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Date:   Tue, 7 Jul 2020 19:49:25 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Nick Terrell <nickrterrell@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, Chris Mason <clm@fb.com>,
        linux-kbuild@vger.kernel.org, x86@kernel.org,
        gregkh@linuxfoundation.org, Petr Malat <oss@malat.biz>,
        Kernel Team <Kernel-team@fb.com>,
        Adam Borowski <kilobyte@angband.pl>,
        Patrick Williams <patrickw3@fb.com>, rmikey@fb.com,
        mingo@kernel.org, Patrick Williams <patrick@stwcx.xyz>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Norbert Lange <nolange79@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nick Terrell <terrelln@fb.com>
Subject: Re: [PATCH v6 2/8] lib: prepare xxhash for preboot environment
Message-ID: <202007071947.5E9723AF48@keescook>
References: <20200707034604.1539157-1-nickrterrell@gmail.com>
 <20200707034604.1539157-3-nickrterrell@gmail.com>
 <20200707215925.GA1591079@rani.riverdale.lan>
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On Tue, Jul 07, 2020 at 05:59:25PM -0400, Arvind Sankar wrote:
> On Mon, Jul 06, 2020 at 08:45:58PM -0700, Nick Terrell wrote:
> > From: Nick Terrell <terrelln@fb.com>
> > 
> > Don't export symbols if XXH_PREBOOT is defined.
> > 
> > This change is necessary to get xxhash to work in a preboot environment,
> > which is needed to support zstd-compressed kernels.
> 
> The usual way to do it is by adding -D__DISABLE_EXPORTS to the CFLAGS, which will
> cause EXPORT_SYMBOL to be stubbed out. Doesn't that work here?

This is quite rare, actually:

$ git grep DISABLE_EXPORTS
arch/s390/purgatory/Makefile:CFLAGS_sha256.o := -D__DISABLE_EXPORTS
arch/x86/boot/compressed/kaslr.c:#define __DISABLE_EXPORTS
arch/x86/purgatory/Makefile:CFLAGS_sha256.o := -D__DISABLE_EXPORTS
drivers/firmware/efi/libstub/Makefile: -D__DISABLE_EXPORTS
include/linux/export.h:#if !defined(CONFIG_MODULES) || defined(__DISABLE_EXPORTS)

But yes, it seems that would be the better approach.

-- 
Kees Cook