linux-kernel - Re: [PATCH] arch/x86/boot: use prefix map to avoid embedded paths

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Date:   Thu, 26 Sep 2019 11:25:30 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ross Burton <ross.burton@...el.com>
Cc:     x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch/x86/boot: use prefix map to avoid embedded paths

Don't forget to CC: lkml and x86@...nel.org on x86 patches.

On Thu, Sep 26, 2019 at 10:11:32AM +0100, Ross Burton wrote:
> From: Bruce Ashfield <bruce.ashfield@...il.com>
> 
> It was observed that the kernel embeds the path in the x86 boot
> artifacts.
> 
> From https://bugzilla.yoctoproject.org/show_bug.cgi?id=13458:
> 
> [
>    If you turn on the buildpaths QA test, or try a reproducible build, you
>    discover that the kernel image contains build paths.
> 
>    $ strings bzImage-5.0.19-yocto-standard |grep tmp/
>    out of pgt_buf in
>    /data/poky-tmp/reproducible/tmp/work-shared/qemux86-64/kernel-source/arch/x86/boot/compressed/kaslr_64.c!?
> 
>    But what's this in the top-level Makefile:
> 
>    $ git grep prefix-map
>    Makefile:KBUILD_CFLAGS  += $(call
>    cc-option,-fmacro-prefix-map=$(srctree)/=)
> 
>    So the __FILE__ shouldn't be using the full path.  However
>    arch/x86/boot/compressed/Makefile has this:
> 
>    KBUILD_CFLAGS := -m$(BITS) -O2
> 
>    So that clears KBUILD_FLAGS, removing the -fmacro-prefix-map option.
> ]
> 
> Other architectures do not clear the flags, but instead prune before
> adding boot or specific options. There's no obvious reason why x86 isn't
> doing the same thing (pruning vs clearing) and no build or boot issues
> have been observed.
> 
> So we make x86 can do the same thing, and we no longer have embedded paths.
> 
> Signed-off-by: Bruce Ashfield <bruce.ashfield@...il.com>
> Signed-off-by: Ross Burton <ross.burton@...el.com>
> ---
>  arch/x86/boot/compressed/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 6b84afdd7538..b246f18c5857 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -38,6 +38,7 @@ KBUILD_CFLAGS += $(call cc-option,-fno-stack-protector)
>  KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
>  KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
>  KBUILD_CFLAGS += -Wno-pointer-sign
> +KBUILD_CFLAGS += $(call cc-option,-fmacro-prefix-map=$(srctree)/=)
>  
>  KBUILD_AFLAGS  := $(KBUILD_CFLAGS) -D__ASSEMBLY__
>  GCOV_PROFILE := n
> -- 

What about arch/x86/boot/Makefile ?

-- 
Regards/Gruss,
    Boris.

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