msys2-runtime: Cygwin: Fix segfault when XSAVE area sizes are unaligned (#6102)
During signal delivery, Cygwin saves the CPU's extended register state (floating-point, SSE, AVX, etc.) to a stack buffer using the xsave64 instruction, which requires its destination to be 64-byte aligned. Before executing xsave64, the code queries the CPU (via cpuid) for the required buffer size, then subtracts that size (plus a fixed overhead) from the stack pointer.
The stack alignment arithmetic assumes that cpuid returns a size that is a multiple of 64. Until recently, this held true for all x86 CPUs. On recent AMD and Intel CPUs, however, the PKU feature (Protection Keys for Userspace, a memory-protection mechanism) adds an XSAVE component of only 8 bytes, which makes the total size no longer a multiple of 64. The subtraction then places the xsave64 buffer at a misaligned address, causing a segfault.
This was first observed when running Cygwin/MSYS2 under Wine on Linux, where the host kernel exposes the PKU feature directly. The same problem could surface on future Windows versions that expose PKU or other small XSAVE components.
The fix rounds up the cpuid-reported size to the next 64-byte multiple before using it in the stack allocation. The existing code already guarantees correct alignment for any buffer size that is a multiple of 64, so this rounding is sufficient.
Fixes: c607889824 ("Cygwin: sigfe: Fix a bug that signal handler destroys fpu states")
Source: https://cygwin.com/pipermail/cygwin/2025-June/258375.html
This corresponds to https://github.com/msys2/msys2-runtime/pull/329
