linux-kernel - Re: [PATCH 05/15] x86: Implement function

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Date:   Sun, 18 Apr 2021 15:57:19 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        David Laight <David.Laight@...lab.com>,
        Borislav Petkov <bp@...en8.de>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        X86 ML <x86@...nel.org>, Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        linux-hardening@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 05/15] x86: Implement function_nocfi

On Sun, Apr 18, 2021 at 9:17 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Sat, Apr 17 2021 at 17:11, Andy Lutomirski wrote:
> > On Sat, Apr 17, 2021 at 4:53 PM Thomas Gleixner <tglx@...utronix.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.

Sami, do you want to do this as part of your series or should I write a patch?