Fix llvm-special-case-list-fuzzer regexp exception

[vlad.tsyrklevich]

Summary

Original oss-fuzz report:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3727#c2

The minimized test case that causes this failure:
5b 5b 5b 3d 47 53 00 5b 3d 5d 5b 5d 0a [[[=GS.[=][].

Note the string "=GS\x00". The failure happens because the code is
searching the string against an array of known collated names. "GS\x00"
is a hit, but since len takes into account an extra NUL byte, indexing
into cp->name[len] goes one byte past it's allocated memory. Fix this to
use a strlen(cp->name) comparison to account for NUL bytes in the input.

[vlad.tsyrklevich]

• Add regression test

[pcc]

LGTM

Should this fix also be sent upstream?

lib/Support/regcomp.c

  - p_b_coll_elem - parse a collating-element name and look it up
  */
 static char     /* value of collating element */
 p_b_coll_elem(struct parse *p,
     int endc)     /* name ended by endc,']' */
 {
   char *sp = p->next;
   struct cname *cp;
-  int len;
+  size_t len;
 
   while (MORE() && !SEETWO(endc, ']'))
     NEXT();
   if (!MORE()) {
     SETERROR(REG_EBRACK);
     return(0);
   }
   len = p->next - sp;
   for (cp = cnames; cp->name != NULL; cp++)
-    if (strncmp(cp->name, sp, len) == 0 && cp->name[len] == '\0')
+    if (strncmp(cp->name, sp, len) == 0 && strlen(cp->name) == len)
       return(cp->code);  /* known name */
   if (len == 1)
     return(*sp);  /* single character */
   SETERROR(REG_ECOLLATE);     /* neither */
   return(0);
 }
 
 /*

unittests/Support/RegexTest.cpp

 
 TEST_F(RegexTest, MatchInvalid) {
   Regex r1;
   std::string Error;
   EXPECT_FALSE(r1.isValid(Error));
   EXPECT_FALSE(r1.match("X"));
 }
 
+// https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3727
+TEST_F(RegexTest, OssFuzz3727Regression) {
+  // Wrap in a StringRef so the NUL byte doesn't terminate the string
+  Regex r(StringRef("[[[=GS\x00[=][", 10));
+  std::string Error;
+  EXPECT_FALSE(r.isValid(Error));
+}
+
 }