linux-kernel - Re: [PATCH v4 6/8] x86: bump ZO_z_extra

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Date:   Wed, 1 Apr 2020 11:33:10 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Nick Terrell <nickrterrell@...il.com>
Cc:     Nick Terrell <terrelln@...com>, linux-kernel@...r.kernel.org,
        Chris Mason <clm@...com>, linux-kbuild@...r.kernel.org,
        x86@...nel.org, gregkh@...uxfoundation.org,
        Petr Malat <oss@...at.biz>, Kees Cook <keescook@...omium.org>,
        Kernel Team <Kernel-team@...com>,
        Adam Borowski <kilobyte@...band.pl>,
        Patrick Williams <patrickw3@...com>, rmikey@...com,
        mingo@...nel.org, Patrick Williams <patrick@...cx.xyz>,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: [PATCH v4 6/8] x86: bump ZO_z_extra_bytes margin for zstd

On Tue, Mar 31, 2020 at 10:39:11PM -0700, Nick Terrell wrote:
> From: Nick Terrell <terrelln@...com>
> 
> Bump the ZO_z_extra_bytes margin for zstd.
> 
> Zstd needs 3 bytes per 128 KB, and has a 22 byte fixed overhead.
> Zstd needs to maintain 128 KB of space at all times, since that is
> the maximum block size. See the comments regarding in-place
> decompression added in lib/decompress_unzstd.c for details.
> 
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Nick Terrell <terrelln@...com>
> ---
>  arch/x86/boot/header.S | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
> index 97d9b6d6c1af..b820875c5c95 100644
> --- a/arch/x86/boot/header.S
> +++ b/arch/x86/boot/header.S
> @@ -536,8 +536,14 @@ pref_address:		.quad LOAD_PHYSICAL_ADDR	# preferred load addr
>  # the size-dependent part now grows so fast.
>  #
>  # extra_bytes = (uncompressed_size >> 8) + 65536
> +#
> +# ZSTD compressed data grows by at most 3 bytes per 128K, and only has a 22
> +# byte fixed overhead but has a maximum block size of 128K, so it needs a
> +# larger margin.
> +#
> +# extra_bytes = (uncompressed_size >> 8) + 131072
>  
> -#define ZO_z_extra_bytes	((ZO_z_output_len >> 8) + 65536)
> +#define ZO_z_extra_bytes	((ZO_z_output_len >> 8) + 131072)
>  #if ZO_z_output_len > ZO_z_input_len
>  # define ZO_z_extract_offset	(ZO_z_output_len + ZO_z_extra_bytes - \
>  				 ZO_z_input_len)
> -- 

So why is this change unconditional if only this compression alg. needs
it?

-- 
Regards/Gruss,
    Boris.

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