quoted

+#define compat_sys_shmat               sys_shmat

What's special about compat_sys_shmat?

It's about SHMLBA definition.
For aarch32 glibc defines it as (__getpagesize () << 2).
For ILP32 there's no definition, and so generic one is used: (__getpagesize ()).

In kernel, for ARM64, COMPAT_SHMLBA defined just as 0x4000. Both
compat and non-compat shmat syscalls pass identical arguments to
do_shmat, except shmlba. Effectively, library expects shmlba to
be 0x1000, as sys_shmat does. And compat_sys_shmat expects 0x4000.

I think, both kernel and library parts are to be fixed. In library
we'd use definition identical to ARM. For kernel we'd use compat
syscall.

My question. Why aarch64 defines COMPAT_SHMLBA as 0x4000? If there's
no specific reason for it, it looks like a bug, and we should
define it like in arch/arm:
        #define SHMLBA  (4 * PAGE_SIZE)          /* attach addr a multiple of this */

Maybe that's why AARCH32 is limited to 4K pages in config.