tarfs: Fix length calculation for file read

When the offset is within a block rather than at its beginning, the
length should be clamped to the remaining length of the block, not its
total length.

Reported by:	n0ps1ed
MFC after:	1 week
Reviewed by:	markj, emaste
Differential Revision:	https://reviews.freebsd.org/D60904

sys/fs/tarfs/tarfs_subr.c

@@ -428,7 +428,7 @@ tarfs_read_file(struct tarfs_node *tnp, size_t len, struct uio *uiop)
 	TARFS_DPF(VNODE, "%s(%s, %zu, %zu)\n", __func__,
 	    tnp->name, uiop->uio_offset, resid);
 	for (i = 0; i < tnp->nblk && resid > 0; ++i) {
-		if (uiop->uio_offset > tnp->blk[i].o + tnp->blk[i].l) {
+		if (uiop->uio_offset >= tnp->blk[i].o + tnp->blk[i].l) {
 			/* skip this block */
 			continue;
 		}
@@ -456,7 +456,8 @@ tarfs_read_file(struct tarfs_node *tnp, size_t len, struct uio *uiop)
 		while (resid > 0 &&
 		    uiop->uio_offset < tnp->blk[i].o + tnp->blk[i].l) {
 			/* now actual data */
-			copylen = tnp->blk[i].l;
+			copylen = tnp->blk[i].l -
+			    (uiop->uio_offset - tnp->blk[i].o);
 			if (copylen > resid)
 				copylen = resid;
 			auio = *uiop;