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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/99805] [9/10/11 Regression] filesystem::path::parent_path
 got a wrong path
Date: Thu, 01 Apr 2021 20:46:58 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99805

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Wow, this is tricksy.

The bug happens when parsing the string into a path. The path is split into
components and the offset of each component from the beginning of the strin=
g is
stored, so that parent_path() can efficiently erase the part of the native()
string that refers to the last component. The offset is calculated like thi=
s:

          for (auto& c : buf)
            {
              auto pos =3D c.str.data() - _M_pathname.data();
              ::new(output++) _Cmpt(c.str, c.type, pos);
              ++_M_cmpts._M_impl->_M_size;
            }

The bug is that for the COW std::string _M_pathname.data() causes the strin=
g to
be reallocated, and so changes the address at which the string is stored. So
the string_view c.str no longer refers to the same data as _M_pathname. We =
get
a bogus offset for the components, and when we try to remove the last compo=
nent
to get the parent_path(), we don't remove anything from the string.=