[analyzer] Add FIXMEs for alpha.unix.cstring.OutOfBounds false positives.

Caused by incorrect strlcat() modeling in r332303,
cf. https://bugs.llvm.org/show_bug.cgi?id=37687#c8

llvm-svn: 359237

clang/lib/StaticAnalyzer/Checkers/CStringChecker.cpp

@@ -1528,6 +1528,10 @@ void CStringChecker::evalStrlcat(CheckerContext &C, const CallExpr *CE) const {
   if (CE->getNumArgs() < 3)
     return;
 
+  // FIXME: strlcat() uses a different rule for bound checking, i.e. 'n' means
+  // a different thing as compared to strncat(). This currently causes
+  // false positives in the alpha string bound checker.
+
   //char *strlcat(char *s1, const char *s2, size_t n);
   evalStrcpyCommon(C, CE,
                    /* returnEnd = */ false,

clang/test/Analysis/bsd-string.c

@@ -15,6 +15,7 @@ void f1() {
 void f2() {
   char buf[5];
   strlcpy(buf, "abcd", sizeof(buf)); // expected-no-warning
+  // FIXME: This should not warn. The string is safely truncated.
   strlcat(buf, "efgh", sizeof(buf)); // expected-warning{{Size argument is greater than the free space in the destination buffer}}
 }