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Date:   Mon, 16 May 2022 22:37:23 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, 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-arm-kernel@lists.infradead.org, llvm@lists.linux.dev
Subject: Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG
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On Mon, May 16, 2022 at 12:39:19PM -0700, Sami Tolvanen wrote:

> > > With the current compiler patch, LLVM sets up function arguments after
> > > the CFI check. if it's a problem, we can look into changing that.
> >
> > Yes, please fix that. Again see that same patch for why this is a
> > problem. Objtool can trivially find retpoline calls, but finding this
> > kCFI gadget is going to be hard work. If you ensure they're
> > unconditionally stuck together, then the problem goes away find one,
> > finds the other.
> 
> You can use .kcfi_traps to locate the check right now, but I agree,
> it's not quite ideal.

Oohh, indeed. Looking at that, I think .kcfi_traps would be better as
relative offsets; eg. 'addr = (void*)s + *s' like. Halfs the amount of
storage needed for it.

Also, that code can use a few {} extra.