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Date:   Tue, 17 May 2022 11:38:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Laight <David.Laight@aculab.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        "x86@kernel.org" <x86@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "linux-hardening@vger.kernel.org" <linux-hardening@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "llvm@lists.linux.dev" <llvm@lists.linux.dev>
Subject: Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG
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On Tue, May 17, 2022 at 08:48:41AM +0000, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 17 May 2022 09:41
> ...
> > > If we use %eax instead of %r10d for the hash transfer (as per Joao), and
> > > use int3 instead of ud2, then we can shrink the fineibt sequence to:
> > >
> > > __cfi_\func:
> > > 	endbr				# 4
> > > 	xorl	$0x12345678, %eax	# 5
> > > 	jz	1f			# 2
> > > 	int3				# 1
> > > \func:
> > > 	...
> > >
> > > Which is 12 bytes, and needs a larger preamble (up from 9 in the current
> > > proposal).
> > 
> > On all that; perhaps it would be good to have a compiler option to
> > specify the preamble size. It can enforce the minimum at 7 to have at
> > least the required:
> > 
> > 	movl $0x12345678, %eax
> > 	int3
> > 	int3
> > 
> > but any larger number will just increase the preamble with int3 padding
> > at the top.
> > 
> > That can go right along with the option to supress endbr when preamble
> > :-)
> 
> You also need a compiler option to specify the register.
> While (I think) %eax is usable in kernel, it isn't in userspace.
> It is used in varargs calls to pass (IIRC) the number of fp
> args that are passed in registers.

You're mistaken, the compiler doesn't emit the FineIBT code *at*all*.
That's all patched in later. For kCFI the mov is never executed and is
only there to make it a valid instruction.