Re: [PATCH] libstdc++: fix memory clobbering in std::vector [PR110879]

[Vladimir Palevich <palevichva@gmail.com>]

On Thu, 17 Aug 2023 at 01:51, Jonathan Wakely <jwakely@redhat.com> wrote:
>
> On 09/08/23 01:34 +0300, Vladimir Palevich wrote:
> >Because of the recent change in _M_realloc_insert and _M_default_append, call
> >to deallocate was ordered after assignment to class members of std::vector
> >(in the guard destructor), which is causing said members to be call-clobbered.
> >This is preventing further optimization, the compiler is unable to move memory
> >read out of a hot loop in this case.
> >This patch reorders the call to before assignments by putting guard in its own
> >block. Plus a new testsuite for this case.
> >I'm not very happy with the new testsuite, but I don't know how to properly
> >test this.
> >
> >Tested on x86_64-pc-linux-gnu.
> >
> >Maybe something could be done so that the compiler would be able to optimize
> >such cases anyway. Reads could be moved just after the clobbering calls in
> >unlikely branches, for example. This should be a fairly common case with
> >destructors at the end of a function.
> >
> >Note: I don't have write access.
> >
> >-- >8 --
> >
> >Fix ordering to prevent clobbering of class members by a call to deallocate
> >in _M_realloc_insert and _M_default_append.
> >
> >libstdc++-v3/ChangeLog:
> >    PR libstdc++/110879
> >    * include/bits/vector.tcc: End guard lifetime just before assignment to
> >    class members.
> >    * testsuite/libstdc++-dg/conformance.exp: Load scantree.exp.
> >    * testsuite/23_containers/vector/110879.cc: New test.
> >
> >Signed-off-by: Vladimir Palevich  <palevichva@gmail.com>
> >---
> > libstdc++-v3/include/bits/vector.tcc          | 220 +++++++++---------
> > .../testsuite/23_containers/vector/110879.cc  |  35 +++
> > .../testsuite/libstdc++-dg/conformance.exp    |  13 ++
> > 3 files changed, 163 insertions(+), 105 deletions(-)
> > create mode 100644 libstdc++-v3/testsuite/23_containers/vector/110879.cc
> >
> >diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
> >index ada396c9b30..80631d1e2a1 100644
> >--- a/libstdc++-v3/include/bits/vector.tcc
> >+++ b/libstdc++-v3/include/bits/vector.tcc
> >@@ -488,78 +488,83 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> >       private:
> >       _Guard(const _Guard&);
> >       };
> >-      _Guard __guard(__new_start, __len, _M_impl);
> >
> >-      // The order of the three operations is dictated by the C++11
> >-      // case, where the moves could alter a new element belonging
> >-      // to the existing vector.  This is an issue only for callers
> >-      // taking the element by lvalue ref (see last bullet of C++11
> >-      // [res.on.arguments]).
> >+      {
> >+      _Guard __guard(__new_start, __len, _M_impl);
> >
> >-      // If this throws, the existing elements are unchanged.
> >+      // The order of the three operations is dictated by the C++11
> >+      // case, where the moves could alter a new element belonging
> >+      // to the existing vector.  This is an issue only for callers
> >+      // taking the element by lvalue ref (see last bullet of C++11
> >+      // [res.on.arguments]).
> >+
> >+      // If this throws, the existing elements are unchanged.
> > #if __cplusplus >= 201103L
> >-      _Alloc_traits::construct(this->_M_impl,
> >-                             std::__to_address(__new_start + __elems_before),
> >-                             std::forward<_Args>(__args)...);
> >+      _Alloc_traits::construct(this->_M_impl,
> >+                               std::__to_address(__new_start + __elems_before),
> >+                               std::forward<_Args>(__args)...);
> > #else
> >-      _Alloc_traits::construct(this->_M_impl,
> >-                             __new_start + __elems_before,
> >-                             __x);
> >+      _Alloc_traits::construct(this->_M_impl,
> >+                               __new_start + __elems_before,
> >+                               __x);
> > #endif
> >
> > #if __cplusplus >= 201103L
> >-      if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> >-      {
> >-        // Relocation cannot throw.
