From 789c57bc5fe023fc6dc72ade4afcb0916ff788d3 Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Tue, 4 May 2021 15:49:38 +0100
Subject: [PATCH] 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.
---
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 fba7c6f33541..84356adc7ae5 100644
--- a/libstdc++-v3/include/bits/basic_string.h
+++ b/libstdc++-v3/include/bits/basic_string.h
@@ -531,7 +531,12 @@ _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.
--
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