Avoid integer overflows with correct signing (tracking HB) and assertions

src/hb/ot_layout_gsubgpos.rs

@@ -170,6 +170,9 @@ pub fn match_lookahead(
     start_index: usize,
     end_index: &mut usize,
 ) -> bool {
+    // Function should always be called with a non-zero starting index
+    // c.f. https://github.com/harfbuzz/rustybuzz/issues/142
+    assert!(start_index >= 1);
     let mut iter = skipping_iterator_t::new(ctx, start_index - 1, true);
     iter.set_glyph_data(0);
     iter.enable_matching(match_func);
@@ -312,9 +315,9 @@ impl<'a, 'b> skipping_iterator_t<'a, 'b> {
     }
 
     pub fn prev(&mut self, unsafe_from: Option<&mut usize>) -> bool {
-        let stop = 0;
+        let stop: usize = 0;
 
-        while self.buf_idx > stop as usize {
+        while self.buf_idx > stop {
             self.buf_idx -= 1;
             let info = &self.ctx.buffer.out_info()[self.buf_idx];
 
@@ -858,7 +861,7 @@ fn apply_lookup(
 
     // All positions are distance from beginning of *output* buffer.
     // Adjust.
-    let mut end = {
+    let mut end: isize = {
         let backtrack_len = ctx.buffer.backtrack_len();
         let delta = backtrack_len as isize - ctx.buffer.idx as isize;
 
@@ -867,7 +870,7 @@ fn apply_lookup(
             match_positions[j] = (match_positions[j] as isize + delta) as _;
         }
 
-        backtrack_len + match_end - ctx.buffer.idx
+        backtrack_len as isize + match_end as isize - ctx.buffer.idx as isize
     };
 
     for record in lookups {
@@ -928,8 +931,8 @@ fn apply_lookup(
         //
         // It should be possible to construct tests for both of these cases.
 
-        end = end.saturating_add_signed(delta);
-        if end < match_positions[idx] {
+        end += delta;
+        if end < match_positions[idx] as isize {
             // End might end up being smaller than match_positions[idx] if the recursed
             // lookup ended up removing many items.
             // Just never rewind end beyond start of current position, since that is
@@ -938,8 +941,8 @@ fn apply_lookup(
             // https://bugs.chromium.org/p/chromium/issues/detail?id=659496
             // https://github.com/harfbuzz/harfbuzz/issues/1611
             //
-            delta += match_positions[idx] as isize - end as isize;
-            end = match_positions[idx];
+            delta += match_positions[idx] as isize - end;
+            end = match_positions[idx] as isize;
         }
 
         // next now is the position after the recursed lookup.
@@ -977,7 +980,7 @@ fn apply_lookup(
         }
     }
 
-    ctx.buffer.move_to(end);
+    ctx.buffer.move_to(end.try_into().unwrap());
 }
 
 /// Value represents glyph class.
@@ -1316,6 +1319,9 @@ pub fn ligate_input(
                 if this_comp == 0 {
                     this_comp = last_num_comps;
                 }
+                // Avoid the potential for a wrap-around bug when subtracting from an unsigned integer
+                // c.f. https://github.com/harfbuzz/rustybuzz/issues/142
+                assert!(comps_so_far >= last_num_comps);
                 let new_lig_comp = comps_so_far - last_num_comps + this_comp.min(last_num_comps);
                 _hb_glyph_info_set_lig_props_for_mark(cur, lig_id, new_lig_comp);
             }
@@ -1344,6 +1350,9 @@ pub fn ligate_input(
                 break;
             }
 
+            // Avoid the potential for a wrap-around bug when subtracting from an unsigned integer
+            // c.f. https://github.com/harfbuzz/rustybuzz/issues/142
+            assert!(comps_so_far >= last_num_comps);
             let new_lig_comp = comps_so_far - last_num_comps + this_comp.min(last_num_comps);
             _hb_glyph_info_set_lig_props_for_mark(info, lig_id, new_lig_comp)
         }