> >-        __new_finish = _S_relocate(__old_start, __position.base(),
> >-                                   __new_start, _M_get_Tp_allocator());
> >-        ++__new_finish;
> >-        __new_finish = _S_relocate(__position.base(), __old_finish,
> >-                                   __new_finish, _M_get_Tp_allocator());
> >-      }
> >-      else
> >+      if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> >+        {
> >+          // Relocation cannot throw.
> >+          __new_finish = _S_relocate(__old_start, __position.base(),
> >+                                     __new_start, _M_get_Tp_allocator());
> >+          ++__new_finish;
> >+          __new_finish = _S_relocate(__position.base(), __old_finish,
> >+                                     __new_finish, _M_get_Tp_allocator());
> >+        }
> >+      else
> > #endif
> >-      {
> >-        // RAII type to destroy initialized elements.
> >-        struct _Guard_elts
> >         {
> >-          pointer _M_first, _M_last;  // Elements to destroy
> >-          _Tp_alloc_type& _M_alloc;
> >-
> >-          _GLIBCXX20_CONSTEXPR
> >-          _Guard_elts(pointer __elt, _Tp_alloc_type& __a)
> >-          : _M_first(__elt), _M_last(__elt + 1), _M_alloc(__a)
> >-          { }
> >-
> >-          _GLIBCXX20_CONSTEXPR
> >-          ~_Guard_elts()
> >-          { std::_Destroy(_M_first, _M_last, _M_alloc); }
> >-
> >-        private:
> >-          _Guard_elts(const _Guard_elts&);
> >-        };
> >-
> >-        // Guard the new element so it will be destroyed if anything throws.
> >-        _Guard_elts __guard_elts(__new_start + __elems_before, _M_impl);
> >-
> >-        __new_finish = std::__uninitialized_move_if_noexcept_a(
> >-                         __old_start, __position.base(),
> >-                         __new_start, _M_get_Tp_allocator());
> >-
> >-        ++__new_finish;
> >-        // Guard everything before the new element too.
> >-        __guard_elts._M_first = __new_start;
> >-
> >-        __new_finish = std::__uninitialized_move_if_noexcept_a(
> >-                          __position.base(), __old_finish,
> >-                          __new_finish, _M_get_Tp_allocator());
> >-
> >-        // New storage has been fully initialized, destroy the old elements.
> >-        __guard_elts._M_first = __old_start;
> >-        __guard_elts._M_last = __old_finish;
> >-      }
> >-      __guard._M_storage = __old_start;
> >-      __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> >+          // RAII type to destroy initialized elements.
> >+          struct _Guard_elts
> >+          {
> >+            pointer _M_first, _M_last;  // Elements to destroy
> >+            _Tp_alloc_type& _M_alloc;
> >+
> >+            _GLIBCXX20_CONSTEXPR
> >+            _Guard_elts(pointer __elt, _Tp_alloc_type& __a)
> >+            : _M_first(__elt), _M_last(__elt + 1), _M_alloc(__a)
> >+            { }
> >+
> >+            _GLIBCXX20_CONSTEXPR
> >+            ~_Guard_elts()
> >+            { std::_Destroy(_M_first, _M_last, _M_alloc); }
> >+
> >+          private:
> >+            _Guard_elts(const _Guard_elts&);
> >+          };
> >+
> >+          // Guard the new element so it will be destroyed if anything throws.
> >+          _Guard_elts __guard_elts(__new_start + __elems_before, _M_impl);
> >+
> >+          __new_finish = std::__uninitialized_move_if_noexcept_a(
> >+                           __old_start, __position.base(),
> >+                           __new_start, _M_get_Tp_allocator());
> >+
> >+          ++__new_finish;
> >+          // Guard everything before the new element too.
> >+          __guard_elts._M_first = __new_start;
> >+
> >+          __new_finish = std::__uninitialized_move_if_noexcept_a(
> >+                            __position.base(), __old_finish,
> >+                            __new_finish, _M_get_Tp_allocator());
> >+
> >+          // New storage has been fully initialized, destroy the old elements.
