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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Tue, 7 Jul 2020 23:23:29 -0400
To:     Kees Cook <keescook@chromium.org>
Cc:     Arvind Sankar <nivedita@alum.mit.edu>,
        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
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On Tue, Jul 07, 2020 at 07:49:25PM -0700, Kees Cook wrote:
> 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

Looks like Ard added it a couple of years back [0] but it got used only
for the EFI stub and not the decompressors.

[0] http://lkml.kernel.org/r/20180704083651.24360-3-ard.biesheuvel@linaro.org