linux-kernel - [PATCH v2] lib/lz4: explicitly support in-place decompression

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Date:   Sun, 22 Nov 2020 11:07:49 +0800
From:   Gao Xiang <hsiangkao@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-erofs@...ts.ozlabs.org,
        Gao Xiang <hsiangkao@...hat.com>,
        Yann Collet <yann.collet.73@...il.com>,
        Nick Terrell <terrelln@...com>, Miao Xie <miaoxie@...wei.com>,
        Chao Yu <yuchao0@...wei.com>,
        Li Guifu <bluce.liguifu@...wei.com>,
        Guo Xuenan <guoxuenan@...wei.com>
Subject: [PATCH v2] lib/lz4: explicitly support in-place decompression

LZ4 final literal copy could be overlapped when doing
in-place decompression, so it's unsafe to just use memcpy()
on an optimized memcpy approach but memmove() instead.

Upstream LZ4 has updated this years ago [1] (and the impact
is non-sensible [2] plus only a few bytes remain), this commit
just synchronizes LZ4 upstream code to the kernel side as well.

It can be observed as EROFS in-place decompression failure
on specific files when X86_FEATURE_ERMS is unsupported,
memcpy() optimization of commit 59daa706fbec ("x86, mem:
Optimize memcpy by avoiding memory false dependece") will
be enabled then.

Currently most modern x86-CPUs support ERMS, these CPUs just
use "rep movsb" approach so no problem at all. However, it can
still be verified with forcely disabling ERMS feature...

arch/x86/lib/memcpy_64.S:
        ALTERNATIVE_2 "jmp memcpy_orig", "", X86_FEATURE_REP_GOOD, \
-                     "jmp memcpy_erms", X86_FEATURE_ERMS
+                     "jmp memcpy_orig", X86_FEATURE_ERMS

We didn't observe any strange on arm64/arm/x86 platform before
since most memcpy() would behave in an increasing address order
("copy upwards" [3]) and it's the correct order of in-place
decompression but it really needs an update to memmove() for sure
considering it's an undefined behavior according to the standard
and some unique optimization already exists in the kernel.

[1] https://github.com/lz4/lz4/commit/33cb8518ac385835cc17be9a770b27b40cd0e15b
[2] https://github.com/lz4/lz4/pull/717#issuecomment-497818921
[3] https://sourceware.org/bugzilla/show_bug.cgi?id=12518
Cc: Yann Collet <yann.collet.73@...il.com>
Cc: Nick Terrell <terrelln@...com>
Cc: Miao Xie <miaoxie@...wei.com>
Cc: Chao Yu <yuchao0@...wei.com>
Cc: Li Guifu <bluce.liguifu@...wei.com>
Cc: Guo Xuenan <guoxuenan@...wei.com>
Signed-off-by: Gao Xiang <hsiangkao@...hat.com>
---
changes since v1:
 - refine commit message;
 - Cc more people.

Hi Andrew,

Could you kindly consider picking this patch up, although
the impact is EROFS but it touchs in-kernel lz4 library
anyway...

Thanks,
Gao Xiang

 lib/lz4/lz4_decompress.c | 6 +++++-
 lib/lz4/lz4defs.h        | 1 +
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/lib/lz4/lz4_decompress.c b/lib/lz4/lz4_decompress.c
index 00cb0d0b73e1..8a7724a6ce2f 100644
--- a/lib/lz4/lz4_decompress.c
+++ b/lib/lz4/lz4_decompress.c
@@ -263,7 +263,11 @@ static FORCE_INLINE int LZ4_decompress_generic(
 				}
 			}

-			LZ4_memcpy(op, ip, length);
+			/*
+			 * supports overlapping memory regions; only matters
+			 * for in-place decompression scenarios
+			 */
+			LZ4_memmove(op, ip, length);
 			ip += length;
 			op += length;

diff --git a/lib/lz4/lz4defs.h b/lib/lz4/lz4defs.h
index c91dd96ef629..673bd206aa98 100644
--- a/lib/lz4/lz4defs.h
+++ b/lib/lz4/lz4defs.h
@@ -146,6 +146,7 @@ static FORCE_INLINE void LZ4_writeLE16(void *memPtr, U16 value)
  * environments. This is needed when decompressing the Linux Kernel, for example.
  */
 #define LZ4_memcpy(dst, src, size) __builtin_memcpy(dst, src, size)
+#define LZ4_memmove(dst, src, size) __builtin_memmove(dst, src, size)

 static FORCE_INLINE void LZ4_copy8(void *dst, const void *src)
 {
-- 
2.18.4