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alloc/foreign: don't use a null base pointer

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We can't use null for the proxy view passed to the successor allocators
because it triggers undefined behaviour.
This commit is contained in:
Danny Robson 2018-03-05 15:57:32 +11:00
parent 923ba6b745
commit b23c274742
1 changed files with 25 additions and 14 deletions
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39
alloc/raw/aligned/foreign.hpp
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@ -37,19 +37,31 @@ namespace util::alloc::raw::aligned {
///
/// This approach will explode if a child allocator wants to write to the
/// range, so the user is mostly restricted to very simple allocators
/// (like 'linear'). This is why we use a view based at nullptr; any
/// mistaken access is quite a lot more likely to fault here.
/// (like 'linear').
///
/// We supply a proxy view at the (probably) invalid address:
/// (void*)alignment
/// This removes any potential bias in the successor allocators. We very
/// specifically cannot use nullptr because it breaks under optimisations
/// due to undefined behaviour of null pointers.
///
/// The proxy view will _probably_ result in a segfault if the successor
/// allocator attempts to read/write directly to it given typical
/// alignments will give addresses far below typical mappings.
template <typename ChildT>
class foreign {
public:
template <typename ...Args>
foreign (util::view<std::byte*> _data, std::size_t _alignment, Args &&...args):
m_successor (
view<std::byte*> {0, _data.size ()},
view<std::byte*> {
reinterpret_cast<std::byte*> (_alignment),
reinterpret_cast<std::byte*> (_alignment + _data.size ()),
},
_alignment,
std::forward<Args> (args)...
),
m_offset (m_successor.data () - _data.data ()),
m_offset (_data.data () - m_successor.data ()),
m_alignment (_alignment)
{ ; }
@ -60,14 +72,14 @@ namespace util::alloc::raw::aligned {
auto ptr= reinterpret_cast<std::byte*> (
m_successor.allocate (size)
);
return ptr - m_offset;
return ptr + m_offset;
}
auto
deallocate (void *ptr, std::size_t size)
{
return m_successor.deallocate (
reinterpret_cast<std::byte*> (ptr) + m_offset, size
reinterpret_cast<std::byte*> (ptr) - m_offset, size
);
}
@ -76,18 +88,18 @@ namespace util::alloc::raw::aligned {
auto offset (const void *ptr) const
{
return m_successor.offset (
reinterpret_cast<const std::byte*> (ptr) + m_offset
reinterpret_cast<const std::byte*> (ptr) - m_offset
);
}
auto data (void) { return m_successor.data () - m_offset; }
auto data (void) const { return m_successor.data () - m_offset; }
auto data (void) { return m_successor.data () + m_offset; }
auto data (void) const { return m_successor.data () + m_offset; }
auto begin (void) { return m_successor.begin () - m_offset; }
auto begin (void) const { return m_successor.begin () - m_offset; }
auto begin (void) { return m_successor.begin () + m_offset; }
auto begin (void) const { return m_successor.begin () + m_offset; }
auto end (void) { return m_successor.end () - m_offset; }
auto end (void) const { return m_successor.end () - m_offset; }
auto end (void) { return m_successor.end () + m_offset; }
auto end (void) const { return m_successor.end () + m_offset; }
auto reset (void) { return m_successor.reset (); }
@ -96,7 +108,6 @@ namespace util::alloc::raw::aligned {
auto remain (void) const { return m_successor.remain (); }
private:
direct<ChildT> m_successor;
std::ptrdiff_t m_offset;
std::size_t m_alignment;