> >+          __guard_elts._M_first = __old_start;
> >+          __guard_elts._M_last = __old_finish;
> >+        }
> >+      __guard._M_storage = __old_start;
> >+      __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> >+      }
> >+      // deallocate should be called before assignments to _M_impl,
> >+      // to avoid call-clobbering
> >
> >       this->_M_impl._M_start = __new_start;
> >       this->_M_impl._M_finish = __new_finish;
> >@@ -728,49 +733,54 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
> >             private:
> >               _Guard(const _Guard&);
> >             };
> >-            _Guard __guard(__new_start, __len, _M_impl);
> >-
> >-            std::__uninitialized_default_n_a(__new_start + __size, __n,
> >-                                             _M_get_Tp_allocator());
> >-
> >-            if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> >-              {
> >-                _S_relocate(__old_start, __old_finish,
> >-                            __new_start, _M_get_Tp_allocator());
> >-              }
> >-            else
> >-              {
> >-                // RAII type to destroy initialized elements.
> >-                struct _Guard_elts
> >+
> >+            {
> >+              _Guard __guard(__new_start, __len, _M_impl);
> >+
> >+              std::__uninitialized_default_n_a(__new_start + __size, __n,
> >+                                               _M_get_Tp_allocator());
> >+
> >+              if _GLIBCXX17_CONSTEXPR (_S_use_relocate())
> >                 {
> >-                  pointer _M_first, _M_last;  // Elements to destroy
> >-                  _Tp_alloc_type& _M_alloc;
> >-
> >-                  _GLIBCXX20_CONSTEXPR
> >-                  _Guard_elts(pointer __first, size_type __n,
> >-                              _Tp_alloc_type& __a)
> >-                  : _M_first(__first), _M_last(__first + __n), _M_alloc(__a)
> >-                  { }
> >-
> >-                  _GLIBCXX20_CONSTEXPR
> >-                  ~_Guard_elts()
> >-                  { std::_Destroy(_M_first, _M_last, _M_alloc); }
> >-
> >-                private:
> >-                  _Guard_elts(const _Guard_elts&);
> >-                };
> >-                _Guard_elts __guard_elts(__new_start + __size, __n, _M_impl);
> >-
> >-                std::__uninitialized_move_if_noexcept_a(
> >-                  __old_start, __old_finish, __new_start,
> >-                  _M_get_Tp_allocator());
> >-
> >-                __guard_elts._M_first = __old_start;
> >-                __guard_elts._M_last = __old_finish;
> >-              }
> >-            _GLIBCXX_ASAN_ANNOTATE_REINIT;
> >-            __guard._M_storage = __old_start;
> >-            __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> >+                  _S_relocate(__old_start, __old_finish,
> >+                              __new_start, _M_get_Tp_allocator());
> >+                }
> >+              else
> >+                {
> >+                  // RAII type to destroy initialized elements.
> >+                  struct _Guard_elts
> >+                  {
> >+                    pointer _M_first, _M_last;  // Elements to destroy
> >+                    _Tp_alloc_type& _M_alloc;
> >+
> >+                    _GLIBCXX20_CONSTEXPR
> >+                    _Guard_elts(pointer __first, size_type __n,
> >+                                _Tp_alloc_type& __a)
> >+                    : _M_first(__first), _M_last(__first + __n), _M_alloc(__a)
> >+                    { }
> >+
> >+                    _GLIBCXX20_CONSTEXPR
> >+                    ~_Guard_elts()
> >+                    { std::_Destroy(_M_first, _M_last, _M_alloc); }
> >+
> >+                  private:
> >+                    _Guard_elts(const _Guard_elts&);
> >+                  };
> >+                  _Guard_elts __guard_elts(__new_start + __size, __n, _M_impl);
> >+
> >+                  std::__uninitialized_move_if_noexcept_a(
> >+                    __old_start, __old_finish, __new_start,
> >+                    _M_get_Tp_allocator());
> >+
> >+                  __guard_elts._M_first = __old_start;
> >+                  __guard_elts._M_last = __old_finish;
> >+                }
> >+              _GLIBCXX_ASAN_ANNOTATE_REINIT;
> >+              __guard._M_storage = __old_start;
> >+              __guard._M_len = this->_M_impl._M_end_of_storage - __old_start;
> >+            }
> >+            // deallocate should be called before assignments to _M_impl,
> >+            // to avoid call-clobbering
> >
> >             this->_M_impl._M_start = __new_start;
> >             this->_M_impl._M_finish = __new_start + __size + __n;
> >diff --git a/libstdc++-v3/testsuite/23_containers/vector/110879.cc b/libstdc++-v3/testsuite/23_containers/vector/110879.cc
> >new file mode 100644
> >index 00000000000..d38a5a0d1a3
> >--- /dev/null
> >+++ b/libstdc++-v3/testsuite/23_containers/vector/110879.cc
> >@@ -0,0 +1,35 @@
> >+// -*- C++ -*-
> >+
> >+// Copyright (C) 2023 Free Software Foundation, Inc.
