Tagfilter extension

When we encounter a tag that causes an HTML 5 parser's content model
flag [1] to be changed to RCDATA, CDATA or RAWTEXT [2] [3], we escape
the tag by replacing its opening "<" with "&lt;".  This causes the tag
to appear verbatim in the page it's placed on.

We do this to prevent users breaking the page content, where the parser
would not interpret further tags as inserted by cmark as HTML until a
matching close tag was hit.  (Such a closing tag could exist if a user
entered it themselves, but it'd cause all cmark-generated markup in
between to be rendered raw, and is unlikely to be desireable behaviour.)

[1] https://www.w3.org/TR/2009/WD-html5-20090423/syntax.html#tokenization
[2] https://www.w3.org/TR/2009/WD-html5-20090212/serializing-html-fragments.html#parsing-html-fragments
[3] https://github.com/google/gumbo-parser/blob/aa91b27b02c0c80c482e24348a457ed7c3c088e0/src/parser.c#L4023-L4053

Makefile

@@ -81,7 +81,7 @@ afl:
 	    -o test/afl_results \
 	    -x test/afl_dictionary \
 	    -t 100 \
-	    $(CMARK) -e table -e strikethrough -e autolink $(CMARK_OPTS)
+	    $(CMARK) -e table -e strikethrough -e autolink -e tagfilter $(CMARK_OPTS)
 
 clang-check: all
 	${CLANG_CHECK} -p build -analyze src/*.c
@@ -156,8 +156,8 @@ $(ALLTESTS): $(SPEC) $(EXTENSIONS_SPEC)
 leakcheck: $(ALLTESTS)
 	for format in html man xml latex commonmark; do \
 	  for opts in "" "--smart"; do \
-	     echo "cmark -t $$format -e table -e strikethrough -e autolink $$opts" ; \
-	     valgrind -q --leak-check=full --dsymutil=yes --suppressions=suppressions --error-exitcode=1 $(PROG) -t $$format -e table -e strikethrough -e autolink $$opts $(ALLTESTS) >/dev/null || exit 1;\
+	     echo "cmark -t $$format -e table -e strikethrough -e autolink -e tagfilter $$opts" ; \
+	     valgrind -q --leak-check=full --dsymutil=yes --suppressions=suppressions --error-exitcode=1 $(PROG) -t $$format -e table -e strikethrough -e autolink -e tagfilter $$opts $(ALLTESTS) >/dev/null || exit 1;\
 	  done; \
 	done;
 

extensions/CMakeLists.txt

@@ -5,6 +5,7 @@ set(LIBRARY_SOURCES
     table.c
     strikethrough.c
     autolink.c
+    tagfilter.c
     ext_scanners.c
     ext_scanners.re
     ext_scanners.h

extensions/core-extensions.c

@@ -2,11 +2,13 @@
 #include "autolink.h"
 #include "strikethrough.h"
 #include "table.h"
+#include "tagfilter.h"
 
 int core_extensions_registration(cmark_plugin *plugin) {
   cmark_plugin_register_syntax_extension(plugin, create_table_extension());
   cmark_plugin_register_syntax_extension(plugin,
                                          create_strikethrough_extension());
   cmark_plugin_register_syntax_extension(plugin, create_autolink_extension());
+  cmark_plugin_register_syntax_extension(plugin, create_tagfilter_extension());
   return 1;
 }

extensions/tagfilter.c

@@ -0,0 +1,59 @@
+#include "tagfilter.h"
+#include <parser.h>
+
+static const char *blacklist[] = {
+    "title",   "textarea", "style",  "xmp",       "iframe",
+    "noembed", "noframes", "script", "plaintext", NULL,
+};
+
+static int is_tag(const unsigned char *tag_data, size_t tag_size,
+                  const char *tagname) {
+  size_t i;
+
+  if (tag_size < 3 || tag_data[0] != '<')
+    return 0;
+
+  i = 1;
+
+  if (tag_data[i] == '/') {
+    i++;
+  }
+
+  for (; i < tag_size; ++i, ++tagname) {
+    if (*tagname == 0)
+      break;
+
+    if (tag_data[i] != *tagname)
+      return 0;
+  }
+
+  if (i == tag_size)
+    return 0;
+
+  if (cmark_isspace(tag_data[i]) || tag_data[i] == '>')
+    return 1;
+
+  if (tag_data[i] == '/' && tag_size >= i + 2 && tag_data[i + 1] == '>')
+    return 1;
+
+  return 0;
+}
+
+static int filter(cmark_syntax_extension *ext, const unsigned char *tag,
+                  size_t tag_len) {
+  const char **it;
+
+  for (it = blacklist; *it; ++it) {
+    if (is_tag(tag, tag_len, *it)) {
+      return 0;
+    }
+  }
+
+  return 1;
+}
+
+cmark_syntax_extension *create_tagfilter_extension(void) {
+  cmark_syntax_extension *ext = cmark_syntax_extension_new("tagfilter");
+  cmark_syntax_extension_set_html_filter_func(ext, filter);
+  return ext;
+}

