The entry stubs called the title's entry with rbp left as the process
started it (0 on Linux) or as a loop counter left it (0 on Windows). The
title's start code pushes rbp, so the frame-pointer chain of its main
thread ended in a null frame pointer. Unreal Engine walks that chain to
capture call stacks (for an ensure, for example) and reads the return
address at [rbp + 8], so the walk read address 8 and crashed.
Both stubs now push a zero frame record (saved rbp 0, return address 0)
and point rbp at it before the call, so frame-pointer walks stop at the
entry. The Linux stub pushes it below the aligned stack pointer, keeping
the 16-byte alignment at the call. The Windows stub writes it into free
slots of its own frame at rsp+0x40.
PPSA15657 crashed in its stack walk at the title screen in about half of
its runs.
Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_0134RWCyiyydJBbfKz8g9Bwe
MarcZX10