>
> Do you actually have a FSF copyright assignment for GCC?
>
> If not, then this should not be here, as you can't assign it to the
> FSF.
>
> You've already added a Signed-off-by tag, which I assume means you're
> contributing under the terms of the DCO: https://gcc.gnu.org/dco.html
>
> In which case, this isn't copyright FSF.
>
> I've stopped putting the copyright and license header in tests, I
> don't consider a 5 line function that does nothing interesting to be
> copyrightable, or worth adding all this boilerplate to.
>
> If you don't mind, I'll just remove this boilerplate from the test.

I've just copypasted this boilerplate from other test without thinking.
You are correct, I don't have a FSF copyright assignment. I'm contributing
under the terms of the DCO.
You can just remove this boilerplate.

>
>
> >+// This file is part of the GNU ISO C++ Library.  This library is free
> >+// software; you can redistribute it and/or modify it under the
> >+// terms of the GNU General Public License as published by the
> >+// Free Software Foundation; either version 3, or (at your option)
> >+// any later version.
> >+
> >+// This library is distributed in the hope that it will be useful,
> >+// but WITHOUT ANY WARRANTY; without even the implied warranty of
> >+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> >+// GNU General Public License for more details.
> >+
> >+// You should have received a copy of the GNU General Public License along
> >+// with this library; see the file COPYING3.  If not see
> >+// <http://www.gnu.org/licenses/>.
> >+
> >+// { dg-do compile }
> >+// { dg-options "-O3 -fdump-tree-optimized" }
> >+
> >+#include <vector>
> >+
> >+std::vector<int> f(std::size_t n) {
> >+  std::vector<int> res;
> >+  for (std::size_t i = 0; i < n; ++i) {
> >+    res.push_back(i);
> >+  }
> >+  return res;
> >+}
> >+
> >+// Reads of _M_finish should be optimized out.
> >+// This regex matches all reads from res variable except for _M_end_of_storage field.
> >+// { dg-final { scan-tree-dump-not "=\\s*\\S*res_(?!\\S*_M_end_of_storage;)" "optimized" } }
>
> If this was added to gcc/testsuite/g++.dg/ instead of
> libstdc++-v3/testsuite then we wouldn't need scandump.exp and
> scantree.exp in the library tests.

Sure, you can move this test to a more appropriate place.

>
> >diff --git a/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp b/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp
> >index e8c6504a7f7..1d0588aeadc 100644
> >--- a/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp
> >+++ b/libstdc++-v3/testsuite/libstdc++-dg/conformance.exp
> >@@ -18,6 +18,19 @@
> >
> > # libstdc++-v3 testsuite that uses the 'dg.exp' driver.
> >
> >+# Damn dejagnu for not having proper library search paths for load_lib.
> >+# We have to explicitly load everything that gcc-dg.exp wants to load.
> >+
> >+proc load_gcc_lib { filename } {
> >+    global srcdir loaded_libs
> >+
> >+    load_file $srcdir/../../gcc/testsuite/lib/$filename
> >+    set loaded_libs($filename) ""
> >+}
> >+
> >+load_gcc_lib scandump.exp
> >+load_gcc_lib scantree.exp
> >+
> > # Initialization.
> > dg-init
> >
>