[all-commits] [llvm/llvm-project] d129ea: [libcxx] Align `__recommend() + 1` by __endian_fa...

[Vitaly Buka via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d129ea8d2fa347e63deec0791faf389b84f20ce1
      https://github.com/llvm/llvm-project/commit/d129ea8d2fa347e63deec0791faf389b84f20ce1
  Author: Vitaly Buka <vitalybuka at google.com>
  Date:   2024-05-02 (Thu, 02 May 2024)

  Changed paths:
    M libcxx/include/string

  Log Message:
  -----------
  [libcxx] Align `__recommend() + 1`  by __endian_factor (#90292)

This is detected by asan after #83774

Allocation size will be divided by `__endian_factor` before storing. If
it's not aligned,
we will not be able to recover allocation size to pass into
`__alloc_traits::deallocate`.

we have code like this 
```
 auto __allocation = std::__allocate_at_least(__alloc(), __recommend(__sz) + 1);
    __p               = __allocation.ptr;
    __set_long_cap(__allocation.count);

void __set_long_cap(size_type __s) _NOEXCEPT {
    __r_.first().__l.__cap_     = __s / __endian_factor;
    __r_.first().__l.__is_long_ = true;
  }

size_type __get_long_cap() const _NOEXCEPT {
    return __r_.first().__l.__cap_ * __endian_factor;
  }

inline ~basic_string() {
    __annotate_delete();
    if (__is_long())
      __alloc_traits::deallocate(__alloc(), __get_long_pointer(), __get_long_cap());
  }
```
1. __recommend() -> even size
2. `std::__allocate_at_least(__alloc(), __recommend(__sz) + 1)` - > not
even size
3. ` __set_long_cap() `- > lose one bit of size for __endian_factor == 2
(see `/ __endian_factor`)
4. `__alloc_traits::deallocate(__alloc(), __get_long_pointer(),
__get_long_cap())` -> uses even size (see `__get_long_cap`)



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