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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Tue, 17 May 2022 13:36:14 -0700
Message-ID: <CABCJKuePNjvcKqZ3ZRmv9ws_2-L2B+a1y_rSXS_cjprf=grLrw@mail.gmail.com>
Subject: Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     David Laight <David.Laight@aculab.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.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>
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On Mon, May 16, 2022 at 11:44 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, May 16, 2022 at 03:03:02PM -0700, Sami Tolvanen wrote:
> > On Mon, May 16, 2022 at 2:44 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > On Mon, May 16, 2022 at 09:32:55PM +0000, David Laight wrote:
> > >
> > > > > The compiler always generates this specific instruction sequence.
> > > >
> > > > Yes, but there are several ways to encode 'cmpl imm,-6(reg)'.
> > >
> > > Yes, but we don't care. This *always* uses the 32bit immediate form.
> > > Even if the immediate is small.
> >
> > Yes, that part is not a problem, but it's a valid point that LLVM
> > might not always use r8-r15 here, so I will have to check for the REX
> > prefix before blindly attempting to decode the instruction.
>
> LLVM has always used r11 for indirect calls, will that change?

No, this won't change register allocation, but I will have to ensure
that the compiler won't do any unnecessary register shuffling for the
check itself.

Sami