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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>,
        Sami Tolvanen <samitolvanen@google.com>,
        X86 ML <x86@kernel.org>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        linux-hardening@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH 05/15] x86: Implement function_nocfi
In-Reply-To: <CALCETrVEhL9N_DEM8=rbAzp8Nb2pDitRCZGRAVcE288MBrJ4ug@mail.gmail.com>
References: <20210416203844.3803177-1-samitolvanen@google.com> <20210416203844.3803177-6-samitolvanen@google.com> <20210416211855.GD22348@zn.tnic> <CABCJKud8TvzhcjHCpsrtCJ4B50ZUfaL48F42EhZ2zWKLteAc0Q@mail.gmail.com> <20210416220251.GE22348@zn.tnic> <CALCETrVTtKqD6fonUmT_qr0HJ0X9TWzLGq-wpm+A7XKyjn3W5g@mail.gmail.com> <202104161519.1D37B6D26@keescook> <CALCETrV6WYx7dt56aCuUYsrrFya==zYR+p-YZnaATptnaO7w2A@mail.gmail.com> <202104161601.CFB2CCF84F@keescook> <CALCETrWUS52tzLNiWL5sAVVB5-ko1EW73-TEiO=eZ5jF_QyGPQ@mail.gmail.com> <877dl0sc2m.ffs@nanos.tec.linutronix.de> <CALCETrVEhL9N_DEM8=rbAzp8Nb2pDitRCZGRAVcE288MBrJ4ug@mail.gmail.com>
Date:   Sun, 18 Apr 2021 18:17:06 +0200
Message-ID: <871rb7sh31.ffs@nanos.tec.linutronix.de>
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On Sat, Apr 17 2021 at 17:11, Andy Lutomirski wrote:
> On Sat, Apr 17, 2021 at 4:53 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>> which works for
>>
>>       foo = function_nocfi(bar);
>
> I agree in general.  But right now, we have, in asm/proto.h:
>
> void entry_SYSCALL_64(void);
>
> and that's pure nonsense.  Depending on your point of view,
> entry_SYSCALL_64 is a symbol that resolves to an integer or it's an
> array of bytes containing instructions, but it is most definitely not
> a function void (void).  So, regardless of any CFI stuff, I propose
> that we standardize our handling of prototypes of symbols that are
> opaque to the C compiler.  Here are a couple of choices:
>
> Easy one:
>
> extern u8 entry_SYSCALL_64[];
>
> Slightly more complicated:
>
> struct opaque_symbol;
> extern struct opaque_symbol entry_SYSCALL_64;
>
> The opaque_symbol variant avoids any possible confusion over the weird
> status of arrays in C, and it's hard to misuse, since struct
> opaque_symbol is an incomplete type.

Makes sense.