Avoid stack-overflow and add some defensive conditions

src/blocks.c

@@ -224,18 +224,18 @@ static void remove_trailing_blank_lines(cmark_strbuf *ln) {
 // Check to see if a node ends with a blank line, descending
 // if needed into lists and sublists.
 static bool S_ends_with_blank_line(cmark_node *node) {
-  if (S_last_line_checked(node)) {
-    return(S_last_line_blank(node));
-  } else if ((S_type(node) == CMARK_NODE_LIST ||
-              S_type(node) == CMARK_NODE_ITEM) && node->last_child) {
+  while (!S_last_line_checked(node)) {
     S_set_last_line_checked(node);
-    return(S_ends_with_blank_line(node->last_child));
-  } else {
-    S_set_last_line_checked(node);
-    return (S_last_line_blank(node));
+    if (S_type(node) != CMARK_NODE_LIST && S_type(node) != CMARK_NODE_ITEM)
+      break;
+    if (!node->last_child)
+      break;
+    node = node->last_child;
   }
+  return S_last_line_blank(node);
 }
 
+
 // returns true if content remains after link defs are resolved.
 static bool resolve_reference_link_definitions(cmark_parser *parser) {
   bufsize_t pos;

src/buffer.c

@@ -199,7 +199,8 @@ void cmark_strbuf_unescape(cmark_strbuf *buf) {
   bufsize_t r, w;
 
   for (r = 0, w = 0; r < buf->size; ++r) {
-    if (buf->ptr[r] == '\\' && cmark_ispunct(buf->ptr[r + 1]))
+    if (buf->ptr[r] == '\\' && r + 1 < buf->size &&
+        cmark_ispunct(buf->ptr[r + 1]))
       r++;
 
     buf->ptr[w++] = buf->ptr[r];

src/inlines.c

@@ -934,9 +934,11 @@ static cmark_node *handle_pointy_brace(subject *subj, int options) {
     if (c == '!' && (subj->flags & FLAG_SKIP_HTML_COMMENT) == 0) {
       c = subj->input.data[subj->pos+1];
       if (c == '-' && subj->input.data[subj->pos+2] == '-') {
-	if (subj->input.data[subj->pos+3] == '>') {
+	if (subj->pos + 3 < subj->input.len &&
+            subj->input.data[subj->pos+3] == '>') {
 	  matchlen = 4;
-	} else if (subj->input.data[subj->pos+3] == '-' &&
+	} else if (subj->pos + 4 < subj->input.len &&
+                   subj->input.data[subj->pos+3] == '-' &&
                    subj->input.data[subj->pos+4] == '>') {
           matchlen = 5;
         } else {

src/scanners.c

@@ -13,9 +13,20 @@ bufsize_t _scan_at(bufsize_t (*scanner)(const unsigned char *), cmark_chunk *c,
   } else {
     unsigned char lim = ptr[c->len];
 
-    ptr[c->len] = '\0';
-    res = scanner(ptr + offset);
-    ptr[c->len] = lim;
+    // The re2c scanners are built with yyfill:enable = 0, so they
+    // require a NUL sentinel at ptr[c->len].  In the common case the
+    // chunk is backed by a cmark_strbuf which is already NUL-terminated
+    // at that position, so we avoid the transient write entirely (it
+    // would otherwise mutate shared backing storage and break both
+    // const-correctness and reentrancy).  Only when the sentinel is
+    // missing do we fall back to patching the byte around the call.
+    if (lim == '\0') {
+      res = scanner(ptr + offset);
+    } else {
+      ptr[c->len] = '\0';
+      res = scanner(ptr + offset);
+      ptr[c->len] = lim;
+    }
   }
 
   return res;

src/scanners.re

@@ -12,9 +12,20 @@ bufsize_t _scan_at(bufsize_t (*scanner)(const unsigned char *), cmark_chunk *c,
         } else {
 	  unsigned char lim = ptr[c->len];
 
-	  ptr[c->len] = '\0';
-	  res = scanner(ptr + offset);
-	  ptr[c->len] = lim;
+	  // The re2c scanners are built with yyfill:enable = 0, so they
+	  // require a NUL sentinel at ptr[c->len].  In the common case the
+	  // chunk is backed by a cmark_strbuf which is already NUL-terminated
+	  // at that position, so we avoid the transient write entirely (it
+	  // would otherwise mutate shared backing storage and break both
+	  // const-correctness and reentrancy).  Only when the sentinel is
+	  // missing do we fall back to patching the byte around the call.
+	  if (lim == '\0') {
+	    res = scanner(ptr + offset);
+	  } else {
+	    ptr[c->len] = '\0';
+	    res = scanner(ptr + offset);
+	    ptr[c->len] = lim;
+	  }
         }
 
 	return res;