linux-hardening - Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM

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Date:   Sat, 16 Oct 2021 14:16:43 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Sami Tolvanen <samitolvanen@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v5 03/15] linkage: Add DECLARE_NOT_CALLED_FROM_C

On Sat, Oct 16, 2021 at 12:17:40AM +0200, Thomas Gleixner wrote:
> For actually callable functions, by some definition of callable,
> e.g. the clear_page_*() variants a proper attribute would be definitely
> preferred.

See my last email, clear_page_*() has nothing to do with CFI in the
first place.

> That attribute should tell the compiler that the function is using the
> register arguments correctly but is not suitable for direct invocation
> because it clobbers registers.
> 
> So the compiler can just refuse to call such a function if used directly
> without an inline asm wrapper which describes the clobbers, right?
> 
> But thinking more about clobbers. The only "annotation" of clobbers we
> have today are the clobbers in the inline asm, which is fragile too.
> 
> Something like
> 
>  __attribute__ ((clobbers ("rcx", "rax")))
> 
> might be useful by itself because it allows validation of the clobbers
> in the inline asm wrappers and also allows a analysis tool to look at
> the ASM code and check whether the above list is correct.
> 
> Hmm?

Functions are allowed to clobber rcx and rax anyway.

The clear_page_*() functions follow the C ABI, like (almost) every other
asm function in the kernel.  I think there's a misunderstanding here, as
most of this doesn't have anything to do with CFI anyway.

-- 
Josh