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prevector: avoid misaligned member accesses

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`#pragma pack(1)` prevents aligning the struct and its members to their
required alignment. This can result in code that performs non-aligned
reads and writes to integers and pointers, which is problematic on some
architectures.

It also triggers UBsan — see
  https://github.com/bitcoin/bitcoin/pull/17156#issuecomment-543123631
and #17510.
This commit is contained in:
Anthony Towns 2019-12-10 17:05:21 +10:00
parent 1189b6acab
commit 9d933ef919
1 changed files with 9 additions and 5 deletions
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14
src/prevector.h
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@ -15,7 +15,6 @@
#include <type_traits> #include <type_traits>
#include <utility> #include <utility>
#pragma pack(push, 1)
/** Implements a drop-in replacement for std::vector<T> which stores up to N /** Implements a drop-in replacement for std::vector<T> which stores up to N
* elements directly (without heap allocation). The types Size and Diff are * elements directly (without heap allocation). The types Size and Diff are
* used to store element counts, and can be any unsigned + signed type. * used to store element counts, and can be any unsigned + signed type.
@ -147,14 +146,20 @@ public:
}; };
private: private:
size_type _size = 0; #pragma pack(push, 1)
union direct_or_indirect { union direct_or_indirect {
char direct[sizeof(T) * N]; char direct[sizeof(T) * N];
struct { struct {
size_type capacity;
char* indirect; char* indirect;
size_type capacity;
}; };
} _union = {}; };
#pragma pack(pop)
alignas(char*) direct_or_indirect _union = {};
size_type _size = 0;
static_assert(alignof(char*) % alignof(size_type) == 0 && sizeof(char*) % alignof(size_type) == 0, "size_type cannot have more restrictive alignment requirement than pointer");
static_assert(alignof(char*) % alignof(T) == 0, "value_type T cannot have more restrictive alignment requirement than pointer");
T* direct_ptr(difference_type pos) { return reinterpret_cast<T*>(_union.direct) + pos; } T* direct_ptr(difference_type pos) { return reinterpret_cast<T*>(_union.direct) + pos; }
const T* direct_ptr(difference_type pos) const { return reinterpret_cast<const T*>(_union.direct) + pos; } const T* direct_ptr(difference_type pos) const { return reinterpret_cast<const T*>(_union.direct) + pos; }
@ -523,6 +528,5 @@ public:
return item_ptr(0); return item_ptr(0);
} }
}; };
#pragma pack(pop)
#endif // BITCOIN_PREVECTOR_H #endif // BITCOIN_PREVECTOR_H