extensions/tagfilter.h

@@ -0,0 +1,8 @@
+#ifndef TAGFILTER_H
+#define TAGFILTER_H
+
+#include "core-extensions.h"
+
+cmark_syntax_extension *create_tagfilter_extension(void);
+
+#endif

test/CMakeLists.txt

@@ -63,14 +63,14 @@ IF (PYTHONINTERP_FOUND)
     )
 
   add_test(extensions_executable
-    ${PYTHON_EXECUTABLE} "${CMAKE_CURRENT_SOURCE_DIR}/spec_tests.py" "--no-normalize" "--spec" "${CMAKE_CURRENT_SOURCE_DIR}/extensions.txt" "--program" "${CMAKE_CURRENT_BINARY_DIR}/../src/cmark -e table -e strikethrough -e autolink"
+    ${PYTHON_EXECUTABLE} "${CMAKE_CURRENT_SOURCE_DIR}/spec_tests.py" "--no-normalize" "--spec" "${CMAKE_CURRENT_SOURCE_DIR}/extensions.txt" "--program" "${CMAKE_CURRENT_BINARY_DIR}/../src/cmark -e table -e strikethrough -e autolink -e tagfilter"
     )
 
   add_test(roundtrip_extensions_executable
     ${PYTHON_EXECUTABLE}
     "${CMAKE_CURRENT_SOURCE_DIR}/roundtrip_tests.py"
     "--spec" "${CMAKE_CURRENT_SOURCE_DIR}/extensions.txt"
-    "--program" "${CMAKE_CURRENT_BINARY_DIR}/../src/cmark -e table -e strikethrough -e autolink"
+    "--program" "${CMAKE_CURRENT_BINARY_DIR}/../src/cmark -e table -e strikethrough -e autolink -e tagfilter"
     )
 
   add_test(regressiontest_executable

test/afl_test_cases/test.md

@@ -40,4 +40,10 @@ google ~~yahoo~~
 
 google.com http://google.com google@google.com
 
+and <xmp> but
+
+<surewhynot>
+sure
+</surewhynot>
+
 [f]: /u "t"

test/extensions.txt

@@ -424,6 +424,50 @@ Full stop outside parens shouldn't be included http://google.com/ok.
 ````````````````````````````````
 
 
+## HTML tag filter
+
+
+```````````````````````````````` example
+This is <xmp> not okay, but **this** <strong>is</strong>.
+
+<p>This is <xmp> not okay, but **this** <strong>is</strong>.</p>
+
+Nope, I won't have <textarea>.
+
+<p>No <textarea> here either.</p>
+
+<p>This <random /> <thing> is okay</thing> though.</p>
+
+Yep, <totally>okay</totally>.
+
+<!-- HTML comments are okay, though. -->
+<!- But we're strict. ->
+<! No nonsense. >
+<!-- Leave multiline comments the heck alone, though, okay?
+Even with {"x":"y"} or 1 > 2 or whatever. Even **markdown**.
+-->
+<!--- Support everything CommonMark's parser does. -->
+<!---->
+<!--thistoo-->
+.
+<p>This is &lt;xmp> not okay, but <strong>this</strong> <strong>is</strong>.</p>
+<p>This is &lt;xmp> not okay, but **this** <strong>is</strong>.</p>
+<p>Nope, I won't have &lt;textarea>.</p>
+<p>No &lt;textarea> here either.</p>
+<p>This <random /> <thing> is okay</thing> though.</p>
+<p>Yep, <totally>okay</totally>.</p>
+<!-- HTML comments are okay, though. -->
+<p>&lt;!- But we're strict. -&gt;
+&lt;! No nonsense. &gt;</p>
+<!-- Leave multiline comments the heck alone, though, okay?
+Even with {"x":"y"} or 1 > 2 or whatever. Even **markdown**.
+-->
+<!--- Support everything CommonMark's parser does. -->
+<!---->
+<!--thistoo-->
+````````````````````````````````
+
+
 ## Interop
 
 Autolink and strikethrough.