[gcc r9-10060] libstdc++: Fix undefined behaviour in std::string

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:47c48fcfbec4d606cea7ea885323f453d8336604

commit r9-10060-g47c48fcfbec4d606cea7ea885323f453d8336604
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 4 15:49:38 2021 +0100

    libstdc++: Fix undefined behaviour in std::string
    
    This fixes a ubsan error when constructing a string with a null pointer:
    
    bits/basic_string.h:534:21: runtime error: applying non-zero offset 18446744073709551615 to null pointer
    
    The _M_construct function only cares whether the second pointer is
    non-null, so create a non-null value without undefined arithmetic.
    
    We can also pass the random_access_iterator_tag directly to the
    _M_construct function, to avoid going via the tag dispatching
    _M_construct_aux, because we know we have pointers not integers here.
    
    libstdc++-v3/ChangeLog:
    
            * include/bits/basic_string.h (basic_string(const CharT*, const A&)):
            Do not do arithmetic on null pointer.
    
    (cherry picked from commit 789c57bc5fe023fc6dc72ade4afcb0916ff788d3)

Diff:
---
 libstdc++-v3/include/bits/basic_string.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/bits/basic_string.h b/libstdc++-v3/include/bits/basic_string.h
index 6c11135fa8c..5977b5cb7c0 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -528,7 +528,12 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
 #endif
       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
       : _M_dataplus(_M_local_data(), __a)
-      { _M_construct(__s, __s ? __s + traits_type::length(__s) : __s+npos); }
+      {
+	const _CharT* __end = __s ? __s + traits_type::length(__s)
+	  // We just need a non-null pointer here to get an exception:
+	  : reinterpret_cast<const _CharT*>(__alignof__(_CharT));
+	_M_construct(__s, __end, random_access_iterator_tag());
+      }
 
       /**
        *  @brief  Construct string as multiple characters.