This is necessary to make sure that functions that can have stack protection are not used to update the stack guard.
In that case, the stack guard check would fail when it shouldn't.
guard_setup() calls elf_aux_info(), which, in turn, calls memcpy() to update stack_chk_guard. If either elf_aux_info() or memcpy() have stack protection enabled, __stack_chk_guard will be modified before returning from them, causing the stack protection check to fail.
This change uses a temporary buffer to delay changing __stack_chk_guard until elf_aux_info() returns.