wasm: fix re2 bug

When feeding a `char *` into `re->Match()`, it was converted to a StringPiece,
taking its size as `strlen()`. For our (long) input, that wasn't resulting in
the correct size, and did then freak out the re2 match input validation if the
regular expression has an end anchor, but the endpos wasn't the same as its
length. Since the endpos was taken from `s->len`, and the "length" taken via
the mentioned StringPiece's strlen() call, they did indeed not match.

Worked around by feeding it a properly-constructed std::string instead. I'm a
C++ novice at best, but it does the trick, and I'm reasonable certain it's less
wrong than before.

Fixes open-policy-agent#6376.

Signed-off-by: Stephan Renatus <[email protected]>

internal/compiler/wasm/opa/callgraph.csv

@@ -1068,7 +1068,6 @@ opa_regex_find_all_string_submatch,memcpy
 opa_regex_find_all_string_submatch,compile\28char\20const*\29
 opa_regex_find_all_string_submatch,opa_array
 opa_regex_find_all_string_submatch,memset
-opa_regex_find_all_string_submatch,strlen
 opa_regex_find_all_string_submatch,re2::RE2::Match\28re2::StringPiece\20const&\2c\20unsigned\20long\2c\20unsigned\20long\2c\20re2::RE2::Anchor\2c\20re2::StringPiece*\2c\20int\29\20const
 opa_regex_find_all_string_submatch,fullrune
 opa_regex_find_all_string_submatch,chartorune

wasm/src/regex.cc

@@ -108,20 +108,21 @@ opa_value *opa_regex_find_all_string_submatch(opa_value *pattern, opa_value *val
         return NULL;
     }
 
-    opa_string_t *s = opa_cast_string(value);
+    std::string val(opa_cast_string(value)->v, opa_cast_string(value)->len);
     opa_array_t *result = opa_cast_array(opa_array());
     int nsubmatch = re->NumberOfCapturingGroups() + 1;
     re2::StringPiece submatches[nsubmatch];
 
     // The following is effectively refactored RE2::GlobalReplace:
 
-    const char* p = s->v;
-    const char* ep = p + s->len;
+    const char* beginpos = val.c_str();
+    const char* p = beginpos;
+    const char* ep = p + val.size();
     const char* lastend = NULL;
     int pos = 0;
 
     while (p <= ep && (num_results == -1 || result->len < num_results)) {
-        if (!re->Match(s->v, static_cast<size_t>(p - s->v), s->len, re2::RE2::UNANCHORED, submatches, nsubmatch))
+        if (!re->Match(val, static_cast<size_t>(p - beginpos), val.size(), re2::RE2::UNANCHORED, submatches, nsubmatch))
         {
             break;
         }