Merge pull request commonmark#254 from github/empty-input

Check for empty buffer when rendering

Author: jgm

src/render.c

@@ -172,7 +172,7 @@ char *cmark_render(cmark_node *root, int options, int width,
   }
 
   // ensure final newline
-  if (renderer.buffer->ptr[renderer.buffer->size - 1] != '\n') {
+  if (renderer.buffer->size == 0 || renderer.buffer->ptr[renderer.buffer->size - 1] != '\n') {
     cmark_strbuf_putc(renderer.buffer, '\n');
   }
 

test/cmark-fuzz.c

@@ -3,24 +3,28 @@
 #include "cmark.h"
 
 int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
-  int options = 0;
-  if (size > sizeof(options)) {
-    /* First 4 bytes of input are treated as options */
-    int options = *(const int *)data;
+  struct __attribute__((packed)) {
+    int options;
+    int width;
+  } fuzz_config;
+
+  if (size >= sizeof(fuzz_config)) {
+    /* The beginning of `data` is treated as fuzzer configuration */
+    memcpy(&fuzz_config, data, sizeof(fuzz_config));
 
     /* Mask off valid option bits */
-    options = options & (CMARK_OPT_SOURCEPOS | CMARK_OPT_HARDBREAKS | CMARK_OPT_SAFE | CMARK_OPT_NOBREAKS | CMARK_OPT_NORMALIZE | CMARK_OPT_VALIDATE_UTF8 | CMARK_OPT_SMART);
+    fuzz_config.options &= (CMARK_OPT_SOURCEPOS | CMARK_OPT_HARDBREAKS | CMARK_OPT_SAFE | CMARK_OPT_NOBREAKS | CMARK_OPT_NORMALIZE | CMARK_OPT_VALIDATE_UTF8 | CMARK_OPT_SMART);
 
     /* Remainder of input is the markdown */
-    const char *markdown = (const char *)(data + sizeof(options));
-    const size_t markdown_size = size - sizeof(options);
-    cmark_node *doc = cmark_parse_document(markdown, markdown_size, options);
+    const char *markdown = (const char *)(data + sizeof(fuzz_config));
+    const size_t markdown_size = size - sizeof(fuzz_config);
+    cmark_node *doc = cmark_parse_document(markdown, markdown_size, fuzz_config.options);
 
-    free(cmark_render_commonmark(doc, options, 80));
-    free(cmark_render_html(doc, options));
-    free(cmark_render_latex(doc, options, 80));
-    free(cmark_render_man(doc, options, 80));
-    free(cmark_render_xml(doc, options));
+    free(cmark_render_commonmark(doc, fuzz_config.options, fuzz_config.width));
+    free(cmark_render_html(doc, fuzz_config.options));
+    free(cmark_render_latex(doc, fuzz_config.options, fuzz_config.width));
+    free(cmark_render_man(doc, fuzz_config.options, fuzz_config.width));
+    free(cmark_render_xml(doc, fuzz_config.options));
 
     cmark_node_free(doc);
   }