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Date:   Tue, 3 Jan 2023 08:55:26 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Dan Li <ashimida.1990@gmail.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
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        Will Deacon <will@kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Kees Cook <keescook@chromium.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Frederic Weisbecker <frederic@kernel.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Marco Elver <elver@google.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>,
        Song Liu <song@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: [RFC/RFT] CFI: Add support for gcc CFI in aarch64
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On Mon, Dec 19, 2022 at 04:04:55PM +0100, Peter Zijlstra wrote:
> On Mon, Dec 19, 2022 at 05:32:04AM -0800, Dan Li wrote:
> > Hi Peter,
> > 
> > On 12/19, Peter Zijlstra wrote:
> > > On Sun, Dec 18, 2022 at 10:17:58PM -0800, Dan Li wrote:
> > > 
> > > > 1. When a typeid mismatch is detected, the cfi_check_failed function
> > > >    will be called instead of the brk instruction. This function needs
> > > >    to be implemented by the compiler user.
> > > >    If there are user mode programs or other systems that want to use
> > > >    this feature, it may be more convenient to use a callback (so this
> > > >    compilation option is set to -fsanitize=cfi instead of kcfi).
> > > 
> > > This is not going to be acceptible for x86_64.
> > 
> > I'm not familiar enough with the x86_64 platform, could you please
> > tell me why this is not acceptable? Is there a similar situation
> > on the arm64 platform?
> 
> Mostly because the call would be a 5 byte instruction while the trap
> (UD2) is only 2 bytes.
> 
> I suspect Argh64 has a similar problem if the to be called function is
> outside the immediate range (26 bits or thereabout), in which case you
> end up with a multi-instruction sequence to construct the call target or
> so.

Either that or a direct branc to a PLT.

> A trap is always a single instruction.

Indeed.

I strongly prefer the BRK for the reasons I've given in my other reply, which
include code size.

Thanks,
Mark.