linux-kernel - [PATCH v5 6/8] x86: bump ZO_z_extra

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Message-Id: <20200408215711.137639-7-nickrterrell@gmail.com>
Date:   Wed,  8 Apr 2020 14:57:09 -0700
From:   Nick Terrell <nickrterrell@...il.com>
To:     Nick Terrell <terrelln@...com>
Cc:     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>,
        Nick Terrell <nickrterrell@...il.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: [PATCH v5 6/8] x86: bump ZO_z_extra_bytes margin for zstd

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.

The existing code is written so that all the compression algorithms use
the same ZO_z_extra_bytes. It is taken to be the maximum of the growth
rate plus the maximum fixed overhead. The comments just above this diff
state that:

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 735ad7f21ab0..6dbd7e9f74c9 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -539,8 +539,14 @@ 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)
-- 
2.26.0