Disable unsigned integer sanitizer for basic_string::replace()

[tomcherry]

Summary

basic_string::replace() has the below line

sz += n2 - __n1;

which fails overflow checks if n1 > n2, as the negative result
from the subtraction then overflows the original __sz when added to
it.

This behavior is valid as unsigned integer overflow is defined to wrap
around the maximum value and that produces the correct final value for
__sz. Therefore, we disable this check on this function.

[srhines]

You probably want to remove the Change-Id section above in your description (or at least drop that when you finally submit this to libc++).

[EricWF]

include/string

2559

Please put this comment inside one of the function bodies, right above the offending line.

I think putting it here will result in it maybe becoming unmaintained.

[EricWF]

Side note: There are plenty of tests in the test-suite that trigger this overflow, so no new tests are needed.

When I have time I'm going to enable -fsanitize=unsigned-integer-overflow once I have time to clean up any existing failures.

[tomcherry]

Moved comments to an appropriate line

[EricWF]

@Tom Do you have commit acces or would you like me to commit this for you?

[tomcherry]

I don't have commit access, so please commit for me.

Thank you

[EricWF]

Committed in r297355.

include/string

_LIBCPP_ASSERT(__get_const_db()->__find_c_from_i(&__pos) == this,
         " referring to this string");
 #endif
     difference_type __p = __pos - begin();
     insert(static_cast<size_type>(__p), __n, __c);
     return begin() + __p;
 }
 
 // replace
+// __sz += __n2 - __n1; in the two functions below can cause unsigned integer overflow,
+// but this is a safe operation, so we disable the check.
 
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos, size_type __n1, const value_type* __s, size_type __n2)
+    _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 {
     _LIBCPP_ASSERT(__n2 == 0 || __s != nullptr, "string::replace received nullptr");
     size_type __sz = size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     __n1 = _VSTD::min(__n1, __sz - __pos);
     size_type __cap = capacity();
     if (__cap - __sz + __n1 >= __n2)
__finish:
     else
         __grow_by_and_replace(__cap, __sz - __n1 + __n2 - __cap, __sz, __pos, __n1, __n2, __s);
     return *this;
 }
 
 template <class _CharT, class _Traits, class _Allocator>
 basic_string<_CharT, _Traits, _Allocator>&
 basic_string<_CharT, _Traits, _Allocator>::replace(size_type __pos, size_type __n1, size_type __n2, value_type __c)
+    _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK
 {
     size_type __sz = size();
     if (__pos > __sz)
         this->__throw_out_of_range();
     __n1 = _VSTD::min(__n1, __sz - __pos);
     size_type __cap = capacity();
     value_type* __p;
     if (__cap - __sz + __n1 >= __n2)