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Date: Mon, 12 Oct 2026 11:16:01 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Kees Cook <kees@kernel.org>
Cc: Thomas Gleixner <tglx@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
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	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
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	Justin Stitt <justinstitt@google.com>, x86@kernel.org,
	llvm@lists.linux.dev, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH 2/3] x86/kcfi: Emit the FineIBT paranoid "cs" prefix as a
 byte
Message-ID: <20261012091601.GN4121339@noisy.programming.kicks-ass.net>
References: <20261012090424.i.497-kees@kernel.org>
 <20261012090436.1676525-2-kees@kernel.org>
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In-Reply-To: <20261012090436.1676525-2-kees@kernel.org>

On Mon, Oct 12, 2026 at 02:04:25AM -0700, Kees Cook wrote:
> The "cs" prefix on the LEA in the FineIBT paranoid caller template is
> padding: it places the LEA's 0xf0 immediate byte where it doubles as the
> LOCK prefix that makes the failing Jcc raise #UD. GNU as warns about it
> in every CONFIG_FINEIBT=y build:
> 
>   Warning: segment override on `lea' is ineffectual

Shees...