[PATCH v1 1/1] libstdc++: Set _M_string_length before calling _M_dispose()

[Kefu Chai]

This always sets _M_string_length in the constructor specialized for
range of input_iterator, for the cases like istringstream.

We copy from source range to the local buffer, and then reallocate
to larger one if necessary, when disposing the old buffer. And the
old buffer could be provisioned by the local buffer or an allocated
buffer. _M_is_local() is used to tell if the buffer is the local one
or not. In addition to comparing the buffer address with the local buffer,
this function also performs the sanity check if _M_string_length is
greater than _S_local_capacity, if the check fails
__builtin_unreachable() is called. But we failed to set _M_string_length
in this constructor is specialized for std::input_iterator. So,
if UBSan is enabled when compiling the source, there are chances that
the uninitialized data in _M_string_length is greater than
_S_local_capacity, and the application aborts a runtime error or
exception emitted by the UBSan.

In this change, to avoid the false alarm, _M_string_length is
updated with the length of number of bytes copied to local buffer, so
that _M_is_local() is able to check based on the correct length.

This issue only surfaces when constructing a string with a range of
input_iterator, and the uninitialized _M_string_length is greater than
_S_local_capacity, i.e., 15.

libstdc++-v3/ChangeLog:

        * include/bits/basic_string.tcc (_M_construct): Set
        _M_string_length before calling _M_dispose().

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
---
 libstdc++-v3/include/bits/basic_string.tcc | 1 +
 1 file changed, 1 insertion(+)

diff --git a/libstdc++-v3/include/bits/basic_string.tcc b/libstdc++-v3/include/bits/basic_string.tcc
index 99fdbeee5ad..ec2198ee20b 100644
--- a/libstdc++-v3/include/bits/basic_string.tcc
+++ b/libstdc++-v3/include/bits/basic_string.tcc
@@ -177,6 +177,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    __p[__len++] = *__beg;
 	    ++__beg;
 	  }
+	_M_length(__len);
 
 	struct _Guard
 	{
-- 
2.40.1