov = &iov;
	auio.uio_iovcnt = 1;
	auio.uio_rw = UIO_READ;
	auio.uio_segflg = UIO_SYSSPACE;
	auio.uio_td = td;
	auio.uio_offset = 0;
	auio.uio_resid = *buflen;

	if ((ioflg & IO_NODELOCKED) == 0)
		vn_lock(vp, LK_SHARED | LK_RETRY);

	ASSERT_VOP_LOCKED(vp, "IO_NODELOCKED with no vp lock held");

	/* authorize attribute retrieval as kernel */
	error = VOP_GETEXTATTR(vp, attrnamespace, attrname, &auio, NULL, NULL,
	    td);

	if ((ioflg & IO_NODELOCKED) == 0)
		VOP_UNLOCK(vp);

	if (error == 0) {
		*buflen = *buflen - auio.uio_resid;
	}

	return (error);
}

/*
 * XXX failure mode if partially written?