[PATCH] elf: dl-minimal malloc needs to respect fundamental alignment

[Florian Weimer]

On 06/23/2016 06:15 PM, H.J. Lu wrote:

> Malloc alignment has been an isssie.  GCC defines
> MALLOC_ABI_ALIGNMENT as
>
> #ifndef MALLOC_ABI_ALIGNMENT
> #define MALLOC_ABI_ALIGNMENT BITS_PER_WORD
> #endif
>
> which is incorrect for Linux and unusable:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36159
>
> I'd like to see a reasonable solution to address malloc alignment
> on Linux, not just in ld.so.

But this glibc bug is fixed:

   https://sourceware.org/bugzilla/show_bug.cgi?id=6527

As far as I know, we know match or exceed the C11 max_align_t 
requirements.  What we cannot do is to bump max_align_t alignment 
because it affects ABI.  Overall, this entire concept of fundamental 
alignment is somewhat ill-defined.

Over-aligned allocations with default operator new in C++ are a 
different matter and require C++ ABI changes, so that the C++ runtime 
can call the appropriate aligned allocation function.

There are popular interposed mallocs which do not provide sufficient 
alignment, matching max_align_t, as well.

Florian