Re: [PATCH v6 2/8] lib: prepare xxhash for preboot environment

Kees Cook
2020-07-08T02:49:30Z

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