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Date: Sun, 26 Oct 2025 10:10:32 +0000
To: cygwin AT cygwin DOT com
Subject: Re: XSAVE-related segfaults observed under wine
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From: Pip Cet via Cygwin <cygwin AT cygwin DOT com>
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Pip Cet <pipcet AT protonmail DOT com> writes:

> This is about a bug which was observed when running msys2 binaries in
> Wine under recent Linux kernels on new Intel/AMD CPUs that support the
> PKU/PKRU feature, but it appears to be a bug in Cygwin; specifically,
> in:

I don't think this was ever merged, but I'm not aware of any remaining
objections. Rebased patch follows:

From 7ed7da412248ec0b2d27157980d4c2f6fe63398c Mon Sep 17 00:00:00 2001
From: Pip Cet <pipcet AT protonmail DOT com>
Date: Sun, 26 Oct 2025 10:01:33 +0000
Subject: [PATCH] Cygwin: Fix segfault when XSAVE area sizes are unaligned

On recent AMD and Intel CPUs, the PKU/PKRU feature reports an XSAVE
storage area of just 8 bytes. This is exposed to Cygwin code when we
run in Wine under Linux.

The xsave64 instruction requires a memory area aligned to 64 bytes; in
the setting above, we would no longer meet that requirement, causing
segfaults and abnormal program termination.

This fix aligns the size of the structure we allocate on the stack
before performing an xsave64 to the next 64-byte multiple; the old
code already guarantees that structures of such size are aligned
properly for xsave64. This fixes the issue on Linux and hopefully
avoid it on future Windows systems which expose such features.

Signed-off-by: Pip Cet <pipcet AT protonmail DOT com>
---
 winsup/cygwin/scripts/gendef | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/winsup/cygwin/scripts/gendef b/winsup/cygwin/scripts/gendef
index d60d45431..2dc9bce53 100755
--- a/winsup/cygwin/scripts/gendef
+++ b/winsup/cygwin/scripts/gendef
@@ -243,6 +243,8 @@ sigdelayed:
 	xorl	%ecx,%ecx
 	cpuid	# get necessary space for xsave
 	movq	%rbx,%rcx
+	addq	\$63, %rbx
+	andq	\$-64, %rbx # align to next 64-byte multiple
 	addq	\$0x48,%rbx # 0x18 for alignment, 0x30 for additional space
 	subq	%rbx,%rsp
 	movl	%ebx,0x24(%rsp)
-- 
2.51.1


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