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From:   Arvind Sankar <nivedita@alum.mit.edu>
Date:   Tue, 14 Jul 2020 17:42:42 -0400
To:     Nick Terrell <nickrterrell@gmail.com>
Cc:     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>,
        Kees Cook <keescook@chromium.org>,
        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>,
        Alex Xu <alex_y_xu@yahoo.ca>, Nick Terrell <terrelln@fb.com>
Subject: Re: [PATCH v7 2/7] lib: add zstd support to decompress
Message-ID: <20200714214242.GA1000731@rani.riverdale.lan>
References: <20200708185024.2767937-1-nickrterrell@gmail.com>
 <20200708185024.2767937-3-nickrterrell@gmail.com>
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On Wed, Jul 08, 2020 at 11:50:19AM -0700, Nick Terrell wrote:
> From: Nick Terrell <terrelln@fb.com>
> 
> * Add unzstd() and the zstd decompress interface.
> * Add zstd support to decompress_method().
> 
> The decompress_method() and unzstd() functions are used to decompress
> the initramfs and the initrd. The __decompress() function is used in
> the preboot environment to decompress a zstd compressed kernel.
> 
> The zstd decompression function allows the input and output buffers to
> overlap because that is used by x86 kernel decompression.
> 
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
> Signed-off-by: Nick Terrell <terrelln@fb.com>
> + *
> + * __DISABLE_EXPORTS stops zstd and xxhash from declaring themselves
> + * as modules by disabling the EXPORT_SYMBOL macro.
> + */

Hi Nick, this doesn't actually work, because misc.c includes export.h
via
	"misc.h" -> <linux/linkage.h> -> <linux/export.h>
and EXPORT_SYMBOL is already defined before __DISABLE_EXPORTS is defined
here.

Adding -D__EXPORT_SYMBOLS to KBUILD_CFLAGS in arch/x86/boot/compressed/Makefile
fixes that, and then you don't have to define it here.

Also, __DISABLE_EXPORTS does not currently inhibit MODULE_INFO, so a
.modinfo section gets emitted into misc.o. I *think* this is harmless
and it will get discarded when building compressed/vmlinux, but thought
I'd point it out.