linux-kernel - Re: [PATCH v7 02/10] module: allow symbol exports to be disabled

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Date:   Tue, 2 Jan 2018 18:47:14 -0500 (EST)
From:   Nicolas Pitre <nicolas.pitre@...aro.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
cc:     linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Garnier <thgarnie@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Russell King <linux@...linux.org.uk>,
        Paul Mackerras <paulus@...ba.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Petr Mladek <pmladek@...e.com>, Ingo Molnar <mingo@...hat.com>,
        James Morris <james.l.morris@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-mips@...ux-mips.org,
        linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, x86@...nel.org,
        matt@...eblueprint.co.uk
Subject: Re: [PATCH v7 02/10] module: allow symbol exports to be disabled

On Tue, 2 Jan 2018, Ard Biesheuvel wrote:

> To allow existing C code to be incorporated into the decompressor or
> the UEFI stub, introduce a CPP macro that turns all EXPORT_SYMBOL_xxx
> declarations into nops, and #define it in places where such exports
> are undesirable. Note that this gets rid of a rather dodgy redefine
> of linux/export.h's header guard.
[...]

> --- a/include/linux/export.h
> +++ b/include/linux/export.h
> @@ -83,6 +83,15 @@ extern struct module __this_module;
>   */
>  #define __EXPORT_SYMBOL(sym, sec)	=== __KSYM_##sym ===
>  
> +#elif defined(__DISABLE_EXPORTS)
> +
> +/*
> + * Allow symbol exports to be disabled completely so that C code may
> + * be reused in other execution contexts such as the UEFI stub or the
> + * decompressor.
> + */
> +#define __EXPORT_SYMBOL(sym, sec)
> +

I think you should rather put this first thing in the #if sequence so to 
override the defined(__KSYM_DEPS__) case too.  No need to create build 
dependencies for module symbols that you're going to stub out 
afterwards anyway.


Nicolas